NBSIR 73130 Cfi)

Report on Dental Research at

the National Bureau of Standards

Dr. James M. Cassel, et al

Dental Research Section

National Bureau of Standards

Washington, D. C. 20234

January 1973

Semi-annual Progress Report for Period

July - December 1972

Prepared for American Dental Association

National Institute of Dental Research

Veterans Administration

U.S. Army Medical Research & Development

Command

NBSIR 73-130

REPORT ON DENTAL RESEARCH AT

THE NATIONAL BUREAU OF STANDARDS

Dr. James M. Cassel, et al

Dental Research Section

National Bureau of Standards

Washington, D. C. 20234

January 1973

Semi-annual Progress Report for Period

July - December 1972

This is a progress report. The work is incomplete and is continuing.

Results and conclusions are not necessarily those that will be included

in the final report. For this reason, before reference to this report

is made, permission should be requested from the Chief of the Dental

Research Section, National Bureau of Standards, Washington, D. C. 20234

Prepared for American Dental Association

National Institute of Dental Research

Veterans Administration

U.S. Army Medical Research & Development

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

Command

TABLE OF CONTENTS

Page

1. Introduction 1

2. Reports Issued 1

3. Papers Published 2

4. Work In Progress 3

4.1 Properties and Interactions of Tissue Structures and

Restorative Materials 3

4.1.1 Physical, Electrical, Chemical and Mechanical

Properties of Tissues 3

4.1.1.1 Dimensional Changes in Tooth Structure 3

4.1.1.2 Piezoelectric and Dynamic Mechanical

Measurements of Bone, Dentin and

Collagen 5

4.1.1.3 Characteristics and Thermal Stability

of Hard and Soft Tissue Collagen 7 4.1.2 Grafting of Polymers to Natural Tissue 10

4.1.2.1 Grafting to Collagen and Soft Tissues 10

4.1.2.2 Grafting to Hard Tissues 13

4.1.3 Surface Characterization by Water Adsorption 15

4.2 Crystallographic , Solubility and Chemical Studies on

Ca and/or P Minerals 17

4.2.1 Crystal Structure Determinations and Crystal

Chemistry 17

4.2.2 Physical Chemical Studies of Calcium Salts 23

4.3 Structure and Thermodynamic Studies of Noble Metals

and Their Alloys 26

4.4 Developmental, Clinical and Standards-Related Research 27

4.4.1 Three-Dimensional Panoramic X-Ray 27

4.4.2 Composite Restorative and Pit and Fissure

Sealant Materials 28

4.4.3 Dental Ceramics 35

4.4.4 Dental Amalgam Studies 36

4.4.4.1 Studies of Dental Amalgam with Various

Additions 36

4.4.4.2 Study of y2 Phase Distribution at

Amalgam Surfaces 36

4.4.4.3 Correlation of Dimensional Change on Hardening in Amalgam with Marginal

Breakage and Recurrent Caries 36

4.4.5 Dental Cements 37

4.4.6 Polymeric Stabilization of Orthopedic Implants 40

4.4.7 Specifications and Standards 41

4.4.8 Miscellaneous 41

Appendix:

NBS Report 10916 The Crystal Structure of V^Rh^.

NBS Report 10978 Ca(H2P0^)2, A Crystal Structure Containing Unusual

Hydrogen Bonding.

i
NBS Report 10979 Clinical Evaluation of a Radiopaque Composite

Restorative Material After Three and One-half

Years

.

NBS Report 10980 Clinical Evaluation of a Tooth-Restoration

Coupling Agent.

NBS Report 10981 Ca^(PO,) 0, Tetracalcium Diphosphate Monoxide;

Crystal Structure and Relationships to Ca_(PO,) OH

and K^NaCSO^)^.

NBS Report 10982 The Formation of Fluorapatite and Calcium Fluoride

in Topical Fluoride Treatments.

NBS Report 10983 Experimental Study of the Surface and Lattice

Effects on the Solubility of Hydroxy apatite

.

ii
REPORT ON DENTAL RESEARCH

AT THE

NATIONAL BUREAU OF STANDARDS

1. INTRODUCTION

Dental research at the National Bureau of Standards during the

half year ending December 31, 1972, included investigations of a wide range of materials used in restorative dentistry and of hard

and soft natural tissues. The rfesearch program is supported by the

Council on Dental Research of the American Dental Association, the

Collaborative Research Office of the National Institute of Dental

Research, the Dental Research Division of the U. S. Army Medical Research and Development Command, and the Veterans Administration.

Support from the National Institute of Dental Research is also provided in the form of grants to Research Associates sponsored by the

American Dental Association. This support is designated NIDR (Grant)

where reference to it is made. In the summaries of work in progress,

sponsors of various projects are identified. However, since most of

the projects are closely related both technically and administratively, reports on all projects are presented for the information of all

sponsors. Detailed reports issued during the period on some phases

of the research are listed and are appended to this report.

2. REPORTS ISSUED

NBS Report 10916 The Crystal Structure of V^Rh^.

NBS Report 10978 CaCH^PO^) A Crystal Structure Containing Unusual

Hydrogen Bonding.

NBS Report 10979 Clinical Evaluation of a Radiopaque Composite Restorative Material After Three and One-half Years.

NBS Report 10980 Clinical Evaluation of a Tooth-Restoration Coupling

Agent

1
NBS Report 10981 Ca^(P0,)20, Tetracalcium Diphosphate Monoxide;

Crystal Structure and Relationships to Cac.(PO,)„OH

and K^NaCSO^)^.

NBS Report 10982 The Formation of Fluorapatite and Calcium Fluoride

in Topical Fluoride Treatments.

NBS Report 10983 Experimental Study of the Surface and Lattice Effects on the Solubility of Hydroxyapatite.

3. PAPERS PUBLISHED

Dickens, B. ; Bowen, J. S. ; and Brown, W. E. A Refinement of the Crystal

Structure of CaHPO (synthetic monetite) . Acta Cryst. B28: 797-806, March

1972.

Dickens, B. and Bowen, J. S. Refinement of the Crystal Structure of

Ca(H2PO^)2*H20. Acta Cryst. B27 : 2247-2255 , Nov. 1971.

Bowen, R. L. and Argentar, H. A Stabilizing Comonomer: I. Synthesis and

Confirmation of Structure. J. Dent. Res. 51:1071-1074, July-Aug. 1972.

Dickson, G. and Cassel, J. M. Dental Materials Research. Proc. 50th Anniv.

Symposium, National Bureau of Standards 1969, NBS Spec. Pub. 354, July 1972.

Bowen, R. L. ; Barton, J. A., Jr.; and Mullineaux, A. L. Composite Restorative Materials, In: Dental Materials Research, Proc. 50th Annlv. Symposium, National Bureau of Standards 1969, NBS Spec. Pub. 354, July 1972, p. 93-100.

Brauer, G. M. Cements Containing 0-ethoxybenzoic Acid (EBA) . In: Dental

Materials Research, Proc. 50th Annlv. Symposium, National Bureau of

Standards, NBS Spec. Pub. 354, July 1972, pgs. 101-111.

Oglesby, P. L. Viscoelastic Behavior. In: Dental Materials Research, Proc.

50th Anniv. Symposium, National Bureau of Standards 1969, NBS Spec. Pub.

354, July 1972, pgs. 127-144.

Dickson, G. Ultrasonic Methods for Determination of Mechanical Properties.

In: Dental Materials Research, Proc. 50th Anniv. Symposium, National Bureau

of Standards 1969, NBS Special Pub. 354, 1972, pp. 161-168.

Huget, E. F. : Brauer, G. M. ; Kumpula, J. W. and Clvjan, S. Filled ColdCuring Acrylic Resin as a Splinting Material. US Patent 3,675,327, July 1972.

Paf fenbarger , G. C. The Role of Dental Materials in the Prevention of

Dental Diseases. Int. Dent. J. 22:343-349, September 1972.

Dickens, B. and Brown, W. E. The Crystal Structure of CaKAsO -SH 0.

Acta Cryst. B28 : 3056-3065 , October 1972.

Loebenstein, W. V. Heats of Adsorption of Ammonia and Carbon Dioxide on Tooth Components. J. Dent. Res. 51:1529-1536, Nov. -Dec. 1972.

Bowen, R. L. and Argentar, H. A Stabilizing Comonomer: II. Stabilization

and Polymerization Characteristics. J. Dent. Res. 51:1614-1618, Nov. -Dec.

1972.

2
4. WORK IN PROGRESS

4.1. Properties and Interactions of Tissue

Structures and Restorative Materials

4.1.1 Physical, Electrical, Chemical and

Mechanical Properties of Tissues

Sponsor: NIDR

4.1.1.1 Dimensional Changes in Tooth Structure

One of the properties of tooth or bone structure which must be

given consideration in any restoration procedure is the capacity to

expand or contract as a function of temperature. While the determination of this property for enamel is straightforward over all

temperatures to which the mouth conceivably may be exposed (near 0°C

to 60°C) , a time dependent aspect combined with orientation effects

have made the similar measurement on dentin and bone more complex.

Aside from the relation to the clinical situation, these types of

measurements are important because they assist in defining the nature

of the natural composite systems in these respective tissues.

Measurements of the thermal expansion of bone were made over

the temperature range of 10 to 70°C. Results for bovine femur are similar in many respects to those obtained for human tooth dentin.

The larger specimens available from bone facilitate the study of the

anisotropic characteristics of the thermal expansion. Cubiform

specimens were prepared so that measurements could be made on a single

specimen in the axial, circumferential and radial directions of the

femur. Average results for the temperature range of 10 to 50° C are given in the table:

Direction Coefficient X 10 per °C

Rapid Slow Approximate

Heating Heating Range

Axial 7.9 7.2 ± 1.0

Circumferential 2.7 1.4 t 2.0

Radial -0.8 -1.4 ± 5.0

Volume (calculated) 9.8 7.2

The coefficient in the axial direction is the same within ex- perimental variation as that found in the axial direction for human

3
tooth root dentin during rapid temperature changes. Time dependent

dimensional changes are present in bone as in dentin, but are con- fined primarily to the circumferential and radial directions. The

time scale for these effects appears to be shorter in bone than in

dentin. At the fastest rates of temperature change used in this investigation (about 4°C/min) , the thermal expansion curves in the

circumferential and radial directions are not linear and variations

with time and previous thermal history are readily apparent. These

variations are indicated by the large ranges of values on which the

average coefficients shown in the table are based. Between 50 and

70° C, the coefficient in the axial direction remained essentially

the same as for lower temperatures. In the other directions, the

rate of time dependent shrinkage on heating generally increased above

50°C. The observed differences between human tooth dentin and bovine

femur are probably in part the result of more uniformity of structural

orientation in the bone specimens.

The time dependent dimensional changes observed in bovine femur

support the previously suggested mechanism for such changes in dentin,

namely that they result from changes in the water content of the

organic component of the material. Large changes in water content

~6

(190 X 10 g/g per °C based on dry weight) were determined for the

bovine femur, but these cannot be quantitatively related to the linear

dimensional changes. The methods do not permit simultaneous measurements of dimensions and water content and are not of high enough

precision to establish quantitative relationships even if simultaneous determinations could be made. However, the water content

measurements do provide data for calculating the volumetric thermal

expansion of the organic component of the bone. Using the data in

the table, a measured value of 12.5% for the water content of the

bone at 23°C, published values for the expansion of water and the

mineral component of the bone and a published value of 35 to 65 for

the ratio of the organic to the mineral content of dry bone, the

calculated value for the volumetric thermal expansion of the organic

~6

component of the bone is 560 x 10 /°C. This is in agreement with

~6

the average value of 540 x 10 previously determined by Weir for

kangaroo tail tendon collagen (NBS J. Res. 41:279, 1948).

4
4.1.1.2 Piezoelectric and Dynamic Mechanical

Measurements of Bone, Dentin and Collagen

The construction and assembly of piezoelectric and mechanical

properties measurement apparatus is being brought to completion.

This apparatus is to be used in an extensive and long-term study of

the properties of bone and collagen over a wide range of temperature,

humidity and frequency. Several project goals are: (1) to obtain

a molecular explanation for the piezoelectric effect, (2) to obtain

a collection of basic characterization data, (3) to observe the effect of chemical alteration (such as fluorine substitution) in bone

and collagen, and (4) to investigate the differences in the effects

which occur in bone and in collagen. Two important long-range goals are the use of the piezoelectric effect as a diagnostic tool and an investigation of the role of the piezoelectric effect in caries

formation and osteogenesis.

The schematic diagram of figure 1 shows the components of the

apparatus. Essentially, the system consists of a transducer driver

for applying an alternating stress to the sample, a voltage detection

circuit for measuring the piezoelectric voltage, a phase measuring

circuit for determining the phase difference between applied stress

and resultant observed piezoelectric voltage or resultant strain,

and a displacement transducer to detect sample strain. Measurement

of the phase angle will provide an observation of the viscoelastic

effects in terms of the loss tangent tan 6.

All components of the system, except the amplifier in the bucking voltage circuit, have been constructed as purchased. The feasibility of the system has been established in an observation of the

stress-strain properties of a sample of dry bovine femur. In this

experiment, a 220-volt rms signal at 150 Hz was applied to a ceramic

transducer driver. This resulted in an alternating stress of about

1 lb. being applied to the sample and a corresponding strain in the

-3

sample of approximately 5 x 10 %. Sample displacement was approximately 5 X 10 ^ cm which gave an adequate signal of 50 millivolts

from the displacement transducer.

Before complete experimental runs can be made, an extensive

calibration of the system must be undertaken. The phase shift in

5
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-6-
each amplifier must be determined and the accuracy of the -1 inverting amplifier and the tt/2 phase shifter must be known. Also, the

load cell and the displacement transducer must be calibrated. These

calibrations will require the assembly of specific calibration circuits.

At this time, we are proceeding with the calibration of the

phase shift in the amplifiers. A result from a measurement of the

-3

phase shift in the -1 amplifier is 10 radians at 1000 Hz, but only

10 ^ radians at 10 Hz, and less than 10 ^ radians at frequencies below

-3

10 Hz. Since we are interested in a resolution of 10 radians, cor- rection for the amplifier phase shift becomes significant only above

10 Hz.

For the immediate future, calibration will continue for several

months and the sample holder design will be formulated.

4.1.1.3 Characteristics and Thermal Stability

of Hard and Soft Tissue Collagen

Compared with collagen from other sources, the collagen derived

from bovine dentin (and, presumably, human dentin) has been shown by

Veis to have infinitely less capacity to absorb large quantities of

water as the system is made acidic or basic. This fact has been attributed to a combination of the morphology of dentinal collagen, i.e.

its thin fibers packed in a very tightly interwoven array, and an apparently greater degree of crosslinking. By comparing the interaction of collagens from different tissue sources with a variety of

solvents, we are seeking to provide better understanding on a macromolecular level of these collagenous tissues and to relate this information to that being obtained on a molecular level by amino acid

sequence studies in other laboratories.

It is well known that compounds which form strong hydrogen bonds

can disrupt the hydrogen bonding that exists in synthetic and natural

polypeptides. However, care must be exercised that the chemical agents

do not degrade the peptide bonds as well. Thus, certain strong or- ganic acids, e.g. trifluoroacetic acid, in addition to disrupting

the inter or intra-molecular hydrogen bonding of peptides also can cause degradation of the polyamide backbone. It has been reported

7
that certain highly fluorinated alcohols and diols possess excellent

hydrogen bonding capacities while causing little, if any, of the degradative side effects. For example, it is claimed that even leather,

i.e. tanned collagen, may be altered (improved scuff resistance, oil

and water repellency, etc.) by treatment with certain polyfluorinated

gem diols ( U. S. Patent 3,094,370). Since these diols and other

fluorinated alcohols are such potent solvents for a wide variety of

polar polymers (polyamides, polyesters, polyethers, etc.), it is of

interest to determine their effect on collagen, especially dentinal

collagen.

We have found that highly fluorinated alcohols such as 2,2,2-

trif luoroethanol , 1,1, 3-trihydrotetraf luoropropanol , hexafluoroisopropanol and the gem diol of sym-dichlorotetraf luoroacetone cause significant changes in the morphology of kangaroo tail tendon,

achilles tendon, and collagen film.

For example, at room temperature 2 , 2

, 2-trifluoroethanol will

cause an initial rapid lateral swelling and then subsequent longitudinal contraction (denaturation) in kangaroo tail tendon. Achilles

tendon, on the other hand, exhibits only slow lateral expansion in

this solvent. Bovine dentinal collagen (demineralized) exhibits very

little swelling, but some curling at the edges after 24 hours at room

temperature.

A more potent solvent is hexafluoroisopropanol. Kangaroo tail

tendon is most affected by this agent (very rapid lateral swelling,

followed by longitudinal contraction, gelation and possibly some solution degradation). Achilles tendon behaves in a similar fashion

at about the same rate. Dentinal collagen does not appear to swell

in hexafluoroisopropanol even after prolonged exposure (several weeks).

However, this form of collagen does appear to be undergoing some

change, perhaps in morphology, as witnessed by the edges of the collagen curled inward and the material becoming translucent. After re- moval from the solvent and washing with distilled water and air drying, the treated collagen seemed tougher than the untreated material.

On immersion in water, the curling effect returned. Aqueous solutions

of hexaf luoroisopropanol had similar effect on all forms of collagen.

8
In the case of aqueous solution of the gem diol of symdichlorotetraf luoroacetone , pronounced rapid swelling followed by

longitudinal contraction was observed with kangaroo tail tendon and

achilles tendon. With dentinal collagen, not much swelling was observed, but, again, the edges of the collagen curl inward, probably

indicative of strong internal stresses occurring in the structure.

As reported earlier, certain forms of collagen exhibit interesting swelling behavior when immersed in certain organic compounds

(monomers, such as 2-chloroethyl methacrylate and alkyl iodides) in

the presence of water. In the case of the methacrylate monomers (e.g.

2-chloroethyl methacrylate, trif luoroethyl methacrylate, ethylene

dimethacrylate) only lateral swelling has been observed. Presently,

it is suspected that traces of impurities of methacrylic acid and

the corresponding alcohol may be contributing, if not causing, the

swelling phenomenon. However, methacrylate monomers that have been

washed repeatedly with water to remove these impurities still ex- hibit this swelling potency. An apparent decrease in pH with time

suggests that some hydrolysis of the ester may be occurring. We

-4

find that only a very small amount of methacrylic acid in water <10

mol/1 can induce significant swelling in kangaroo tail tendon.

Dentinal collagen does not appear to swell significantly under these

conditions

.

Achilles tendon, again, exhibits intermediate swelling behavior.

In none of the above experiments was any longitudinal contraction

observed.

A third potent swelling system is certain alkyl iodides in the

presence of water. Particularly effective are methyl iodide, ethyl

iodide and perf luoroheptyl iodide. All cause gradual but very pronounced lateral swelling in kangaroo tail tendon. No longitudinal

contraction is observed. In fact, after several months the tendon

appears to be converted to a film-like form of collagen. Thus far,

dentinal collagen has shown only slight swelling and some tendency

to curl in these solvent systems. Achilles tendon exhibits intermediate behavior in this solvent system.

9
A study of mineralized forms of collagen such as turkey leg

tendon, and bone by differential scanning calorimetry (DSC), re- vealed no melt endotherms below 90° C. A sample of expanded kangaroo

tail tendon that had been swollen by octaf luoropentyl methacrylatetreatment and then washed and subjected to vacuum freeze drying

exhibited a melt endotherm at 55°C compared with 59°C for a control

specimen.

4.1.2 Grafting of Polj^ners to Natural Tissue

4.1.2.1 Grafting to Collagen and Soft Tissues

Sponsor: U. S. Army

The physical properties of previously synthesized steer hide

collagen graft copolymers were studied. Water sorption at 50% RH was always lowered on treatment of the collagen. This even applied

for products containing hydrophilic groups in the side chain. Thus,

the collagen-hydroxyethyl methacrylate product had a water uptake

of 7.4% compared to 19.2% for collagen powder. Water sorption was dependent on the presence of hydrophilic groups and also on the

amount of copolymer incorporated in the product. Lowest water uptake

of about 4% was obtained for products containing fluorinated acrylates

or the higher molecular weight esters of acrylic or methacrylic acid.

The reaction products of collagen with acrylic acid, methacrylic

acid, hydroxyethyl methacrylate and cyanoethyl acrylate were hydrophilic and gave a zero contact angle within 15 seconds. Exact contact

angles were difficult to determine because of the porous and uneven

surface of the specimens. The collagen-lauryl methacrylate and

collagen-2-ethylhexyl acrylate had approximate contact angles of 85°

whereas those of the fluorinated acrylates and methacrylate products

had contact angle values over 90°.

A procedure was worked out to graft monomers containing basic

groups such as dimethylaminoethyl methacrylate onto collagen. These

could not be grafted by the procedure previously used. The aqueous

solution of the monomer is acidified with 2N nitric acid prior to

the addition of the monomer to the collagen. Although a considerable

10
amount of soluble homopolymer formed, some grafting also took place

as indicated by a weight increase after extraction of homopolymer.

A paper entitled "Grafting of Acrylates and Vinyl Chains Upon

Collagen with Ceric Initiator" has been accepted for publication in

the Journal of Applied Polymer Science.

Work was continued on the grafting to soft tissue (ratskin).

Over 30 acrylates, methacrylates or vinyl monomers reacted with ratskin. By far the largest increase in weight (163%) was obtained

with the highly fluorinated pentadecaoctylf luoroacrylate . The coefficient of variation for the weight increases averaged 28%, a satisfactory reproducibility considering that the ratskins vary in surface

topology and chemical composition because of their natural origin.

Reaction of diallyl phosphite or tri-allyl phosphate with skin produced modified surfaces containing phosphite or phosphate groups,

respectively

.

Reducing the reaction time below 3 hours decreased the amount

of polymer formed. However, significant weight increases occurred

when the reaction time was lowered to 20 minutes. Hence, relatively

short reaction times should be sufficient to modify the chemical and

biological characteristics of ratskin surfaces. By using selected

monomers, the ratskin surface could be made hydrophilic or hydrophobic.

Mineral oil completely wets untreated ratskin. Oil repellency was incorporated in the subdermal surface on polymerization with pentadecafluorooctyl acrylate, but efforts to obtain oleophobic epidermal

surfaces were not successful. This behavior toward non-polar liquids

differs from that of grafted collagen powder or film and presumably

results from the keratinous nature of the epidermal surface.

Modification of wetting characteristics was more easily accomplished

with collagen than with ratskin. Water uptake for the treated specimens

differed little from that of the controls. The uptake from 0 to 50%

RH was generally in the 7 to 10% range. An exception was ratskinpentadecafluorooctyl acrylate which had a water sorption value at

50% RH of 3.1%. These results differ from those obtained with collagen powder where grafting always lowered water sorption. The high

11
yields for grafting onto collagen indicate that the reaction takes

place throughout the specimen, whereas the much lower weight increase

with skin is a result of mainly surface grafting. Thus, it is not

surprising that water sorption, which is predominantly a function

of water absorption throughout the specimen, is not altered greatly

for the grafted ratskins.

Growth of microorganisms is retarded or completely inhibited

after reacting the skins with monomers. Mold growth was not found

after one week or one month on skins treated with methyl methacrylate

,

glycidyl methacrylate, isodecyl acrylate, hexafluoroisopropyl acrylate or acrylonitrile . Profuse growth was observed on the ratskin

controls. Some growth took place at the edges where the skins had

been cut after grafting treatment and where, apparently, the tissue was not exposed to monomer. This indicates again that grafting oc- curred predominantly at the surfaces.

Recently, a persulfate-bisulfite redox system has been suggested

for grafting side chains to chromium- tanned collagen. Using this

redox system, we obtained yields of methyl methacrylate grafted to

ratskin somewhat larger than those obtained in the eerie ammonium

nitrate-initiated poljnnerization. Even larger increases in weight

were obtained with butyl acrylate, especially on addition of 10%

acrylic acid, both for 3-hour and for 20-minute reaction times. A

methyl methacrylate-10% acrylic acid comonomer gave a 10% increase

in weight for a 20-minute reaction period.

. This reaction was conducted in a carbon dioxide atmosphere.

Substituting for nitrogen in the eerie ion initiated polymerization did not affect the yield. Carbon dioxide is readily available

as "dry ice" and may, therefore, be preferable for practical

applications

Studies have been initiated to graft to ratskin using donoracceptor complexes. Polymerization is limited to certain comonomer

pairs having electron donor and acceptor properties, respectively.

Weight increases have been obtained with methyl methacrylate-styrene

comonomers. It is anticipated that the side chains in the product

12
have alternating styrene-methacrylate units. Future investigations

will determine the feasibility of using ultraviolet radiation to

initiate grafting. It has been proven conclusively that the ratskin

surface is altered by the reaction with various monomers. Investigations will be initiated to determine if such surfaces improve adhesion to various substrates.

4.1.2.2 Grafting to Hard Tissue

Sponsor: NIDR

The feasibility of grafting selected monomers to hard bone tissue

has been established in previous work conducted in this laboratory.

The investigation now in progress has the following aims: (1) standardization of preparation of test specimen and grafting techniques;

(2) determination of the scope of the eerie ammonium nitrate (CAN)

initiated graft polymerization to bone using selected monomers; (3)

establish the reproducibility of the results obtained by this technique; (4) determine extent of modification in surfaces; (5) investigate the use of other chemical initiator systems such as persulfatebisulfite, polymerization with charge-transfer (donor-acceptor comonomer)

complexes which may lead to stereoregular side chains and may yield

more rapid grafting; (6) investigate use of ultraviolet radiation as the initiator of side chain grafting; (7) assess improvement in adhesion and resistance to environmental attack generated by the polymeric graft technique; (8) characterization of the side chain grafted

to bone; and (9) application of the technique to modify hard tissue

for clinical application.

Experimentally, bone marrow pulverized to powder at dry-ice

temperature was used. The sieyed powder was equilibrated to 50% R.H.

and stored for one hour in the ^ presence of wetting agent. The suspension was then deaerated and a 0.05 M eerie ammonium nitrate solution

and monomer were added. After reaction times varying from 20 minutes

to 3 hours, the bone was filtered off, dried and weighed. Homopolymer

was assiduously removed by extraction with a suitable solvent, usually

acetone. The extent of grafting was assessed gravimetrically by making

comparison to a control experiment (initiator and monomer absent). A

13
small weight loss occurred in the control experiment as a result of

the slight decalcification incurred in the presence of the acidic

(initial pH = 2.2) eerie ammonium nitrate solution.

The higher homologues of the acrylate and methacrylate esters

could be grafted onto powdered bone. Grafting to obtain products

containing fluorinated acrylate or methacrylate side chains is being

studied. Such products should possess a highly hydrophobic surface.

The bone-poljrmer product Can be further modified. For instance,

the glycidyl methacrylate side chain contains an oxirane ring. A

variety of functional groups such as hydroxy or amino groups can be

incorporated into the side chain through a ring opening process.

Generally, it was observed that the presence of mineral in the

tissue restricted the yields of graft polymer (yields based on total

bone powder weight) relative to that achieved with pure collagenous

powder (steer hide powder) . This might be expected since less collagenous surface per gram of substrate would be available, and there

is also less swelling capacity in the mineralized collagen. An ex- ception, however, was found on grafting dimethylaminoethyl methacrylate

which, unless acidified prior to the addition to the reaction mixture,

will not graft to a pure collagen powder surface. With bone, however,

this monomer reacts readily and the mechanism of this reaction needs

to be studied further.

The presence of polymer in the substrate is indicated by the

infrared spectra of the products. However, the absorption spectra

do not permit one to distinguish between homopolymer which was formed

in the reaction in the interstices or at the surface of the bone, from

truly covalently attached polymer.

We have explored in a preliminary way the possibility of using

so-called me chano- chemical techniques for grafting. These techniques

have been used with polyamides and substrates such as wool and cellulose. For exploratory purposes, we have used collagen rather than

bone. We appear to get graft polymerization (there is no initiator

or catalyst in the normal sense of these terms) when polymerization

was attempted in vacuum for relatively long periods (up to several

14
weeks). Modification of the procedure so that reaction was under

nitrogen for relatively short periods of time (one to eight hours)

failed to yield any graft polymer. A more drastic swelling reaction

appears to be a requirement for grafting by this procedure and, if

so, the potential application to bone or dentin would appear to be

very reduced.

4.1.3 Surface Characterization by Water Adsorption

Sponsor: NIDR

Surface phenomena play a critical role in development of caries,

development of periodontal problems, and in the efficacy of a tooth

restoration process. Surface phenomena are also basic to development

of preventive dentistry techniques. It is a dichotomy that we do not

understand the mechanism by which undesired plaque adhere to the toot?h

surface so tenaciously and that we are unable to date to develop an adhesive procedure which will effectively bond dental materials to

tooth structure.

Immediately previous to initiating studies of water adsorption

on natural and synthetic dental materials, we had determined the adsorptive capacity and energy of interaction with gases such as N^,

CO2, NH^ and CH2NH2 and a portion of this research was recently published (J. Dent. Res. 51:1529-36, 1972).

It has been pointed out recently by Brunauer et al (J. Colloid

and Interface Science 29:485, 1969) that there are many important

adsorbents that have part of their pore systems, often the major part,

inaccessible to nitrogen but accessible to water vapor. Inasmuch as the work on CO^ and NH^ adsorption demonstrated the profound influence

of polar groups on the capacity for adsorption by dentin and because

water is the most abundant component of all biological systems, the

water vapor project was initiated.

After careful study, a circulating closed system utilizing a peristaltic type pump to control the rate of circulation of air

saturated with water vapor was selected for the measurements. A

weighing tube containing the adsorbent was mounted up stream from the

15
pump and saturator with the effluent connected back to the pump to

complete the circuit. Relative humidities between 10-97% are used.

Dead volume of the system is very small compared with other systems

and this, together with a minimum of dependence on diffusion, con- tributes appreciably to the superior performance of the apparatus.

Final revisions were made on a manuscript describing the procedure

and Initial results and it is presently slated for publication in

J. Dent. Res., March-April 1973.

A striking observation of the water adsorption characteristics

of collagen-containing adsorbents, e.g. dentin, is the large hysteresis

loops which persist over the entire range of humidities. A manuscript

is under review by the NBS Editorial Board on the extension and ex- perimental verification of an adsorption hysteresis theory.

Among the synthetic dental materials being studied are a poly-

(methyl methacrylate) , hardened silicate cement, and investment material. Surface area, hysteresis boundary curves and scanning curves are determined. Interpretation of these data appears to relate

to dimensional changes as well as sizes, shapes and distribution of

pores within the substrates.

Considerable effort has been spent in revision of a manuscript

to be submitted to J. Colloid and Interface Science. This is en- titled "Surface Area of Aggregates". In its present form, the theory

is entirely free from some objectionable aspects of the original

formulation. As a consequence, it has lent considerable strength

to the possibility of using adsorption theory as a tool to detect

the permeability toward water vapor (or other adsorbate molecules)

possessed by the resinous component of an aggregate. This concept

is now being tested experimentally by means of water vapor adsorption

measurements. The samples consist of different known percentage compositions of the same two components in resinous aggregates, as well as the components, themselves, measured separately. Application of

these results, it is hoped, will be directly translatable to "pit

and fissure" type sealants.

Valve attachments have been designed and constructed to increase

16
the range of applicability of the water vapor adsorption apparatus.

They enable the adsorption to be performed in the absence of air,

when so desired. Any gas can now be used to replace air as the

carrier medium. This is especially useful when room temperature

oxidation of the sample might otherwise occur, giving rise to erroneous results.

4.2 Crystallographic , Solubility and Chemical

S tudies on Ca and/or P Minerals

4.2.1 Crystal Structure Determinations and Crystal Chemistry

Sponsor: NIDR, NIDR (Grant), ADA

This research is an integral part of a major program on the

mechanism as opposed to the etiology of dental caries. Input is also

provided by these studies to the development of adhesion between re- storative materials and tooth structure.

We have continued to obtain structural data on substances of

biological importance or relevance, including substances with deliberately introduced amounts of impurities of biological significance.

Structural parameters give the averaged atomic arrangement and strong

indications of the sites of impurities. Often the reason for the

choice of a particular site by an impurity and the means whereby it

affects the physical and chemical properties of the host substance

become apparent. Along more interpretive lines, systemization of

these crystal structures by geometric properties and crystal energies,

development of epitaxial relationships, and elucidation of the role

of hydrogen bonding will provide a better basis for understanding how

hard tissues are organized.

e-Ca3(PO^)2

B-Ca^CPO^)^ is an important constituent of dental calculus, particularly in those areas where the drinking water contains magnesium

ions. Its presence in carious lesions has also been suspected. The

two main points of interest in B-Ca^(VO^) ^ ^re (1) the role of Mg

in acting as a stabilizing impurity, which when present favors the

formation of Mg-containing e-Ca„(PO, )^ rather than Ca^(PO, )„0H,

17
hydroxyapatite , and (2) the exact nature of the salt when formed at

either '^1000°C under dry conditions or body temperature in an aqueous

environment. We have obtained x-ray data and structural parameters

' 2+ for samples of ^-Ca^iVO^) ^ containing 0-14% of cations as Mg . The

analysis of the pure compound showed that the even requirement of the

C-glide plane in the space group R3c is met by having four normal

cations and one cation which appears as two "half" cations by being

randomly distributed over two sites (i.e. the disorder, which is

necessary to conform to the symmetry requirements of the space group

and to the electrical neutrality requirement, is confined to one

cation) . It is obvious from the structural parameters that this

disorder has a considerable effect on the rest of the structure.

The analyses of the Mg-containing samples showed that (1) the

stabilizing effect of Mg arises because the smaller Mg ion replaces

Ca in an octahedron of oxygens which is actually smaller than a typical coordination of Ca, (2) there is a small amount of additional disorder of the cation found to be disordered in pure ^-Ca^(70^)

and (3) extra precautions are necessary in the least squares refinements to allow for the effect of the cation disorder on the rest of

the structure, i.e. to insure that the numbers obtained in the com- puter programs are not unduly influenced by artifact. Because of

factors (2) and (3) , the computational analysis is more complicated

than usual, but we believe a satisfactory method of procedure in the

least squares refinements has been devised. After we have applied

this procedure to all the Mg-containing S>-Ca^(FO^) ^ structures, we

will have information which will reveal the structural role of differing amounts of impurity. To our knowledge, this study constitutes

the most intensive and accurate studies of impurity effects. The

procedure is applicable to other materials where the effect of impurity

warrants study.

CagH^ (PO^)g • 5H2O (octacalcium phosphate)

CagH2 (PO^) ^ • 5H2O is important as a calcium phosphate which has

a pronounced tendency to enter into an epitaxial relationship with

Cac- (PO , ) „0H, hydroxyapatite, and thus is often intimately involved

jL8
with Ca^(PO,)„OH. The details of a previously determined structure

5 4-5

(1960, W. E. Brown) are too coarse for many applications and we are refining the structure with advanced techniques available to us in

this laboratory. About 2500 weak x-ray reflections have been collected. The crystal used is small, but is the largest found in

years of growing CagH2 (PO^)^* 5H2O.

Isotropic refinement of the structure of CagH2 (PO^) 2 * 5H2O showed

evidence of disorder in the hydrated layer. This was borne out by

subsequent anisotropic refinements. Although the disagreement (P-^)

between observed x-ray quantities and calculated quantities is very

low (4.6%), and the results are therefore excellent at first sight, some changes in the fine details of the structural model as used in

the refinement scheme are necessary before the structural parameters

can be assumed to have physical significance.

The disorder arises because some water molecules are able to

hydrogen bond in more than one way 'and are therefore in slightly

different places in different unit cells. A hydrogen bonding scheme

for the waters and two HPO, groups in Ca„H„(PO, )„'5H-0 has been worked 4° 82 42 2 out and is being tested.

Ca(BF^)2

In conjunction with Professor T. H. Jordan of the Chemistry

Department, Cornell College, Iowa, who has chosen to spend a sabbatical

year in our laboratory, we have begun an investigation into BF^ salts.

These salts are important as possible fluoridating agents because

they slowly hydrolyse in water to produce fluoride ions. The rate

of production of fluoride ions is too slow to allow CaF2 to precipitate and instead there is a tendency to form the desirable product

fluoridated hydroxyapatite Ca^ (PO^) ^(OHjF) . We have collected 6239

x-ray reflections from a spherical single crystal of Ca(BF^)2. These

reflections were merged into a unique set of 2934 reflections of

observable intensity and 454 reflections which were not significantly

above background. The disagreement between equivalent pieces of data

was satisfactorily good at 2.6%.

19
Ca(H2PO^)2'H20

In a previous x-ray diffraction study of Ca(H2PO^)2*H20, ideal

hydrogen positions were calculated because of the imprecise detail

revealed through x-ray diffraction. By neutron diffraction we have

confirmed the bifurcated bonding of one hydrogen atom predicted on

the basis of x-ray diffraction. On completion of least square re- finement, the hydrogen bonding in this salt will be well-characterized

and can be related to that in other phosphates.

From a reported crude determination of the structure of SnHPO^

,

the HPO^ ions apparently form dimers (HPO^)^ by very strong hydrogen

bonds. Such entities may also exist in aqueous solutions of phosphates, and their occurrence in the solid state permits them to be

well-characterized. Once known, these same characteristics can be

sought in aqueous solutions. In conjunction with Prof. Jordan, we

3 have (1) attempted to grow crystals of a size suitable ('v^Smm ) for

neutron diffraction, (2) investigated the infrared and Raman spectra

of SnHPO^ and assigned frequencies to the hydrogen and deuterium

motions, and (3) established by DTA that decomposition occurs at

^250° C and that there is no first order phase transition above -100°C.

The spectral investigation showed that the hydrogen bond between dimers is not as strong as the structure determination suggests.

Further work on crystal growth is in progress. If this effort is

successful, the crystals will be used in single crystal Raman work

and in neutron diffraction.

Rare Earth Apatites

In collaboration with Professor I. Mayer of Hebrew University

in Jerusalem, Israel, who is spending a sabbatical year at NBS, we

have been investigating various rare earth apatites with the general

ion such as La and M is a divalent alkali earth cation such as Ca^ with all cation sites filled in the apatite structure. The

reactions were carried out at 100°C in a sealed platinum tube and

SnHPO

4

„ Na (PO,), F„ , where RE is a trivalent rare earth

i-r2x X 4 6 2'

2|0
the products were first characterized by x-ray powder diffraction.

Lattice constants of a series of rare earth-strontium-sodlum, bariumsodium, and calcium-sodium f luoroapatites prepared In this way have

been determined. The lattice parameters have been observed to de- crease with decreasing ionic radius of the rare earth ion. It is

noteworthy that in the Eu-Ca-Na system, the lattice parameters remain

constant at all compositions and are the same as those of CaT_(PO,),F_

3+ + 10 ^ 6 2 perhaps because the average of the radii of Eu and Na is close

2+

to the radius of Ca , and these ions are distributed among the crys2+ tallographic sites of Ca as follows:

Cations in Cations in

Cations in Compound Triangular Sites Columnar Sites

in the compound [Eu^Ca2Na2] [Ca2Eu2Na2] [Eu2Na2]

[Eu^Ca^Na^] [Ca^EuNa] [Eu2Na2]

" " " [Eu2CagNa2] [Ca^] [Eu2Na2]

In case of the Ba-compounds , apatite-like compounds with rare earths

smaller than Nd could not be prepared, most probably because the

difference in size of the two atoms was too large. Attempts to prepare compounds with composition Ln^Na^ (PO^) ^F2 also failed.

In many cases, single crystals of the above compounds were found

in the preparations. We plan to investigate the exact locations of

the various ions in these compounds.

Several attempts to prepare oxyapatites of the type RE2Cag (PO^) ^0

and RE2Srg (PO^) ^02 were unsuccessful in spite of ^feat precautions.

Epitaxy

Epitaxy is defined as the oriented overgrowth of one substance

on another. Considerable emphasis has been placed in the scientific

literature on elucidating the initial nucleation process, mainly

through developing techniques for the growth and examination of thin

films, and on the mechanisms of misfit accommodations due to lack of

complete correspondence of the two substances involved. Because

growth of calcium phosphates in an aqueous environment takes place

under mild conditions, structural fit is probably more important than

misfit requirements, although the degree of misfit which can be ac- commodated is probably also small. We have written two computer programs with which to search systematically for possible epitaxies in

21
calcium phosphates, with special reference to apatites. The first

program searches out those metric fits (or misfits) which occur within predefined limits. The second, which takes the metric fits and

compares the possible structural fits, is still being developed,

checked and corrected. Development of both programs is taking place

slowly because of the large amount of introspection necessary in

developing a new approach. The first program is based mainly on lattice parameters and pays little attention to chemical information.

It determines which sections or networks in the unit cell lattices

in the two compounds being compared match up with one another within

generally accepted or reasonable criteria. At present, these criteria

o are (1) no network edge is greater than 20A, and (2) no misfit of

networks greater than 15% in length of component vectors defining

networks and no misfit of greater than 10° in the angles between

vectors is allowed. Only those networks which will give unique

planes through the real crystal structure (which usually has lower

symmetry than the lattice) are generated. The results between

hydroxyapatite and various other compounds of possible or known

biological significance are given in the following table:

Other compound Number of uniq

Ca^(PO^)20 146

Ca^(P0^)30H 20

Sn3P0^F3 111

CaHPO, • 2H^0

4 2 621

CaHPO^ 15*

C^8«2(P^>6-^V 13

CaCO^-eH^O 90

Ca(COO)2-H20 134

Ca(C00)2-2.2H20 17

CaCO^ (aragonite) 538

CaCO^ (calcite) 40

BaCa(C03)2 774

CaKAsO, -BH^O

4 2

201

MgNH^PO^-eH^O 331

originally > 1000 so criteria reduced internally by program.

22
Later work has shown the necessity for reduction of the number of

network matches to be considered, so a figure of merit has been as- sessed for each match and the matches sorted in order for each compound.

The second program takes the sorted matches generated by the

first program and checks the match of the structural details. In a recent run (with 16 unsorted matches) the program correctly signalled

what we a priori considered the best match, picked the appropriate

planes through the two structures and correctly matched up the different kinds of atoms, i.e. columnar Ca ions (Ca(I) in Ca^(P0^)20H) with

columnar Ca ions in the second compound, and so on. This was a pleasing result. Because of the computing delay referred to earlier,

recent progress has been slow.

Rotation About Scattering Vector. The crystal of Ca(BF^)2 referred to earlier had weak reflections

where we thought there should be, none. We showed conclusively that

this was an artifact due to double reflection. To do this it was necessary to write a computer program for an accessible time shared

computer and to use the angle settings thus calculated on the x-ray

dif f ractometer.

Energetic Calculations

We have resumed calculations of lattice energies. The first

step, by no means trivial, is to make the computer program made available by W. R. Busing of Oak Ridge National Laboratory fit on the NBS

Univac 1108 computer. Later, we hope to determine and test the transfer

ability of the potential parameters of Ca, PO^, CO^ and perhaps H2O.

This work will be exploratory in that we do not know how feasible it

will be or how precisely the calculations will correspond to experimental determinations. In principle, there could be a tie-in with

a third step of the epitaxy calculations in which energies of epitaxy

could be calculated. Eventually, calculations of this type should

have value relative to the variable composition and solubilities of

apatites such as those in enamel.

4.2.2 Physical Chemical Studies of Calcium Salts

Sponsor: NIDR (Grant), ADA

23
Solubility of CaHP0,'2H^0

4 2 The experimental data on the solubility of CaHPO^ '21120 in the

system Ca(OH) 2-H2PO^-NaCl-H20 were treated by the adjustment procedure.

The results show that both the Debye-Hiickel and Davies equations provided suitable means for calculating the activity coefficients of

the ions up to NaCl concentrations equal to those in serum. However,

it was found that the fit of the data was poorest in the highest

range of pH's covered by our measurements at higher NaCl concentrations. The electroneutrality unbalances calculated from the raw data indicated an unaccounted for anionic species could have caused

. poor adjustments. Reviewing the situation further, it became apparent that this species could be NaHPO^. Preliminary calculations give

the stability constant for such an ion pair in the range of 8-10 L/mole.

Adjustment procedure is being used to estimate its value correctly.

It is hoped that the data will fit better at all pH ranges after

introducing NaHPO^ species. The existence of the ion pair NaHPO^

could have considerable physiological significance because the high Na"*" concentration in serum would require that corrections be made in the

activity of phosphate ions taking into account the species NaHPO^.

The fact that our calculations of (1) the solubility product constant,

and (2) the electroneutrality unbalances both point to the existence

of this species gives us added confidence in the validity of this

new feature in our model.

Topical Fluoridation

Our objective of developing a more effective topical fluoride

agent has been premised on the condition that one should attempt to

form an acidic salt such as CaHPO^*2H20 within the enamel which, after

treatment with a fluoride-containing solution, would be converted to

the insoluble salt f luorapatite , Ca^CPO^)^?. Formation of fluorapatite

in this way would not be limited by the slowness of a solid state

reaction which seems to be the limiting factor in presently used

treatments. The possibility of achieving this objective was considerably improved in a recent experiment when it was found that hydroxyapatite could be nearly completely converted to fluorapatite with a

24
solution containing calcium, phosphate and BF^ ions. CaHPO^- 211^0 forms

during the reaction as an intermediate product which is subsequently

converted to fluorapatite by F ions resulting from the hydrolysis of

BF^ ions. We believe that the formation of fluorapatite is favored over that of CaF2 in this solution by the high calcium and phosphate

concentrations, and the low concentration of F ions because the major

portion of the fluoride is present as the BF^ ions.

Coulometry

The preparation and physical chemical study of hydroxyapatite and

related calcium phosphates has indicated a need for more sensitive

analytical methods of chemical assay. For example, the heats of solution of different apatite preparations were found to vary by as much

as 12 K cal/mol. These same preparations were assayed to have nearly

identical calcium to phosphate ratios within the limits of present

analytical procedures. Therefore, in an effort to improve the accuracy

and precision for calcium analysis, to better than 0.5%, considerable

time has been devoted to the adoption of a published method using

constant current coulometry.

Constant current coulometry relies only on accurate measurements

of current and time. Thus, not only is coulometry independent of

chemical standards, but offers the additional advantages of pretitration of reagent impurities and the ability to titrate microgram quantities. The method is limited almost entirely by the precision with

which the end-point can be located.

The adopted method utilizes the cathodic generation of ethylenediaminetetraacetate anion from its mercury (II) complex.

HgEDTA + 2e > Hg° + EDTA

For each Faraday of current passed through the titration cell, one-half

of an equivalent of titrant is generated. To date, the method has not

yielded the desired precision. However, it is anticipated that the

remaining experimental difficulties will be eliminated.

Growth of Fluorapatite Crystals from Aqueous Solutions

Although fluorapatite, Ca^(P0^)2F, can be grown in good crystalline

form at very high temperatures, its properties and composition are

25
likely to differ somewhat from those grown from aqueous systems

at more moderate temperatures. We have chosen conditions of maximum acidity where fluorapatite is still a stable phase. The solutions used are ones which are near the singular point for

Ca^(P0^)20H, CaF2, and Ca(H2PO^)2 in the system Ca(OH) 2-H2PO^-H20-HF.

The reaction employed is the hydrolysis of CaHPO^ and CaF2 to form

Ca^CPO^)^? and Ca(H2PO^)2. After most of the CaHPO^ has been hydrolyzed, the solution is diluted to form more CaHPO^ from the

Ca(H2PO^)2.

The results to date include successful growth of Ca^CPO^)^?

crystals as large as 100 x 10 ym. Although these are still too

small for single-crystal x-ray studies, they should be useful for

other purposes. Contrary to our experiences with Ca^(P0^)20H and

Cac(PO,)„Cl, both of which show considerable fibrous character in

5 4 3

the crystals grown from aqueous solutions, the Ca^(P0^)2F crystals

appear to be monolithic.

4 . 3 Structure and Thermodynamic Studies

of Noble Metals and Their Alloys

Sponsor: NIDR (Grant)

Electron microprobe data for alloys in the Vanadium-rhodium

alloy system have now been obtained and these data will enable us

to complete the phase diagram. Similar data are currently being obtained from niobium-osmium alloy samples. Meanwhile, we are con- centrating our efforts in the preparation and heat-treating of alloy

samples for the tantalum-palladium, niobium-palladium and niobiumplatinum alloy systems. These systems were only partially determined

in previous studies at other laboratories and it is our Intention

to complete these diagrams.

As our systematic study of these phase diagrams approaches its

logical conclusion, we feel compelled to review the data which have

been acquired to see if useful trends are emerging. It seems desirable already that certain alloy systems should receive further study

in efforts to develop useful dental alloys. These systems offer the

possibilities of improved mechanical properties, adequate corrosion

26
resistance, and biocompatibility with the oral tissues. The latter

point should certainly receive thorough consideration in view of

recent trends toward a more extensive use of implant dentures.

A great deal of our effort at the present time is directed toward

writing and publishing our results in reputable scientific journals

so that the information obtained will be widely disseminated. Details

of the experimental procedures employed are highly desirable in such

reports so that the reader can evaluate the quality of the work and

decide how much reliance he may place in these results. Many figures

and tables are needed and this requires a considerable investment of

time and effort in writing, in proof-reading and in editorial reviews.

Each binary alloy system is to be published separately since the

experimental procedures were varied in order to produce optimum re- sults for each system. Our future plans will emphasize the preparation of suitable publications for all of the data which we have acquiri

We shall ultimately try to incorporate all of the existing data on binary systems involving the transition elements into a comprehensive,

systematic and compact review so that the data can be readily applied

to the development of practical alloys for various applications in

dentistry and elsewhere. Finally, we shall continue our experimental

work in selected systems or within certain composition regions where

previous studies have been inadequate.

4 . 4 Developmental, Clinical, and

Standards-Related Research

4.4.1 Three-Dimensional Panoramic X-Ray

Sponsor: NIDR, ADA

The addition of a third dimension to the panoramic x-ray would

provide additional diagnostic and operative information to the oral

surgeon and to the endodontist and should be of assistance to an orthodontist.

As a first step in the development of such an instrument, we have

reassembled a modified form of the original two-dimensional panoramic

x-ray prototype used some years past at NBS. Our aim is to provide

instrumentation which can be operated with personnel now familiar with

27
the presently used panoramic x-ray equipment. The major change In

procedure will Involve splitting the x-ray beam or devising a procedure for incorporating an additional x-ray source. The two routes

we are exploring are (1) simultaneous non- interfering exposure of

two films and a special viewing technique, e.g. stereo glasses, etc.,

and (2) a lenticular lens technique using one x-ray beam. Because

the latter technique is not envisaged as requiring stereo glasses or stereo viewing equipment and thus has potentially greater usefulness,

we have been expending our effort in this direction.

In this report period, we have prepared a number of lenticular

plate-optical grid-cassette combinations, but the results have not

approached the full potential of this system.

We are modifying some of the stationary x-ray grids which we

will test to see if exposures can be made to register with the lenticular

plates.

While we are emphasizing the lenticular lens approach, we are continuing design and construction of a camera for recording two

separate exposures at different angles (method (1) above) since it

will probably be necessary to compare results and tissue radiation

exposure for the two methods before deciding on the optimal system

for producing stereoscopic x-rays.

4.4.2 Composite Restorative and Pit

and Fissure Sealant Materials

Sponsor: ADA, NIDR (Grant), NIDR

Work has been commenced on developing materials for use in preventive dentistry, and work has continued to further improve composite

restorative materials. The latter were first invented and developed

in the Dental Research Section of the National Bureau of Standards

and manufacturers were provided with synthetic methods and suitable

formulations. These dental filling materials are being sold in this

country under trade names* such as Adapt ic, Blendant, Concise, Compodent , Compolite Composite Restorative, DFR, Exact, HL-72, Posite,

Prestige, and Smile. Foreign markets are developing rapidly. While

the consensus is that these materials are more durable than the silicate

*Identif ication of commercial products in this report is done so only

for clarity and does not imply recommendation or endorsement by tne

National Bureau of Standards.

28
cements they are replacing, it is nonetheless true that certain

properties leave something to be desired. Among these are (a)

storage stability, (b) color stability, (c) dimensional stability

during hardening, and (d) resistance to wear on incisal or occlusal

surfaces of the teeth. The challenge of discovering means for

adequate and stable (e) bonding to the hard tooth tissues has still

not been met.

(a) Stabilizers - Storage stability of the dimethacrylate monomer

liquids or their pastes requires phenolic polymerization inhibitors

or "antioxidants." One such compound, BHM, (3,5-di-_t-butyl-4-hydroxybenzyl methacrylate) was synthesized and evaluated; its properties

were described in "A Stabilizing Comonomer I. Synthesis and Confirmation of Structure" and "II. Stabilization and Pol3nnerization Characteristics", J. Dent. Res. 51:1071, 1614 (July, Nov.) 1972. Attempts

were made to synthesize a similar compound, MP, [2 ,6-di-t-butyl-4- (2-

methacryloxyethoxy)phenol] that, theoretically, should be more effective than BHM. The preparation of MP requires a two-step synthesis.

The first step was done successfully in high yield. The second step,

however, has frustrated numerous attempts at what would appear to be

a straightforward Williamson synthesis procedure. Alternative routes

are being sought for this potentially valuable compound.

(b) Color stability - Color stability of composite materials

appears to be enhanced if a small amount of an ultraviolet absorbing

compound is included in the formulation. It is deemed desirable that

all possible ingredients be capable of copol3nnerization, thus becoming

part of the poljrmeric network and hence unable to diffuse out of the

hardened material. This is expected to minimize untoward tissue re- actions and possibly improve certain physical properties. Accordingly,

an ultraviolet absorbing compound that contains a methacrylate group

in the molecule was synthesized. This compound, HB, [2-hydroxy-4 (2-

methacryloxy ethoxy)benzophenone] and its properties are described

in a manuscript currently being prepared for publication in the Journal

of Dental Research.

Color stability, howeyer, requires more than protection

against UV-induced color-producing chemical reactions. For example.

29
previous mention was made of a yellow complex formed between aromatic

amine accelerators and one or more phthalate monomers that are under

consideration as possible binders for composite restorative materials.

The investigation of the nature of the complex using ultraviolet and

visible absorption spectroscopy has been completed and shows that a very weak complex is formed in cyclohexane (the solvent of choice)

between NjN-dimethyl-p-toluidine , one of the most commonly used amine

accelerators, and each of the three phthalate monomers. In each case,

the absorption maximum (the wavelength at which maximum absorption

occurs) of the complex is at approximately 350 nm (in the ultraviolet

region of the spectrum) with some absorption extending into the visible

region (400-800 nm) . A comparison of solutions containing an equimolar concentration of each ester and one concentration of amine (in

large excess) shows that the absorption of light is greatest in the case of the terephthalate monomer complex and least in the case of

the phthalate monomer complex; i.e., in the order, para derivatives > meta derivatives > ortho derivatives.

Work is in progress in correlating the values of the absorption maxima of complexes formed between various benzene derivatives

and N ,N-dimethyl-£-toluidine with the nature of the substituents on

the benzene ring. Correlations of this type can be useful in predicting discoloration caused by admixture of monomers containing benzene

groups with N,N-dimethyl-£-toluidine or similar amine accelerators.

One means to circumvent the charge transfer complex problem

is to employ somewhat similar isomeric crystalline monomers that possess diminished ir-acid character. Three such monomers, the bis(2-

methacryloxyethyl) ether of hydroquinone , resorcinol and catechol,

respectively, were prepared and studied. Since these three each are crystalline solids that melt slightly above room temperature, they,

like the phthalate isomers, are capable of forming a liquid ternary

eutectic mixture. A manuscript, "Dimethacrylate Monomers of Aromatic

Diethers", has been drafted and is being prepared for publication in

the Journal of Dental Research.

(c) Dimensional stability during hardening - Shrinkage (or the

30
development of stress If strain is prevented) during hardening is one of the major obstacles that must be overcome if high values of

adhesion are to be obtained. Although too preliminary to enumerate

at present, a number of concepts are under study with the aim of

eliminating or compensating for polymerization shrinkage, and a test

method is being developed to evaluate these ideas.

(d) Resistance to wear - Although composite materials were

developed for use in anterior teeth (where silicate cements would

otherwise be used) , it is nonetheless a fact that a number of dentists

are using these materials (prompted by some commercial advertising)

on occlusal surfaces. Phillips and others (J. Pros. Den. 28:164, 1972)

have observed that about half of the composites suffered from occlusal

wear by two years. A number of composite formulations are being

readied in anticipation of measurements of wear by laboratory equipment now under development.

Additional study was made of certain metal powders as re- inforcement materials for experimental composites. Aluminum, zirconium, tantalum and gold powders were evaluated and results described

in a manuscript entitled "Metal-Filled Resin Composites" that is being

prepared for publication in the Journal of Dental Research. Appropriate

coupling agents applied to the metal powders prior to incorporation

into the resin improved strength and other properties. Additional

metal powders (titanium, niobium, iron, zinc, tin and certain alloys

and combinations of metal and metal oxide) were treated with various

coupling agents and various physical properties of their corresponding

composites are being measured. The preliminary observations of such

materials burnished on their surface with a steel instrument, and

viewed with the aid of the scanning electron microscope, supports

the contention that the malleability of certain metal reinforcing

fillers may result in composites with greater resistance to wear and

attrition than the present glass-filled composites. Materials of

this type may find use in posterior teeth containing incipient carious

lesions, when utilizeci- in conjunction with resins that are capable of

sealing developmental pits and fissures.

Not only the wear resistance, but also most other desirable

31
properties of composites and "sealant" resins depend on the attainment of the most thorough cure or greatest possible degree of polymerization of the formulation. In those with an amine-peroxide

initiator system, the ratio of these ingredients is fundamental to

the hardening reaction. In this context, one of the previously

developed tertiary aromatic amine polymerization accelerators (that

is also capable of copolymerizing with the resin monomers) was further

studied during this period. It vas used in a study that was primarily

aimed at determining the optimum benzoyl peroxide to amine ratio for

the self-curing of resins and composite materials. It was found

that about 1.5 molecules of benzoyl peroxide are required for each

molecule of amine accelerator for the optimum hardening of the resin.

This work is described in a manuscript entitled "A Method for Determining the Optimum Peroxide to Amine Ratio for Self-Curing Resins"

that has been accepted for publication in the Journal of Applied

Polymer Science.

(e) Bonding to hard tooth tissues - Attempts were made to ob- serve adsorption of NPG-GMA on synthetic hydro'xy apatite

Repeatedly washed with hot water) from aqueous 10% alcohol solution.

The adsorption was studied spectrophotometrically at 251 my at pHs

-3

of 3.3, 5.8 and 7.6, within the concentration range of 1 x 10 to

—6

1 x 10 m/1. No adsorption was observed under these conditions.

Similarly, there was no adsorption of di (2-methacryloxyethyl) dimethyl

ammonium bromide or of ammonium bromide using an EMF method of measure'

ment. However, when the synthetic hydroxyapatite was exposed to

aqueous solutions of Zn, Cu, Fe, Hg, Co, Ni, Mn, La or Th nitrate or

Cr chloride, it appeared to acquire the color of the respective metal

phosphates, which color did not wash off. This is the beginning of

a study involving pretreatments of the tooth surface with "mordants"

to improve adhesive bonding by way of appropriate coupling agents.

There is extensive laboratory data showing the effectiveness of

the coupling agent, NPG-GMA, in improving adhesion between hard tooth

tissues and resinous materials. Now results of a clinical study are available. Fifty-four pairs of Class III and V restorations were

32
placed in adult patients using an experimental composite filling

material. One of each pair of the cavity preparations was treated

with a 5.0% solution of [N- (2-hydroxy-3-Tnethacrylo 3^propyl)-Nphenylglycine] (NPG-GMA) in acetone. The other of each pair was treated with acetone alone as a control.

The restorations were evaluated by three dentists at 0, 1, 2.5

and 3.5 years on the basis of general sensitivity, secondary caries,

marginal notch and stain and whether one of each pair was better,

the same, or worse in its overall condition than its mate.

There was a significant difference in the incidence of marginal

notching in favor of the NPG-GMA pretreatment ; however, there was

not a significant trend of increased notching with time with either

the pretreated or control restorations.

The increase in marginal staining with those restorations placed

over the NPG-GMA pretreatment ranked less than the controls.

All evaluators indicated a significantly higher number of those

restorations placed over the NPG-GMA solution were better as compared

to those placed over the control liquid. In addition, each evaluator

rated a higher percentage of the NPG-GMA associated restorations as being better at all four time intervals, with one exception, at which

they were rated equal.

The results clinically demonstrate the effectiveness of an adhesion promoting tooth-restoration coupling agent through improved

margins of composite restorations.

Pit and Fissure Sealant Materials

A novel class of methacrylate monomers has been discovered and was studied further during this period. Since these compounds will

probably become the subject of patent application, full details may

be obtained only in confidence; however, generically they comprise

methacrylate or acrylate monomers or their polymers which are capable

of releasing fluoride ion when exposed to moisture. A number of

these monomers and their homopolymers and copolymers were prepared.

The rate of fluoride ion release was measured simultaneously with

weight gain (or loss) as a function of the copolymer immersed in water

33
over a prolonged period. Various combinations of monomers were used.

It is hoped that the incorporation of one or more of these compounds

into pit and fissure "sealant" tesins will prove to be of benefit in

reducing the incidence of carious attack on the surrounding hard tooth

tissues

Clinical Research

Forty-six pairs of Class II and V composite restorations were

placed in adult patients. One, (A), of each pair was restored with a composite material consisting of a resin binder of a ternary eutectic

dimethacrylate and a reinforcing filler of two parts fused silica and one part radiopaque BaF2-containing glass. The other, (B) , of each

pair was restored as a control with a composite material based on an earlier NBS development of Bis-GMA (the adduct of Bis phenol A and

glycidylmethacrylate) as resin binder.

^ The cavity preparations in restorations (A) were treated with

N- (2-hydroxy-3-methacryloxypropyl)-N-phenylglyclne , an adhesion

promoting coupling agent. Those in (B) were treated with a liner known

to have no adhesive promoting ability.

The restorations were examined by three dentists at 0, 1, 2.5

and 3.5 years on the basis of sensitivity, gross fracture, restoration

loose or missing, secondary caries, need for replacement, color, surface texture, surface stain, marginal notch, marginal stain, and

marginal flushness.

Evaluations for color, texture and flushness were made with the

aid of newly developed evaluation guides.

The materials behaved similarly with respect to general sensitivity,

sensitivity to percussion, gross fracture, loose or missing restorations,

secondary caries, need for replacement, surface staining and marginal

flushness

Restorative A appeared slightly improved with respect to color

change, surface texture, marginal notch and marginal stain.

Restorations made from both materials were still clinically sound

in a high percentage of cases after 3.5 years. However, further

34
improvements in color stability are needed.

4.4.3 Dental Ceramics

Sponsor: ADA

The effort this period has been directed toward devising satisfactory strength tests for dental porcelains. Statistical evaluation of various methods for measuring compressive and tensile

strengths and the fracture modulus are being made. Both dimetral

compression and pull tensile tests have been performed.

Dimetral and pull tensile strength and fracture moduli have been

determined on restorative dental porcelain to which anti-balling

liquids have been applied. Also, Rockwell 15N Brale superficial hardness and Knoop micro-hardness measurements have been made. In all

these tests for identical fires, porcelains to which the anti-balling

additive has been applied are as strong as those to which it has not

been applied.

Thermal expansion measurements of additive porcelains between

ice water and hot coffee temperatures reveal that they match or better

the match of non-additive porcelain. Solubility measurements are now

being performed at boiling temperatures in an azeotropic HCl solution.

Measurements of compressive green strength have shown that green

strengths at least two to three times that of the non-additive porcelains are possible. No decay of this strength occurs with time as occurs with non-additive porcelains. Non-additive porcelains lose

about half their green strength on standing 12 hours.

A new additive composition has been devised which gives even greater green strength and more reduced balling. It appears that

this porcelain may almost double tensile strength.

Efforts to obtain a patent for the additive porcelain technique

have continued. The technique is important because it can accomplish

what the current aluminous porcelain seeks to do without the aluminous

porcelain's loss in translucency , but also with reduction in processing

35
difficulty. The additive porcelain technique shows promise of being applicable to Veneer porcelains.

4.4.4 Dental Amalgam Studies

Sponsor: ADA, NIDR (Grant)

4.4.4.1 Studies of Dental Amalgam with Various Additions

The use of copper or gold additions to conventional dental amalgam

alloys has been shown to be effective in suppressing the weak and corrosionprone y2 phase. Unfortunately, the copper-containing alloy has a rather

low tensile strength and the gold-containing alloy has the undesirable

feature of a somewhat higher cost than conventional alloys.

We have made up several experimental alloys in which manganese is

used to suppress the formation of the yl phase. Manganese is an inexpensive

element which is apparently non-toxic in biological systems. In fact, tracie

amounts have been reported to be essential to a normal metabolism. The

alloys have so far exhibited negligible dimensional changes during the setting reaction and give high early strengths. Fully hardened samples have

been tested using the ADA flow test and these results have revealed significantly less flow than is observed in a contemporary commercial alloy

advertised to have unusually low flow properties. Further studies are in

progress and these include studies of corrosion resistance as compared with

the other alloys in which yl phase formation has been suppressed.

4.4.4.2 Study of yl Phase Distribution at Amalgam Surfaces

Our study of the tin distribution on free surfaces of dental amalgam

is continuing. During the past six months, we have extended the work to

several new amalgams and to various types of finishing procedures. The ex- perimental data are still incomplete, however, and we must await the acquisition of these data before we may reach conclusions as to the relative effectiveness of the various finishing procedures and types of alloys in producing a desirable tin distribution. The most desirable tin distribution

will probably be revealed by our planned supplementary clinical study, which

will be designed to reveal differences in corrosion behavior and marginal

breakdown among the various alloys.

4.4.4.3 Correlation of Dimensional Change on Hardening in

Amalgam with Marginal Breakage and Recurrent Caries

36
Clinical restorations continue to be placed and observations on previously placed restorations are being made. Restorations have been

under surveillance for up to 18 months.

4.4.5 Dental Cements

Sponsor: NIDR

Reinforced Polycarboxylate Cements

The aim of this study is to develop improved dental carboxylate

cements by incorporating reinforcing agents or by partial substitution of poly (acrylic acid) by other polycarboxylic acid polymers

such as itaconic or maleic acid containing copolymers. In this study,

powder to liquid ratios were maintained that could be used in dental

practice either as a base or as a cementing media. The hardening

times of the mixed cements varied from 5 to 10 minutes at 37°C, 100

RH.

The results show that addition of certain reinforcing agents

such as stainless steel, silver, aluminum or alumina powder to the

commercial zinc oxide (90%)-magnesium oxide (10%) powder markedly

increases the tensile strength of the cements obtained on mixing

these powders with commercial, aqueous poly (acrylic acid) or acrylic

acid-itaconic acid copolymer solutions. Even greater improvements

in physical properties are obtained with 20 to 30% potassium titanate

in the powder. Thus, the compressive and tensile strength of a product

containing 27% potassium titanate-63% zinc oxide-magnesium oxide powder

2

mixed to a base consistency (powder-liquid ratio 1.5) were 916 kg/cm

2 and 152 kg/cm , respectively. This compares to a compressive strength

2 2 of 600 to 690 kg/cm and tensile strength of 68 kg/cm for commercial

polycarboxylate cements, an increase of over 30% in compressive and over 120% in tensile strength. Further improvements in compressive

strength were reached on addition of 8% graphite, but the resulting

products showed a dark shade which is undesirable for some clinical

applications. Incorporation of small percentages of silicon carbide

whiskers or silane treated glass did not yield products with improved

properties. An effort to use an aromatic nylon type fiber

37
having a very high tensile strength and modulas per unit weight as reinforcing agent proved unsuccessful. The fiber is only available

as a yarn. Chopping the film into small strands proved not only

laborious, but the resulting fibrils clumped together - which could

not be overcome. This clumping made it impossible to obtain a homogeneous mix on blending with zinc oxide. While compressive strength

was not affected by changing the rate of loading from 0.2"/inin to

O.S'Vmin, a 15 to 20% decrease in tensile strength was observed at

the higher loading rates.

On heating commercial polycarboxylate stored in water from 10

to 63°C at a rate of 3°C/min, the cement shrinks considerably. Further

studies to explain this behavior are needed.

Two polycarboxylate copolymers were synthesized. Itaconic acid

(35 mol percent) was copolymerized with glacial acrylic acid (65 mol

percent) in an aqueous solution containing ammonium persulfate initiator. After removal of some of the water, a 40% aqueous solution

of the copolymer was obtained which, on mixing with zinc oxide, hardened

in 2 minutes. A 60% aq solution of the copolymer set even faster,

whereas a 25% solution had a considerably slower setting time. The

copolymer of acrylic acid and maleic anhydride in approximately 2:1

molar ratio was also prepared. A dilute aq solution containing 20%

of copolymer set within 2 minutes. More concentrated solutions form

turbid suspensions. Furthier properties of the cements obtained on mixing with zinc oxide-magnesium oxide and appropriate reinforcing

agents will be studied.

Some further thought \<ras given to the setting mechanism of zinc

polyacrylate and the adhesion of this cement to hard tissues. It

has been assumed that poly (acrylic acid)-zinc oxide cement adheres

to enamel by chelate formation of pendant carboxylate groups with the

calcium present at the enamel surface (D. C. Smith, Brit. Dent. J.

125:381 (1968). A literature survey showed that metal polyelectrolyte

complexes such as the poly (acrylic acid)-copper complex form despite

the large ring size of this chelate. This may result from the large

electrostatic interaction of the polyelectrolyte coil which nullifies

38
the steric effect and allows the formation of a complex as stable

as those composed of five and six-membered rings. However, the complex

formation constants for various metals such as zinc with poly (acrylic

acid) are much smaller than with copper and even smaller with those of

the alkaline earths (Ca, Mg) (Hi P. Gregor, L. B. Luttinger and E. M.

Loebl, J. Phys. Chem. 59:990 (1955). There is considerable contraversy regarding the method used for calculating these values . Nevertheless, the complex stability constant for calcium was lower by an order of several magnitudes from that of copper. It was reported

recently by Jurecic and Fairhurst (lADR Abstracts 1972, No. 379) and

Beech (Arch. Oral Biol. 17:907 (1972)) that the infrared absorption

peak on hardening of polycarboxylate cement and on reaction of poly-

(acrylic acid) with enamel shifts from 1770 cm to 1540 cm ^, indicating the formation of ionized carboxyl groups on reaction with

zinc. Similarly, calcium ions in enamel and the polycarboxylate

cement interact electrostatically. Chelate formation, if it occurs

at all, takes place only to a very limited extent.

These data suggest the setting mechanism of zinc polyacrylate

cement is complex and involves neutralization of the polycarboxylic

acid by the metal oxide and strengthening of the polymeric network

by ionic interactions between carboxylate groups in the side chain

of the polymer and zinc and magnesium ions and by the reinforcing

action of excess metal oxide. The small values of the formation con- stants for zinc and calcium acrylate make it doubtful whether interaction of the metal takes place to any appreciable extent with carboxylic acid groups located in juxtaposition on the same polymer chain

(chelation). However, attachment of metal ions can also occur on carboxyl groups located at some distance on the same polymer chain

(intramolecular interaction) or on different polymer chains (intermolecular interaction or crosslinking) . Present experimental techniques do not allow a ready elucidation of which mode of attachment

prevails both in the setting reaction of the carboxylate cement and

in its attachment to enamel.

39
4. A. 6 Polymeric Stabilization of Orthopedic Implants

Sponsor: Cooperative project with George Washington Medical Center

The aim of this project is to provide the clinician with basic

information as to the nature, optimal working conditions and techniques of resins used as hip prosthesis material and as implants for

other orthopedic applications.

Chemical Properties

Procedural methods were developed for chemical characterization of

the in situ-polymerizing methacrylate cement used to stabilize hip

prostheses. These included infrared, ultraviolet and nuclear magnetic

resonance techniques, refractive index and gas-liquid chromatography.

Methanol and ether extractable materials are also determined on the

powder component.

Methods to determine total benzoyl peroxide from the extracted

residue were not successful. Apparently, some decomposition of

peroxide takes place during extraction or removal of the solvent,

and low values are obtained in the spectrophotometric determination

for benzoyl peroxide in the original polymer powder. A method based

on the addition of benzoyl peroxide to solutions containing various

concentrations of resin powder in chloroform was developed which allowed the estimation of the peroxide. An effort will be made to

determine more accurately the small amount of styrene copolymer.

"Furthermore, the molecular weight of polymer powder and cured resin

and the water sorption and solubility of the cured product will be

studied.

Physical and Mechanical Properties

2

Tensile strengths of 4700 psi (32.4 MN/m ) and 4200 psi (29.0

2 MN/ m ) were determined respectively on transparent and radiopaque

specimens prepared from mixes using manufacturer-recommended powder/

liquid ratios. The incorporation of BaSO, did not affect the

40
compressive strength (13,500 psi; 93.1 MN/m ) or Young's Modulus

2 (330,000 psi; 2275.4 MN/m ). Mixes of the cement showed shrinkage

on polymerization with the amount of bulk shrinkage ranging from

about 1 to 5% depending upon the degree of porosity in the specimen.

Preliminary results indicated that the dough time (time from

initial contact of powder and liquid until cement is ready for insertion) , setting time, handling time (period between dough and

setting time) , and peak temperature of polymerization can be con- trolled to some extent by the technique of preparation of the mix

of cement. Increased temperature of the mixing container reduces

dough, handling and setting times. High powder-liquid ratios apparently reduce dough time, setting time and peak temperature. Continued kneading of the mix reduces setting time. The effects of

variations in relative humidity appear to be small, as do difference

between cements with and without BaSO,

.

4

4.4.7 Specifications and Standards

Chairmen of both the FDI-ISO and ADA-ANSI Alloy for Dental

Amalgam Committee prepared two revised recommendations with accompanying ballots for the FDI-ISO task group consideration. For the ADAANSI, a series of round-robin tests were completed and a revised

specification circulated for consideration and ballot.

Chairman of the ANSC-Z156 Committee on Denture Base Resins

completed a 6th and 7th draft of the 4th revision of specification

No. 12. This revised specification includes pour type resins. Processing techniques were modified and employ tinfoil substitutes to

approximate more closely dental laboratory conditions. The 7th re- vision has been approved by the subcommittee and is being reviewed

by ANSC-Z156 before final approval. Drafts for review of the FDI

specification for Denture Base Resin have been reviewed.

4.4.8 Miscellaneous

A correspondence course on dental materials was reviewed and

suggested revision made to the Navy Graduate Dental School.

41
Assistance was provided graduate dental students of the Navy

(Graduate Dental School on four projects: (1) Effect of heat during

trituration on certain physical properties of dental amalgam, (2)

Evaluation of bonding or amalgamation of amalgam to various alloy

formulations used in pin manufacture, (3) Strain evaluation of

various clasp designs used in removable partial dentures, and (4)

Design of cast surfaces for optimum retention of Pyroplast.

42
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-3110561 September 20, 1972 10 916

Progress Report

on THE CRYSTAL STRUCTURE OF VjRhj

R. M. Waterstrat* and Brian Dickens

*Research Associate, American Dental Association

Research Unit at ttie National Bureau of Standards,

Wastiington, D. C. 20234

**Research Chemist, Dental Research Section, National

Bureau of Standards, Washington, D. C. 20234.

This investigation was supported in part by research grant

DE02455 to the American Dental Association from the National

Institute of Dental Research and is part of the dental research

program conducted by the National Bureau of Standards in cooperation with the American Dental Association, the United

States Army Medical Research and Development Command, the

Dental Sciences Division of the School of Aerospace Medicine,

USAF; the National Institute of Dental Research; and the

Veterans Administration.

NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents intended

for use within the Government. Before material in the reports is formally published it is subjected to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or op.en-literature listing of this Report, either in whole or in part, is not authorized unless permission is obtained in writing from the Office of the Director, National Bureau of Standards, Washington, DC 20234. Such permission is not needed, however, by the Government agency for which

the Report has been specifically prepared if tnat agency wishes to reproduce additional copies tor its own use.

IMPORTANT NOTICE

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Abstract

The crystal structure of the phase V^Rh^ has been

elucidated. The unit cell is orthorhombic with

a = 5.420 A, b = 9.276 A, c = 4.320 I and z = 2; the space

group is either Cm2m or Cmcm. The structure is closepacked and contains both ordered and disordered atomic

sites. It is intermediate between the structures of the

Cu^Au and CuAu types except that it possesses a two-layer

stacking sequence.

The Crystal Structure of V^Rh^

Richard M. Waterstrat and Brian Dickens

Greenfield and Beck''" have reported the existence

of a close-packed hexagonal structure in an alloy having

the nominal composition ^40^^50 ^^^^^ annealing at 1200 °C.

However, the authors state that due to the small size of

their samples they could not claim a very high degree of

accuracy for their stated alloy compositions.

2

In a recent investigation of the entire vanadiumrhodium alloy system, no close-packed hexagonal structure

was found. Instead it was observed that a phase having

the cubic Cu^Au type structure exists over the approximate

compositional range of 24 to 33 atomic % V and a tetragonal CuAu type structure exists over the range of

approximately 41 to 47 atomic % V. These two ordered

phase regions are separated from each other by a narrow

single phase region which occurs at the approximate

composition V^^ 5^^62 5 ^3^-'^5^' close to the

composition V^o^''^60 2"^^^^®^ Greenfield and Beck'''.

Large crystals were obtained by fracturing an alloy

of nominal composition V-j_ r-Rh^-^ n which had been annealed

3 / . D 5 2.5

at 1300 °C for 5 days. A small amount of fine powder

of this alloy was also obtained by grinding the ingot

with a dental diamond grinding wheel and then removing

the admixed diamond crystals by sieving through a 400
-2-

mesh screen. The crystals and the fine powder were both

reannealed in high vacuiim for several minutes at 1300 *'C

in order to relieve any internal strains resulting from

cold working.

An x~ray diffraction powder pattern of the alloy

V^Rh^ showed a strong resemblance to the patterns usually

obtained from close-packed hexagonal structures but

careful examination revealed that many of the principal

"h.c.p. lines" were, in fact, double lines.

Precession photographs obtained from several crystals

using MoKa radiation indicated a hexagonal unit cell with

a s; 5.35 A and c £= 4.30 A but all lines in the powder

pattern could not be indexed on the basis of a hexagonal

unit cell having these dimensions. However, by introducing

a slight distortion to this hexagonal cell we were

successful in indexing all lines in the powder pattern

on the basis of a C-centered orthorhombic unit cell

with a = 5.420 A, b = 9.276 A and c = 4.320 A with 2 formula

o units per cell. The wavelengths used were X (CuKq^) = 1.5417 A

and X(CuK<^.)= 1.5405. The observed hexagonal symmetry of

"-1 the reciprocal lattice must, therefore, have been the result

of twinning. Metal lographic examination of the crystals

revealed a highly striated appearance which seemed to confirm the presence of extensive microtwinning

By analogy with the adjacent CU3AU type and CuAu type
-3-

structures in th6 V-Rh alloy system, it was suspected that

the structure of V^Rh^ would be an ordered close-packed

structure. The observed c-spacing for V^Rh^ implies a two layer stacking sequence. The absence of the 001

reflection makes these two layers identical in scattering

power. A reasonable assumption is that this equivalence

also extends to their compositions , so that each layer

contains one formula unit, V,Rh_ . We were unable to

identify this structure with any previously reported structure having a two-layer stacking sequence. The homologous

3 alloy system niobium-rhodium has been reported to contain

a narrow single phase region at the composition Nb-^Rh^

but the crystal structure of this phase possesses a nine4 layer stacking sequence . A fully ordered arrangement

in the basal planes of Nb^Rh^ would satisfy the composition

requirements of V^Rh^, but would not satisfy the C-centered

condition imposed by the x-ray powder data. C-centering

would require disordering of some sites to make them

crystallographically equivalent. It was therefore critical

to determine whether or not the structure was C-centered.

The powder intensities are only qualitative^ at best, due to

x-ray absorption by the sample and because of the nonlinear film response. On the other hand, the twinned

crystal x-ray photographic data are unreliable due to the

very strong x-ray absorption in these large crystals.
_4»

Smaller crystals were cut from the larger ones but these

crystals gave very poor x-ray patterns due to the presence

of streaking produced by cold work associated with the

cutting procedure. It was therefore decided to examine

the possibility of C-centering by collecting neutron

diffraction data since neutron absorption would not be

serious. Further, as a consequence of the favorable

neutron scattering factors for rhodium and vanadium, the

atomic ordering in the sample could be investigated.

However, the preparation of the relatively large amount

of fine powder required for a neutron diffraction powder

pattern was considered too time-consuming and expensive

(in terms of diamond wheels) . Polycrystalline samples

are composed of grains which are too coarse to provide

the desired resolution. Therefore, it was decided to

elucidate the C-centering by collecting neutron data from

the large twinned crystals, even though analysis of the

atomic ordering would be effectively prevented by the

extensive micro- twinning . The neutron diffraction data,

which included two sets of equivalent reflections, indicated a hexagonal unit cell with no systematic extinctions

similar to the one suggested by the precession photographs

There were no observed reflections with h+k = 2n+l for the

orthorhombic cell. Thus, the powder pattern may be
-5-

referred to a C-centered orthorhombic cell which is being

revealed only by the higher resolution in the powder data.

The hexagonal "single" crystal data was apparently produced by a superposition of several C-centered orthorhombic

reciprocal lattices oriented by means of 60° rotations in

the a*b* plane. The annealing treatment of both powder and

crystals made it probable that both were of the same phase,

and that no effects from cold working during grinding of

the powder remained. Further, if the stoichiometry V^Rh^

is to be fitted in one layer in a hexagonal unit cell

all atoms except one per layer must be disordered, which

is considered unlikely to occur.

Only one layer arrangement is consistent with a

C-centered cell of the size dictated by the powder data

and this is shown in Fig. 1. As regards the stacking

sequence, one may locate the vanadium atoms as shown in

Fig. 1, in which case the space group would be Cm2m, or

one may interchange vanadium and disordered sites in the

second layer, in which case the space group is Cmcm. One

may also move the second layer so that rhodiiom atoms

occupy the sites shown as vanadium in Fig. 1, but this

last model can be ruled out on the basis of a comparison

of observed and calculated intensities. We prefer the

first possibility, Cm2m, (shown in Fig. 1) , since this

model would produce a larger number of unlike near
-6-

neighbors. The x-ray powder data are not sufficientlyaccurate to permit a selection between the Cm2m and Cmcm

models and because there are no hO ^ reflections with

I = 2n4-l, Cmcm can not be ruled out. The relative intensities for both of these two models are given in

Table 1 and the atomic parameters in Table 2. Thus,

the twinning mechanism, which is such that [ 100] sometimes becomes [110]/2, corresponds to an interchange of

ordered and disordered sites. The disordering in site

occupancy which is necessary to account for the stoichiometry was included in the structure factor calculations

by assuming that each disordered site was populated by

one half of a vanadium atom and one half of a rhodium

atom. The accuracy of the x-ray powder data is insufficient to justify a more sophisticated treatment designed

to distinguish between short and long range order.

The crystal structure of V^Rh^ may, therefore, be

described as a new structural type which contains both

ordered and disordered sites in a two-layer stacking

sequence in a pseudo-hexagonal unit cell. The structure

can be visualized as being intermediate between the Cu^Au

and the CuAu types as regards the atomic ordering, but

the Cu^Au and CuAu structures possess a three-layer (cubic)

stacking sequence
-7-

whereas the V^Rli^ structure has a two-layer (hexagonal)

stacking sequence. The existence of V^Rh^ over a narrow

region of the phase diagram separating the Cu^Au and CuAu

type phases is somewhat surprising. One possible explanation for this abrupt occurrence of hexagonal stacking

is that for this type of ordered layer one obtains a slightly larger number of unlike near neighbors in hexagonal stacking rather than cubic. Otherwise there

appears to be no reason why a phase of A^B^ stoichiometry

and identical layer type should not also occur with a cubic stacking sequence.

4 Ritter, Giessen and Grant have previously recognized

the possible existence of both a fully ordered and a partially disordered atomic layering at the A^B^

stoichiometry. These are shown in Fig. 2 (II and III)

4 of their paper. These authors were unable to determine

which of the two types of ordered layers was the correct

one for Nb^Rl^^ owing to the similarity of the atomic

scattering factors for niobium and rhodium atoms. This

restriction does not apply in the case of V^Rh^. Therefore,

by analogy we have selected the layer marked II as being the most

probable description for Nb^Rh^. The reason for the

nine-layer stacking sequence in Nb^Rh^ may be connected

in some way with the greater difference in atomic size

between Nb and Rh atoms (~9%) as compared with V and Rh {<!%) .
-8~

Acknowl edqmen t

We thank E. Prince of the Reactor Division,

National Bureau of Standards, for collecting the neutron

data and for helpful discussion.

This investigation was supported in part by research

grant DE02455 to the American Dental Association from the

National Institute of Dental Research and is part of the

dental research program conducted by the National Bureau

of Standards in cooperation with the American Dental

Association; the United States Army Medical Research and

Development Command; the Dental Sciences Division of the

School of Aerospace Medicine, USAF; the National Institute

of Dental Research; and the Veterans Administration.
-9-

References

1. P. Greenfield and P. A. Beck, Trans. AIME, 206 (1956)

265.

2. R. M. Waterstrat, unpublished data (1972)

.

3. D. L. Ritter, B. C. Giessen and N. J. Grant, Trans.

AIME, 230 (1964) 1250.

4. D. L, Ritter, B. C. Giessen and N. J. Grant, Trans.

AIME, 230 (1964) 1259.
Figure 1.

Proposed crystal structure of V3Rh5. The space group is Cm2m.

Filled circles represent vanadium atom sites; open circles

represent rhodium atom sites; crossed circles represent atomic

sites which are populated equally by vanadium and rhodium atoms.

The interconnected atom sites are at z = 0. Unconnected sites

are at z = 1/2. The unit cell outline is shown by dashed lines.
Table 1. X-ray diffraction data for V^Rh^

(orthorhoiabic cell with a = 5.420 i, b = 9.276 A, c = 4.320 A

assuming X (CuK^v) = 1.5417 I and X (CuKcr, ) = 1.5405 1)

interplanar spacings (A) relative intensities

calc.

hkX* obs

.

calc. obs . + Cin2m Cracm

110 - 4.680 - 1 3

020 4.638 - 5 5

001 - 4.320 - 0 0

111 - 3.174 - 2 1

021 3.153 ' 3.161 MW 10 10

200 2.713 2.710 W 5 5

130 - 2.686 - 1 0

220 2.345 2.340 M 25 25

040 2.319 2.319 MW 12 12

201 2 . 296 0 0

131 2.281 _ 0 1

211** 2.187** 2.228 W 0 0

002 2.162 2.160 M 39 39

221 2.058 2.057 S 100 100

041 2.045 2.043 M 49 49

112 ") 1.961 1

<^ <1

1 . 924 vw

022 J 1.958 1 1 1

240 1.762 1.762 WW 1 <1

202 1.691 1.689 vw 2 2

241 1.634 1.631 vw 3 3

222 1.590 1.587 M+ 13 13

042 1.583 1.581 MW 6 6

400 1.357 1.355 MW 7 7

260 1.344 ' 1.343 - M+ 14 14

223 1.228 1.226 MS+ 14 14

043 1.225 1.223 MW+ 7 7

440 1.171 1.170 W 2 2
-2-

Table 1 (Cont.)

AO"? H:\J ^ X1 • XTT ] 49 1 148 MW4- 8 8

X1 • 1X^4]X 1 140 M 15 15

X] • 131 -1- <^ -L 1 129 MW+ 11 11

081 1.120 1.120 MW 5 5

nn4. W VJ 1 080 1 080 MW 3 3

1 030 1 029 MW 3 3

C\AA ^ .9803

9806

9790

MW «^

r 2

L 1

2

X1

AA'\ M 7 7 u o o 90 9 9 MW 3 3

2-10- 0 .Q119 . 8776 w 1 1

A 9

O ^ X « 9.1C\A O / <J'x M4- 7 7

ART 86 39 r 7

^ <

7

^ X ^

1 S. ~ 86 39 862 3 M4- ^ <1

J (broad line) 8600 I 7

*-r W *^ 84 S3 8446 M 5 5

^ V? rr 8416 841 6 10 10X w o ^9 ^9 ft903 r 3 3

l-ll- ^ .8220 .8182 vw <<['\

<1

AR 9 ft! S7 3

2-10- 2 .8131 .8131 vw 3 3

9 9^

045

.8108

• RlO X 0V_/ ^S

.8096

M

9

5

9

5

444

084

.7944

.7936

.7903

MW

1

2

1

2

660 .7816 .7800 MS 37 37

* All lines compatible with C-centering are listed from 010

to 022. Only the observed lines are listed beyond 022.

Unlisted lines all had calculated relative intensities

less than 1.

+ The observed relative intensities are strongly affected

by an unknown absorption factor which decreases with decreasing interplanar spacing. A direct comparison between
Table 1 (Cont.)

-3-

observed and calculated intensities is, therefore,

valid only for groups of lines having similar "d" values.

** This observed line is probably due to a small percentage

of a second phase in our sample. Its interplanar spacing

does not agree well with the calculated value, and its

relative intensity seems to vary with the annealing

treatment while the relative intensities of all other

lines in the patterns remain nearly constant. The 211

reflection has zero intensity in a C-centered space group.

i
Table 2. Structural parameters for V^Rh^

(i)

(ii)

in space group Cm2m (ci'*)

V special

X pos ition symmetry

V 0 -1/12 0 2a mm

Rh 1/4 1/6 0 4d m

iv+ ^Rh 1/2 -1/12 0 2a mm

V 1/2 1/12 1/2 2b ram Rh 1/4 1/3 1/2 4e m

|V4 iRh 0 1/12 1/2 2b mm

special positions: - 2a: (0,0,0; 1/2,1/2,0) -1- O ^r n 2b: (0,0,0; 1/2,1/2,0) + 0,y,l/2.

4d: (0,0,0; 1/2,1/2,0) + x,y,0; -x,y, 0.

4e: (0,0,0; 1/2,1/2,0) + x,y, 1/2; -x,y, 1/:

in space group

17 Cmcm i^2h^

special

X y z position symmetry

V 0 -1/12 1/4 8f m

Rh 1/4 1/6 1/4 8g m

|V+|Rh 1/2 -1/12 1/4 8g m

special positions: - 8f: (0,0,0; 1/2,1/2,0) + 0,y,z; 0,-y,-z; 0,y,l/2-z;

0, -y, 1/24-z.

8g: (0,0,0; 1/2,1/2,0) + x,y,l/4; -x,y,l/4; x,-y,3/4;

-x,-y,3/4.

All temperature factors assumed to be 1 k"^ and isotropic.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-3110561 December 29, 1972 10 978

Progress Report

on Ca(H2P04)2, A CRYSTAL STRUCTURE

CONTAINING UNUSUAL HYDROGEN BONDING

B. Dickens, E. Prince, L. W. Schroeder*, and W. E. Brown

* Research Associate of the American Dental Association

Research Unit at the National Bureau of Standards,

Washington, D. C. 20234.

* Director, American Dental Association Research Unit at the National Bureau of Standards, Washington, D. C.

20234.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

-1-

Abstract

Ca(H2P04)2 crystallizes in the triclinic unit cell

a = 7.5577(5) A, b = 8.2531(6) A, c = 5.5504(3) A, a =

109.87(1)*', 3 = 93.68(1)° and y = 109.15(1)° at 25°C with

Z = 2 . The structure was determined by an automated method

from x-ray data and refined by least-squares to R^(F) = 0.048,

R(F) = 0.020 using 3678 observed x-ray data and to R^(F) =

0.055, R(F) = 0.048 using 843 observed neutron data.

Corrections were made for absorption, isotropic secondary

extinction and anomalous dispersion in the x-ray case and for

anisotropic secondary extinction in the neutron case. Leastsquare refinements proceeded to the limits of the data sets

in space group Pi.

Rudimentary chains Ca. . . (P (1) O4) -H(l) . . . (P (2) O4) . . .run

parallel to [110] . Adjacent chains are joined by Ca. . .0 bonds

on one side and by O (8) -H ( 5) -0 (8

' ) hydrogen bonds on the other.

Alternatively, the structure may be considered to contain

hydrogen bonded layers of PO4 groups; these layers are held

together by hydrogen bonds on one side and by Ca. . .0 bonds on

the other

.

There are two very strong hydrogen bonds in the structure;

0(7)--H(4)—0(7 '),0(7) . . .0(7 ') = 2.434(2) A and O (8) ( 5) —O (8 ') , 0(8)... 0(8') = 2.423 (2) A. These hydrogen bonds join the

P(2)04 groups together to form an infinite chain. The very

strong hydrogen bonds are across nominal centers of symmetry.
-2-

and diffraction results indicate that each of the H(4) and

H(5) protons is effectively centered in a broad symmetric

potential well, possibly with a central barrier. A combination of diffraction and IR spectral considerations suggests

that the central barriers in the wells, if they exist,

probably lie below the zero point energy of the hydrogens.

Introduction

Our earlier studies of calcium phosphates have included

refinements of the crystal structures of the hydrogencontaining salts Ca (HgPO^) g *H20 (Dickens and Bowen, 1971

and CaHP04 (Dickens, Bowen and Brown, 1971a). Here we have

extended our studies of hydrogen bonding in calcium phosphates

and related compounds by determining and refining the crystal

structure of Ca(H2P04)2 using both x-ray and neutron data.

Ca(H2P04)2 alters to randomly oriented crystallites of

Ca (H2PO4) 2 'HgO in moist air (Smith, Lehr and Brown, 1955);

perhaps because of this it has been studied comparatively

little. It is an important constituent of freshly made superphosphate fertilizer.

Experimental details

Clear colorless platy crystals of Ca (H2PO4 ) 2 were prepared according to the method of Smith, Lehr and Brown

(1955). An ellipsoidal crystal with axes of 0.15, 0.15 and

0.18 mm was the result of an attempt to grind a sphere. This . crystal was mounted in a borate glass capillary with clear
-3-

household cement and was used in the x-ray study. Crystal

data are as follows: ideal formula: Ca(H2P04)2; cell at 25°C:

triclinic with a = 7.5577(5) A, b = 8.2531(6) A, c =

5.5504(3) A, a = 109.87(1)°, P = 93.68(1)°, Y = 109.15(1)°,

cell volume = 302.01 A^ ; space-group Pl; cell contents

2 [Ca(H2P04) s] ; calculated density 2.57 g*cm~^7 observed

density 2.546 g«cm"^ (Bassett, 1908). Standard deviations

are given in parentheses after the cell parameters, which

were calculated by least-squares methods from 30 2 9 values

observed at 2 5°C on an x-ray diffractometer equipped with a

highly oriented graphite monochromator . Care was taken' to

use only the Ka^ peak in determining these 2 0 values. The

wave-length X(Mo, Ktti ) = 0.70926 was assumed.

The x-ray data were measured and processed as described

by Dickens and Bowen (1971) with the exception that the

dif fractometer setting program used was written by Lenhert

(1970). 12,043 reflections (maximum sinG/X = 0.907 A"^)

comprising two complete sets from one hemisphere and one set

from the other hemisphere were measured and merged into a

unique set of 37 56. Of these, 3690 were 'observed' and 66

were 'unobserved' (unobserved reflections are those less than

2a (I) above background) . The average disagreement between

equivalent or remeasured reflections was 0.8%. Absorption

corrections were applied for a sphere with a radius the

average of the axes of the elliptical crystal. The maximum
-4-

effect on F due to the asphericity of the crystal, is

estimated at 4%. Absorption due to the capillary was

neglected.

The x-ray structure was solved by methods using

the sequence DATR0N-DATFIX-SIGMA2-PHASE-F0URR in the XRAY

67 system of computer programs (Stewart, 1967) . Seven E

maps were calculated. Calculations of distances between

the three highest peaks (presumed to be one Ca ion and two

P atoms) in these trial structures ruled out all but one.

The oxygen atoms were found from a subsequent electron density

synthesis with phases based on the Ca and P atoms.

The structure with hydrogens excluded was refined isotropically using the program RFINE (Finger, 1968) to R^ =

0.077 and R = 0.064. (R =[Sw (

| Fo | - | Fc |

) ^/Sw (

| Fo | ^ ) ] ^ and

R = E II Fo ]- |fc II /S |fo I . ) The maximum shift/error was -6.4.

The structure was then refined anisotropically to R^ = 0.065,

R = 0.037 (maximum shift/error = 0.90). The scattering

factors used were for Ca^"^ and neutral P, O and H; they were

taken from Cromer and Mann (1968) and Cromer and Liberman

(1970). Only 'observed' reflections were used in the refinements.

The neutron data were taken on a computer controlled

4-circle dif fractometer at the National Bureau of Standards

Research Reactor (Alperin and Prince, 1970) according to the

procedure described by Prince (1972) . The crystal was sealed
-5-

with CaClg in a silica glass enclosure to protect it from

moisture. With a wave-length of 1.2 32 A and a limiting 2 9

angle of 100°, 1212 independent reflections were accessible;

of these 843 had integrated intensities greater than 2a (I).

The crystal was a plate with dimensions approximately 0.5 x

1.0 X 3.0 mm. The hydrogen positions were obtained from a Fourier synthesis based on the neutron data and signs determined from the x-ray parameters. The structure was then

refined further.

consideration of isotropic secondary

In the x-ray refinement of the complete structure,/

extinction was deemed adequate and the refinement on all

parameters but the thermal parameters of hydrogen, which

were fixed at 1 , gave R = 0.033 and R = 0.02 3. The ^ w

isotropic secondary extinction factor, r_, in the notation of

Zachariasen (1967) was 0.33(7) x

10""^ cm. The standard

deviation of an observation of unit weight (SDOUW) , 7.81,

indicated that improper weights were being applied in the

least-squares analysis, so that new weights, (^^^^ +

.00058 F ^)~^, were applied. Here a is the standard devio c

ation of F as estimated from counting statistics. The few

o

reflections with extinction factors less than 0.95 were

rejected from the final refinement. The final values are

R = 0.048, R = 0.020, SDOUW = 1.84, r = 0.74(5) x 10~* cm.

The largest peaks in a difference synthesis calculated after

the last cycle of refinement were 0.63, 0.58 and 0.53

electrons A~^. The lowest value was -0.51 e-A"^. The first
-6-

three are all halfway between P and O in PO4 groups. The

fourth is 0.1 A from Ca.

In the neutron case, anisotropic secondary extinction

corrections were deemed necessary. The value of SDOUW, 6.87,

showed that once again counting statistics were inadequate.

New weights, given by (a ^ + 0.0002 5 F were applied. c o

The structure was then refined to R = 0.055, R = 0.048, w

and SDOUW = 2.50. The scattering lengths used were Ca:0.49;

P:0.51; 0:0.577; H: 0.372. (Neutron Diffraction Commission,

1969.)

The components (xlO^°) of the extinction tensors (Coppens

and Hamilton, 1970) in the neutron case are 0.29(5), 0.27(4),

0.42(6), -0.11(3), 0.24(4), 0.62(3), and indicate considerable

anisotropy. This extinction tensor results from a type II

treatment in which extinction is dominated by domain size.

Refinements using the type I treatment, where mosaic spread

dominates, led to non-positive definite tensors and diverged

even when starting from converged isotropic extinction refinements. This can be understood by considering the isotropic

extinction parameter for the x-ray case, which as given is

equivalent to the mean radius of the domains in the crystal.

However, the refined parameter can also represent a mosaic

spread parameter. Such a calculation leads to a mosaic

spread of ~30 sec, roughly a factor of ten larger than that

usually obtained in cases where type I occurs (Coppens and

Hamilton, 197 0) . Thus, our refinements suggest that extinction in Ca(H2P04)2 is dominated by the effect of the domain

size.
-7-

The largest positive peaks in a difference synthesis

with the neutron data were 27, 2 5 and 2 5 and the largest

negative peaks were -27 and -27 on a scale on which the

amplitude of a hydrogen peak is -600. The first four peaks

are obviously artifacts, since they do not occupy chemically

reasonable positions. The fifth is 1.03 A from 0(1) and

2.04 A from 0(7), and thus could be evidence for very slight

background in the map.

disordering of the hydrogen atoms, but is comparable with the/

The appropriateness of the final weighting schemes was

checked by comparison of F , F and a(F ) with a normal diso c o

tribution as suggested by Abrahams and Keve (1971) using an

unpublished program written by us. The comparison for the

neutron data showed that there was little skewness in the

fit of F^ and F^, implying that the correction of the data

for secondary extinction left essentially random errors

that could be taken up by the model. The value of the standard deviation of an observation of unit weight obtained from

the least-square refinement was confirmed in this comparison.

A similar comparison of the x-ray data showed the F^-F^

distribution to be somewhat skewed with F^ being a little

smaller than F^ suggesting that the extinction correction

may have been somewhat over-estimated. Similar comparisons

of the structural parameters from the x-ray and neutron refinement gave a least-squares line of Dm = 0.18 + 1.12x for

the positional parameters and Dm = 0.12 + 1.14x for the

thermal parameters, where

Dm = II p(l)i |-|p(2)i l| /(g^p(l)i+ a^p(2)i)*, and the p's

are parameters.
-8-

The experimental points fell satisfactorily close to the

line. Dm is the quantity described by Abrahams and Keve

(1971) and x is the appropriate quantile from a normal distribution. The ratio of the neutron to x-ray standard deviations

is so large that the above plot is essentially a test of the

standard deviations assigned to the neutron parameters.

In the neutron case, the largest correlation coefficient

was 0.83 between components of the extinction tensor. Coefficients between the extinction parameters and the scale factor

were in the range 0.56 to 0.69. Other coefficients in the

range 0.56 to 0.69 involved the B's of H5 , but as Fig. 1

shows, the tensor is oriented with respect to the crystal

coordinate system in such a way that fairly large correlations

are expected. In the x-ray refinement the three largest

coefficients were 0.64 to 0.65 between the scale factor and

of Ca(l). The three next largest (~0.6^were between Bi 1 of Ca(l) and the corresponding of P(l) and P(2). The

correlation coefficient between the extinction parameter and

the scale factor was 0.57. Both x-ray and neutron refinements

proceeded satisfactorily to the limits of the data sets

assuming space group Pi. There is therefore no indication

from the diffraction data that the crystals are not centrosymmetric. Furthermore, similar crystals did not show any

piezoelectric resonances when tested by the transmission

method (Blume, 1962)

The structural parameters for Ca(H2P04)2 are given in

Table 1. The observed and calculated structure factors are

given in Table 2 for the x-ray data and in Table 3 for the

neutron data.
-9-

Description of the structure

There are two formula units of Ca(H2P04)2 in a nominally

cantro symmetric unit cell. There are five crystallographically

different hydrogen bonds, two of which cross nominal centers

of symmetry. if these hydrogen bonds are centered,

all atomic sites would be fully occupied

and there would be no positional disorder as there is for

the hydrogen atoms in CaHPO* (Jones and Cruickshank, 1961;

Curry, Denne and Jones, 1968; Dickens, Bowen and Brown, 1971a)

.

The question of whether the hydrogen bonds in Ca(H2P04)2 are

actually centered or not is discussed later.

There are several ways of describing the Ca(H2P04,)3

structure. Many calcium phosphates can be described in terms

of Ca...P04 chains which are linked into sheets, usually

puckered or corrugated but occasionally planar, and which

are bonded to each other either directly or by some chemically

different intermediate layer. In Ca(H2P04:)2 (Fig. 1), rudimentary chains with the repeat unit Ca. . . (P (1) O4) "~ H (1) . .

.

(P (2) O4 ) . . .exist parallel to [llo] . A series of corrugated

sheets may then be imagined parallel to (100) . Adjacent

chains are joined by Ca. . .0 bonds on one side and by

0(8) —H(5) —0(8') bonds on the other. There is hydrogen

bonding along the surfaces of the sheets involving H (3)

between neighboring chains. The corrugated sheets are bonded

together by hydrogen bonds involving H(2) and H(4)

.

An alternative description is that the PO4 groups are
-10-

held in layers parallel to (010) by hydrogen bonds. These

layers are bonded to each other by hydrogen bonds on one

side and by Ca ions on the other.

Two striking features of the structure (Fig. 1) are

(i) H3P(2)04 ions linked into infinite chains by centered

hydrogen bonds involving H(4) and H(5) and {±±) pairs of H2P(1)0

ions linked into dimers across centers of symmetry by the

0(4) —H(2)...0(2) hydrogen bond. The latter feature may be

seen at the lower left front corner of figure 1.

The Ca ion environment

The Ca ion is coordinated (Table 4) to eight oxygen

atoms including the [O(l'), 0(4)] edge of the P(l)04 group.

This edge and the PO4, apexes 0(1), 0(5), and 0(6) are arranged

in an approximate pentagon about Ca. Oxygen atom 0(7) and

the midpoint of the 0(5')...0(2) vector occupy apical positions

of an approximate pentagonal bipyramid. Bipyramidal coordination of Ca is common in calcium phosphates. As judged

from the Ca. . .0 distances (Table 4) six of these Ca...O bonds

are strong; those to 0(4) and 0(6) are weaker.

In the 'normal' Ca...O coordination in sheet-containing

calcium phosphates the shortest distances are from Ca to

oxygen atoms in neighboring chains, the intermediate ones are

to oxygens in the same chain but with no covalently attached

hydrogen, and the longest are to oxygens covalently bonded to

hydrogen (Dickens, Bowen and Brown, 1971a) . While vestiges

of this pattern persist in Ca(H2P04)s» strong hydrogen bonding has apparently modified it, as is the case in CaHP04
-11-

(Dickens, Bowen and Brown, 1971a). The two oxygens 0(4)

and 0(6), which are covalently bonded to hydrogen atoms in

'normal' hydrogen bonds (see later), have the largest Ca...O

distances, 2.665 and 2.826 A, respectively, in some agreement

with the normal pattern. The Ca...0(7) distance, 2.42 9 A,

is relatively short for an oxygen covalently bonded to hydrogen. Oxygens 0(3) and 0(8), which are also bonded to hydrogens,

are not within the primary coordination of Ca.

The PO4 groups and their environments

The details of the two unique PO4 groups and their

environments are given in Table 4. The longest P-O distances

in the phosphate groups are those where the oxygen has a

covalently bonded hydrogen. The P(2)-0(7) and P(2)-0(3) distances

are intermediate in length, which is consistent with the 'half

hydrogens found to be associated with 0(7) and 0(8) . The P-O

bond lengths are expected to be in some proportion to the

strength of the interaction of each oxygen with its environment. In some instances this expectation is realized but m

others it is not.
-12-

The 0-P-O angles in the unusually distorted P(l)04

group may be explained as follows. The smallest angle,

0 (1) , P (1) /O (4) (102.07*'), is in accord with Pauling's rule

(Pauling, 1960) since the 0(1), 0(4) edge is coordinated to

Ca. The O ( 3) , P (1) , 0 (4) angle (103.17°) is expected to be

small because 0(3) and 0(4) are both covalently bonded to

hydrogen and P(l)-0(3) and P(l)-0(4) are both long (see, for

example. Table 11 of Baur and Khan, 1970) . Conversely, the

O (1) , P (1) , 0 (2 ) angle is expected to be large (the observed

value is 117.59°) because P(l)-0(1) and P(l)-0(2) are both

short. In the P(2)04 group, the angles involving 0(5) are

the largest, as expected because P(2)-0(5) is the shortest

P-0 distance in this group. However, it is not clear why

the O (6) , P (2) ,0 (7) angle (102.41°) is considerably

different from the O (6) , P (2 )

, O (8) angle (108.42°), or why

the 0(5) ,P(2) ,0(7) angle is different from the

0(5) ,P(2) ,0(8) angle.

The 0(1), 0(4) edge and the 0(1) and 0(2) apexes in the

P(l)04 group and the 0(5), 0(6) and 0(7) apexes in the P(2)04

group are coordinated to Ca. No edge in the H2P(2)04 ion

is coordinated to Ca.
The hydrogen bonds

In our x-ray studies of calcium phosphates, the

positions of the hydrogen atoms are known only imprecisely

owing to known systematic error in the scattering factors

of hydrogen. It has become our practice to calculate "ideal

hydrogen positions" . In Table 1 we report similarly calculated hydrogen positions for comparison with the positions

from the neutron diffraction results. The calculated

positions were obtained by assuming that the 0-H distance

is 1.000 A and the P-O-H angle is 114°, averages of the values

found by neutron diffraction here and in CaHP04 '21120 (Jones,

1970). The calculated positions for H(l) , H(2) and H(3) are

close to those found by neutron diffraction, and justify the

procedure for use in cases of non-centered hydrogen bonds

where neutron studies may be unavailable.

The variation of the 0-H distance with the 0...0 distance

for the non-centered hydrogen bonds is consistent with the

curve given by Pimentel and McClellan (1971; Fig. 4, therein)

.

The 0(6) -H( 3) distance of 0.97 A is the value expected for

an 0...0 distance of 2.82 A. Likewise, the 0(3) -H(l) and

0(4)-H(2) distances of ~1.03 A are those expected for 0...0

distances of 2.62 and 2.68 A.

While the precision of our neutron study does not equal

that of our x-ray work, the orientations of the ellipsoids

representing the vibrational motion of H(l) and H(2) are
-14-

reasonable in that the smallest principal axis is in each

case approximately parallel to the 0-H bond, and the

largest amplitude of motion is perpendicular to the P-O-H. . .0

plane. This is to be expected since motion of the hydrogen

out of this plane (derived from OH libration about the P-0

bond) is known to be of low frequency (~950 cm~^) and thus

of relatively large amplitude (Chapman and Thirlwell, 1964)

The orientation of the thermal ellipsoid for H(3) is

not as easily understood as those of H(l) and H(2). The

large errors (^20**) in the orientation angles for H(3)

suggest that the motion of this hydrogen may not be adequately

described by the usual thermal ellipsoid.

Hydrogen atoms H(4) and H(5) are both involved in very

short hydrogen bonds, the 0...0 distances being 2.434 and

2.42 3 A, respectively, and link I>{2)0^ groups together to

form a chain with the repeating unit -I^(4)--0 (7 ) P (OgH) 0 (8)--H ( 5)

(see Fig. 1) . The apparent positions of these protons on

centers of symmetry may be examined in terms of three models,

(i) The protons could be ordered in a chain so that each one

is associated with only one PO4 group. Thus, H(4)

and H(5) would not be on centers of symmetry, but the sense

of neighboring chains would be random to give an average

picture of two half-hydrogens about the center. This situation is very unlikely in view of the very short 0...0 distances

asymmetric distributions have been found only for 0...0 distances greater than 2 . 54 A (Pimentel and McClellan, 1971).
"15-

(ii) Each proton could be located in a symmetrical double

minimum potential with the very short 0...0 distance implying a low central barrier and consequently a high tunneling

rate, (iii) Each proton could be in a potential well with a

central barrier, if any, below the zero point energy.

The difficulty of deciding from diffraction data alone

whether the potential has a single or double minimum has

been discussed by Hamilton and Ibers (1968) . Using

the neutron data on Ga (H2PO4) 2, we carried out a series of

refinements in which H(4) and H(5) were displaced off center

along the 0(7)... 0(7') and 0(8)... 0(8') vectors respectively

in steps of 0.05 A and only the vibrational amplitudes of

the protons and associated oxygens 0(7) and 0(8) were allowed

to vary. The apparent amplitudes of vibration for H(4) and

H(5) along the 0...0 vectors decreased as the protons were

moved further off center while the corresponding amplitudes

for 0(7) and 0(8) remained constant. No significant change

in the value of occurred until the protons were 0.17 A off

center, when negative amplitudes of vibration were obtained

for both protons. Fig. 2 gives the results of these refinements and shows that an upper bound for the off-center

distances is ~0.15 A assuming the proton vibrational amplitude

along the 0...0 vector must be at least that of the oxygens.

This is a good assumption since the only motions where the
-16-

proton does not move with the oxygen atoms are modes in

which the oxygen motions maintain the center of symmetry

and these optic modes are known to have very small amplitudes.

This reasoning restricts the protons to within 0.15 A of the

centers.

More insight into the nature of these short hydrogen

bonds can be gained by considering both spectroscopic and

diffraction information. The exact frequency of the OH

stretching vibration for very short (~2.45 A) hydrogen bonds

is still under discussion, but is generally associated with

absorptions in the range 600-1500 cm""^ (Hadzi, 196 5) . The

infrared spectrum of Ca(H2P04)2 in the range 400-4000 cm"''"

is shown in Fig. 3. Our spectrum confirms an earlier one by

Lehr et a_l. (1967) . Although we do not intend to delve into

detailed assignments, some absorption bands can be assigned

by analogy with other phosphates (Chapman and Thirlwell,

1964) and Ca (H3PO4) 2 -HgO (Berry, 1967). The absorptions at 3450

and 332 0 cm"''" are 0-H stretching frequencies expected for

an 0...0 distance of'-2.82 A (Pimentel and McClellan, 1971),

while absorption at 2 950 cm""*" is similar ily expected for

0...0 distances of ~2.6 A. This leaves open the question

of the OH stretching frequency for the very short (^2.45 A)

hydrogen bonds, which are expected to absorb in the 500-

1700 cm"""" region (Hadzi, 1965) . However, many absorptions

including in-plane and out-of-plane OH bends, which hinder
assignments via deuterium shifts, absorb in this region and

make it difficult to uniquely assign the OH stretching

frequencies associated with the short hydrogen bonds. T*he

spectrum indicates that the highest stretching frequency the

OH vibrations in the centered hydrogen bonds could have is

~1400 cm"''" , assuming the absorption at ~1400 cm"''" is due to

OH stretching rather than to OH in-plane bending. One can

construct an approximate double minimum potential for these

protons using both spectroscopic and diffraction data. As

the extreme case, 'we can take an upper bound to the OH

stretching frequency as 1400 cm"''' , and using the relationship (Hamilton and Ibers, 1968)

^ 2 ^ h

Sn^mcv

for the harmonic oscillator in the ground state, we obtain

<u^> « 0.012A^, where <u^> is the mean-square amplitude of

vibration in and v is the frequency in cm"^ . If we make

the assumption that the total mean-square amplitude of each

proton is at least that of the bonded oxygens, the potential

minimum could be 0.15 A off center (Fig. 2) . Fig. 4 shows

that in this extreme case (highest OH stretching frequency,

no allowance for other vibrational modes, i.e., no oxygen

motion considered, harmonic potential) , the central barrier

is below the ground state vibrational level, i.e., the zeropoint energy. Allowance for the anharmonity associated with

hydrogen bonds would lower the barrier.
-18-

A more reasonable potential might be one constructed

assiaming Vq h ~1150 cm""^ because the spectriim shows intense

absorption characteristic of hydrogen bonding in this region.

(This assignment is not unique because vibrations also

absorb in this region.) Then <u^> would be 0.015 , <u^>o

as estimated from the diffraction results is 0.009^^, and the

total mean-square amplitude, <u^>o + <u^>h would be approximately 0.015 + 0.009 = 0.024 . This puts the potential

minimum 0.11 A off center (Fig. 2) and Fig. 4 shows the

central barrier would again be much lower than the ground

state vibrational level.

One might assume a value for the OH stretching frequency lower than that given here, but this leads to a larger

mean- square-amplitude of vibration and thus the separation

of the potential minima must be smaller to fit the diffraction data. Hence, the central barrier would be still lower.

Thus, we conclude that the H(4) and H(5) atoms in Ca(H2P04)2

are in centered hydrogen bonds. The conclusion that the

central barrier lies below the ground state vibrational level

for these hydrogen bonds agrees with the prediction that the

proton displays a centre symmetric distribution when R(0...0)

<2.47 A (Pimentel and McClellan, 1971), and is supported by

recent

^ initio calculations on short hydrogen bonds

(O O ^2.45 A) in HbOs"^ and H3O3" (Kollman and Allen, 1970;

Newton and Ehrenson, 1971) . These calculations indicate that

the central barriers in the potential wells are significantly

below the zero-point energies (~700 cm"''") for 0. . .0 separations

up to 2.487 A. It therefore appears best to consider the
-19-

delocalized proton in hydrogen bonds of this type as a "particle in a box" since the proton moves in a rather broad

and flat potential exhibiting considerable anharmonicity

.

General discussion

The general details found in the structure of CaCHsPO*)

are also present in Ca(HgAs04)3 (Ferraris, Jones and Yerknes

1972) , which was investigated independently. The overall

structural type is expected to be uncommon because the great

excess of anions over cations will make three dimensional

bonding unavailable for most combinations of ions. Possible

candidates for this type of structure will probably consist

chiefly of anions with at least one and more probably two

hydrogens (i.e., HgPO*, H2ASO4 , HgVOl,, HSO4, HCroI , etc.)

with which to form hydrogen bonds. If an acid molecule

(with zero net charge) derived from a tetrahedral anion can

be induced to crystallize in the structure, the cation need

not be more than monovalent. Salts of the type Ca(BF4)2 are

also possibilities. The salts Sr(H2P04)2 and Ba(H2P04)2 are

known to crystallize in two forms each (Lehr et al . , 1967)

.

The reduced cells (Mighell, Santoro and Donnay, 1969) of

their triclinic forms, together with a permutation of the

cell of Ca(H2P04)2, are:
-20-

a b c g 3 Y

Ca(H2P04)a 5.550 A 8.253 A 7.558 A 109.15° 93.68° 109.87°

Sr(H2P04)3 6.70 7.10 7.84 104.7 96.9 109.3

Ba(HsP04)2 7.03 7.24 8.10 105.0 96.2 108.6

While the Sr and Ba salts appear to be structurally related

to each other, they do not seem to be simply related to the

Ca salt. Perhaps Ca is not able to sustain the large cation

coordinations undoubtedly required in the Sr and Ba structures,

and a relationship of distortion obtains similar to the relationship between the Ba3(P04)2 (Zachariasen, 1948) or

Ba3(V04)2 (Susse and Buerger, 1970) type structures and

3Ca3(P04)2 (Gopal and Calvo, 1971; Dickens, Bowen and Brown,

1971b) . In the apatite series, Ca is just within the stability

field which includes Ba and Sr. For cations with ionic radii

less than that of Ca, the -vvagnerite (Mg2P04F) structure is

preferred (Kreidler, 1967)

The linking of PO4 groups through symmetrical hydrogen

bonds to form chains running throughout the crystal is one

of the more novel features of the structure of Ca(H2P04)2.

To our knowledge this is one of only two examples of such

extended linkages involving centered hydrogen bonds, the

arsenate analog, Ca(H2As04)2 (Ferraris et al. , 1972), apparently

being a similar case. Such linkages suggest possible

ferroelectric behavior especially in view of the fact that

k
-21-

other salts of H2PO4", e.g., KH2PO4, are well known ferroelectrics. Present theories of hydrogen-bonded ferroelectr ics

(Kobayashi, 1968) emphasize the ordering of the protons in a double well. In general, these theories lead to the result

that the higher the tunneling rate (i.e., the lower the

barrier) , the lower the temperature of the phase transition,

which is often second order.

In the case of Ca(H2P04)2/ o^i^ unsuccessful attempt to

reduce extinction in the crystal used for neutron diffraction

by dipping it in liquid nitrogen indicates that there is

probably no first-order phase change in Ca(H2P04)2 between

room temperature and ~77°K, although there is still the

possibility of a second-order phase transition. However, the

barriers are seemingly well below the zero-point energies

even at room temperatures, and contraction on cooling would

be expected to decrease the barriers by more overlapping of

the component potential wells. Thus, we conclude that the

existence of a ferroelectric phase of Ca(H2P04)2 is unlikely.

The existence of hydrogen bonded dimers of PO4 groups in

Ca(H2P04)2 niay have some bearing on species present in aqueous

solutions of phosphates. The existence of a dimeric species

[H4(P04)2]^~ in fairly dilute aqueous solutions of alkali

metal orthophosphates has been suggested (Childs, 1969, 1970)

on the basis of studies of ionic equilibria. Similar postulates

have been made for solutions of H3PO4 more concentrated than

0.1 m (Elmore et ajL . , 1965) . The presence of the ion pairs

[CaHP04] ° and [CaH2P04]* has been suggested (Moreno, Gregory

and Brown, 1966) via solubility studies of calcium phosphates.
-22-

Our observation of H2P(1)04 ions linked into dimers and

H2P(2)04 ions linked together by symmetrical hydrogen bonds

to form polymeric chains suggests that the possibility that

small polymers of phosphate ions exist in aqueous solution

be given serious consideration. The related case of the

hydration of H"*" •nH20 to H*'(n+1)H20, has been studied by

Kerbarle ejb aJ.. (1967) who showed by mass spectrometry that no

unique value of n dominates. Indeed, in the crystal structure

studies of (H*«nH20) x" compounds (Lundgren and Olovsson, 1968)

,

H502'*" and higher complexes were found. Thus, further investigations into the existence of dimeric, trimeric, etc., species

involving H2PO4 seems warranted. The dissociation constants

of H3PO4, 10-2-^=, lO-'^-s^ and lo-is-^"^' reveal considerable

affinity of the PO4 group for its second and especially its

third hydrogens. Thus phosphates, being polybasic, might be

expected to contain unusual but chemically important hydrogen

bonds which result in the formation of unusual complexes.

Acknowledgment

We thank J. S. Bowen and P. B. Kingsbury for technical

help. Fig. 1 was drawn using the ORTEP program of C. K.

Johnson. This investigation was supported in part by research

grant DE00572 to the American Dental Association, General

Research Support Grant RR05689 funds made available to the

American Dental Association, and by contract NIDR-02 to the

National Bureau of Standards from the National Institute of

Dental Research.
-23-

References

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A27, 157-165.

Alperin, H. and Prince, E. (1970) . J. Res. Natl. Bur. Stds

74C , 89-95.

Bassett, H. (1908) . Zeit. anorg. Chem. _59. 1-55.

Baur, W. H. and Khan, A. A. (1970) . Acta Cryst. B26 , 1584-1596.

Berry, E. E. (1968) . Spectrochim. Acta 24A , 1727-1734.

Blirnie, R. J. (1962) . Rev. Sci. Instr. 22^, 598-599.

Chapman, A. C. and Thirlwell, L. E. (1964) . Spectrochim.

Acta 2_g, 937-947.

Childs, C. W. (1969) . J. Phys. Chem. 73^. 2956-2960.

Childs, C. W. (1970) . Inorg. Chem. 9, 2465-2469.

Coppens, P. and Hamilton, W, C. (1970) . Acta Cryst. A26 , 71-83.

Cromer, D. T. and Mann, J. B. (1968) . Acta Cryst. A24 , 321-324.

Cromer, D. T. and Liberman, D. (1970) . J. chem. Phys. 53 , 1891-1898.

Curry, N. A., Denne, W. A. and Jones, D. W. (1968) . Bull.

Soc. Chim, Fr., 1748-1750.

Dickens, B. and Bowen, J. S. (1971) . Acta Cryst. B2 7 , 2247-2255.

Dickens, B., Bowen, J. S. and Brown, W. E. (1971). Acta

Cryst. B28, 797-806.
-24-

Dickens, B., Bowen, J. S. and Brown, W. E. (1971)

Abstract G3, American Crystallographic Association winter

meeting, Columbia, S. C.

Elmore, K. L. , Hatfield, J. D. , Dunn, R. L. and

Jones, A. D. (1965). J. Phys. Chem. 96, 3520-3525.

Ferraris, G., Jones, D. W. and Yerkness, J. (1972) . Acta

Cryst. B28 , 209-214.

Finger, L. W. (1968) . Carnegie Institute of Washington,

unpublished.

Gopal, R. and Calvo, C. (1971) . Canadian J. Chem. Soc. 49 , 1036-1046.

Hamilton, W. C. and Ibers, J. A. (1968) . Hydrogen Bonding

in Solids Chap. 3, New York; Benjamin.

Hadzi, D. (1965) . Pure Appl. Chem. 11, 435-453.

Jones, D. W. (1970). (Private communication.)

Jones, D. W. and Cruickshank, D. W. J. (1961) . Z. Krist.

116 , 101-125.

Kerbale, P., Searles, S. K. , Zolla, A., Scarborough, J. and

Arshadi, M. (1967) . J. Am. Chem. Soc. 89, 6393-6399.

Kobayashi, K. K. (1968) . J. Phys. Soc. Japan. Z4, 497-508.

Kollman, P. A. and Allen, L. C. (1970i . J. Am Chem. Soc.

92, 6101-6107.

Kreidler, E. R. (1967) . Ph.D. Thesis, Penn. State University.

Dissertation 68-3549, University Microfilms, Ann Arbor,

Mich.
Lenhert, P. G. (1970) . Abstract M8, American Crystallographic

Association winter meeting. New Orleans, La., March,

Lehr, J. R. , Brown, E. H. , Frazier, A. W. , Smith, J. P.

and nirasher, R. D. (1967) . Crystallographic Properties

of Fertilizer Compounds , Chem. Eng. Bull. 6_, TVA, p. 118.

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International Tables for X-ray Crystallography, Vol. I,

Kynoch Press; Birmingham, p. 534.

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93, 4971-4990.

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Pimentel, G. C. and McClellan, A. L. (1971) . Annual Rev.

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College Park, Maryland 20742.
-26-

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Zachariasen, W. H. (1967) . Acta Cryst. _23, 558-564.

Zachariasen, W. H. (1948) . Acta Cryst. 1, 26 3-265.
Table 1. Atomic parai;etcrs in Cadl^l'O^)^

Atom X y. z 3 Bio B..1

0 0 . 41915 (2 ) 0 . 1 8764 ( 3) 0 . O _i _J \ ^ J 0 .767 (6

)

0.597 (6

)

0 . 2 1 9 (4 ) 0 nsR ( ^

\

0 . 2 3 3 ' 4

)

b 0 .3139(6) 0 .4190 (5) 0 .107 7 (6 ) 1 .0(1) 1 .0(1) 0.9 (1 ) 0.2 (1) 0 .3(2 ) 0.3(1 )

P(l)fb

n0

• 'UC/ \ ^ /

. 2569 (5 )

Q0 . 1 596 (5)0

0 .5171 (5)

Q

1

6 93 (7)

.8 (1)

0

0

. J :?u ^ / ; .7 (1)

0 . 534(7)

0.7 (1)

0.124(5)

0.1 (1)

0

0

.073(5)

.14 (8)

fI 7 n o f

i

0,18(9)

0(1)-

b 0 .42 59 (4)

n0

3 1 on f P ^

. -J J- D KCi } .3169 {4 )

00. -) ± z Z \ ± ; .5122 (5 )g0

fin t ? \ .8(1)n

0

. O J \ ^ } .8 (1) 1.1(1) -0.08 (8)0

0 .20 (8)

u . JO V X

;

0.37 (9)

0(2)-

b

u

0

.J / iJD-l- V^/

. 1&02 (4)

n

0

. XO /D^ \ ^

/

. 1888 (4 ) 0

• /OjO^ \ 1 .7671(5 )

1X1 .0(1)

X

1 .0(1)

U , D J ^ z ^ 0.7 (1)u . J. 4 i X ^ 0.1(1)

nU

0

7 M 1 . z J ( X J

.3 (1)

u , ^ X j 0.1 (1)

0(3)-

b

n -0 .02827 (9) n J. nu U • J\> \^ ) n 7 7 1 n'\ n 3 7 f 1

1

0 .2 903 (4 ) -0 .02 70 (4

)

0 .4384 (6 ) 1 • 8(2) 0 .9 (1) 1 .2 (1) 0.6 (1) 0 .38 (9) 0.3(1)

0(4)-

D

0 . 10443 (9) 0.12477 (9) 0 .2801(1) 0 .98(2) 1 .08(2) 0. 95 (2) 0.11(1) -0 .17(1) 0.49(1)

0 . 1035 (4

)

0 . 1237 (4 ) 0 . 2 7 95 (5 } 0 .9(1) 1 . 1 (1 ) 1.3 (1) 0.1 (1) -0 .1(1) 0.5 (1) a 0 . 306 (3) -0

,

.053 ( 3) 0 . 566 ( 5) 1 .00

H(l)b 0 . 3239 (9) -0 . 0622 (8) 0 .586 (1) 2 .8(3) 2 .1 (3) 2.3 (2) 1.1 (2 ) -0.0 (2) 0.7 (2) 0

,

-0

,

.064 0

,

. 587

a 0,. 021 (4) 0

,

.038 ( 3) 0

,

.235 ( 5) 1

. 00

H(2)b -0,. 0152 (8) 0 0,.237 ( 1) 1,. 5(2 ) 1 . 5 (2 ) 2 .4 (2 ) -0.1 (2) -0, 1 ( 2) 0.5 (7) -0,.013 0..011 0..236

I'(2)-

0 .73538(3) 0

.

.31138(3) 0,. 17145 (4) 0,.651(7) 0,.650(7) 0.624 (7) 0.210(5) 0,.029(5) 0.211(5)

b 0. 7 3 56 (4) 0

.

3116 ^ R

1

0. 1710(6) 0. 6(1) 0

.

8(1) 1.1(1) 0.3(1) 0. 3fl) 0.4(1)

0(5)^ 0,.37118(9) Q _, \J 'i \J / _J \ ]^ ^ in f 7

1

, xu \^ 1 1 _. 05 (2 ^ 0 9<a (7 1 0 , 5n ( 1

)

0..05(1) 0.47 ( 1

)

b 0..6026 (4) 0..3708 (4) 0..0403 (5) 1,.1(1) 1..1 (1) 1.3 (1) 0.6 (1) 0..2 (1) 0.5 (1)

a 0(6)-

b

0., 91733 ( 9) 0.,32349(9) 0

.

.0360 (1) 1,. 12 (2) 1

.

.26 (2) 1.01(2) 0.47 (2) 0..47(1) 0.26 (2)

0,,916 7 (4 j 0..3234 (5) 0..0351 (6) 1..1(1) 1.4(1) 1.3(1)/ 0.4 (1) 0,.6(1) 0.4 (1)

0(7)-

b

nU . 0. 44107 (9) A

u

.

fc> -5 12 ^ TX • n f.7 (7 1 u • Z U \1. ) -0. 01(1) — u . uy ^x ; 0. 8261 (4) 0. 4407 (4) 0. 4553 (5) 0. 5 (1) 1. 3 (1

)

0.9 (1

)

0-2 (1

)

0, 1(1) -0.1 (1

)

0(8)- 0. 6416 {} ) 0. 112 32 ( 3) 0. 1612 (1

)

1. 24 (2 ) 0. 71(2) 1. 19 (2 ) 0.05 (1 ) -0. 23(1) 0.46 (1) u

.

D 4 J 1 (A) 0. 1126 (4) 0

.

ifoin (6

)

1

.

3 (1 ) 0

.

8 (1

;

1 .3 (1

)

0.1 (1 ) -0. 2(1) 0.5 (1

)

0. 899 (3 ) 0. 248(3) -0. 103(5) 1. 00

H(3)b 0. 882 (1) 0. 233(1) -0. 140 (1 ) 3. 6 (3) 2. 7 (3) 2.3 (3) 1.6 (3 ) 1. 1 (2) 0.4 (2)

_c U . bdo 0. 230 — u

.

1 yl "7 J.4 /

.

a 1. 000 0. 500 0. 500 1. 00

H(4)b 1 . 000 0. 500 0. 500 3 . 1 (4) 2. 1 (4) 1.3 (3) 0 .7 (3) 0. 4(3) 0.1 (2) 0. 969 0. 490 0. 493

"0. 500 0. 000 0. 000 1. 00

H(ti)b 0. 500 0. 000 0. 000 2. 5 (3) 2 . 7 (4) 2.6 (5) 1.4 (3) 0. 7 (3) 1.7(3)

c 0. 526 0. 033 0. 020

Figures in parentheses are standard deviations in the last digit (s).

* Thermal parameters are in and have the form exp-[ 1/4 (h^a* i

i +

k^b*^Bpj, + i^c*^B33 + 2hka*b*Bi2 +2hXa*c*Bi3 + 2k jeb*c*B33) ] .

\^ a. X-ray values

b neutron values

c calculated hydrogen positions as described in text.
Table 2. Calculated and observed x-ray structure

factors for Ca(HoP04)2. Columns are I, 10 F^, 10

and 10 a(F^). Unobserved reflections are those with

intensity less than 2 ct(I) and are marked by *.

-I ;ji ir-r

Ik 1 120 3

'3 8 107 10*

Z -ft It '*0

-3 21 23 I

•1 2U 20i

1 3'! 377

S5 S7 1 3 55

5 102 102

-5 2b 2* 1 -9

-» Jlf aoa 7 -T

-5 21* 221 1 -* 15 3*

-J S3 : -2 2»2 t=t\ 7 -1 s:i zf •• 0 133 72 2

1 321.

2 2^7 262 5

-7 120 1J2

3 IM II)

137 134

K -5 > 2 213 ?1«

-1

2

-fc 123 12t

-5 239 2tO

-2 203 203

2 129 L31

-* *S 7S

-3 39 3S -! 175 iTi

109 110 2

-3 i*. 55

-7 ?a 60 1

5 193 ISi I

-7 JOT 21*

•d 2T) 27'> -3 152 155 -2 102 1^6

3 I7s iTg

133 3 -10 bl

-5 223 227 5

-It 95 91 2 -3 53 53 1 -2 358 »ia 9

•1 20 22 I 0 Ihi l"! V

1 229 231 5

2 lao 2"< b

3 219 2?9 5

« 3bb 369 a

5 2TL 27b 6

-T 20» 2U

351

•2 107

ira 5 -6 IBI ISO

-5 103 133

-t Id 19 -3 32b 111

-3 237 235

1 <*22 399

5 152 152

? 326 379

3 17 IT 1*9 liO

5 5!" 3?3

-6 21*3 2«9

-5 l»b 1»9

5 27 35

137 nq

-3 155 15"

-2 ion r;

-3 lua i»b

-6 23 23 -5 139 110 ~t ibJ I6H

-« 93 12
» IS» I** 3

-I <S9 To i n ici«i liO 2 1 I^ ift 1 ? 191 tOl 3 ) 3tJa 209 s

<t 12

U I 5 121 122 J » T2 72 J • 120 120 3

132 133 3

<*h 97 2 -3 90 69 2 -2 Iftit lbt> » -1 «9 2 9 2211 22% % 1 192 13« 3 J *• » 2 i 223 222 S

(19 90 2 1 33 31 1 6 176 17a •« f U h A lb 1% 1 7.-5.L

-« 09 08 3 -5 1'6 17(> •

-"I 2» 2* 1 -i 1^7 lie « -2 2?« 229 t

-1 2S 3i 1 0 113 113 3 1 299 ZbO & 2 55 53 I 3 32 3S 1

<* 158 139 » 5 <t» «3 1 6 93 32 1 T 97 100 2 S 1» S 2 T.-T.L

-4 •» »& 1 -5 T8 77 2 -« 131 133 3 -3 251 29 1 •2 91 92 2

-I 151 153 3 0 67 b9 2 1 39 9 1 2 t&O 163 % 3 •« az 1 97 96 2 5 35 33 i 6 ?5 2* I 7 •2 ftl I 8 SO k9 2 7.-*.L -7 32 21 I -6 198 lOS 3 -5 ^1 52 1 -« 60 60 2 -3 79 79 2 -Z 1» 33 1 •1 165 167 « 0 108 112 2 1 1Q« 136 3 2 179 173 « 3 im IH« 3 • 139 137 i 5 199 160 3 6 00 92 2 7 36 S7 H

<t 61 61 2 7.-5.L

-T 32 33 1 -« 59 S3 2 -5 22 22 1 -•53 5» I -3 79 79 2 •2 19 15 1 -t 69 70 a 0 107 108 2 1 77 2 2 32 31 1 3 75 7» 2 • 63 62 1 5 ^6 «<* 1 6 26 28 1 7 96 2 A 16 15 1 7.-9.t

-7 la Si

-6 21 IS 4 -5 •7 07 1 -a 64 bS 2 -3 "»» 50 1 -2 90 92 2 -I 11* 119 3 " »» 96 2 1 ^0 »7 1 2 2ii0 2'*i b ^75 23 I t 78 79 2 5 I'lfl 110 2 6 75 2 7 157 137 3

•« 10 3 « '2 21 1 -6 \i* to* 2

-•i ^9 92 2

-to 90 2 -1 1«5 157 >t -2 23 22 1 -I 2)7 23» 5 3 1)3 113 3 1 <>« 5b 1 2 251 705 5 » 1''2 1117 i • »5 «i* 1 5 IM Ibl •* 6 60 e>l 2 r 91 30 1 7.-2.L

-A 33 32 I -7 si> I -6 l«l U2 J -5 45 i»y i -« 169 tk« • 0 321 371 3 2M5 2^5 6 222 2?5 2 107 |n5 6 103 101

I<t0 -3 129 -2 20« 0 156

-i 223 2 193 3 170

59

-7 93

-6 169 -5 215

-9 89

-3 139 -2 It?

-1 asb 0 135 1 137 2 273 3 91*

7.2'L

-8 108 106 -7 116 117 •6 166 167

-3 157 159

-1 102 10

-J 135 139 3 -2 79 79 2 8.-19.1. 1 20 20 1 2 108 107 t 3 7» 75 2 « 20 19 I 9>-13*k •19 9 6

I5S 155 « 29 I 16 6 1 31 9 1 55 1 71 2 26 1 6 29 29 1 - 8.-n-L

-5 57 37 1 - -2 21 21 t I -1 92 «2 1 0 23 23 1 1 90 92 2 2 19 19 1 3 36 54 1 9 70 71 2 5 100 tot 3 6 «0 «0 1

60 2 26 1 105 3

an 2 VI 2 IS I

-5 62 61 2

-2 150 152 3 -1 58 56 2 0 78 60 2 1 199 lw8 3 2 170 171 • 3 61 62 2 9 IS9 160 9 5 72 72 2 6 53 53 1 7 69 65 2

05 2 109 2 69 2 156 3 62 2 15 1 30 5 7 56 2

35 1 6t 1 20» 5 130 3 2 199 195 I 5 225 231 5

95 2 - -1 35 33 1 0 203 20* 5 - 1 20 10 1 2 13' 136 3 - 3 215 215 5 5 195 IB*. 1 153 153 3 2 53 55 I 5- 2ft^ 2»9 6 7 92 91 1 • 5 1 112 5 125 9

79 2 62 2 208 5

79 2 126 3 125 3 65 2 49 2 15 1 56 1 U5 3 31 1

76 2 75 2

^•i 2 63 2 60 2

79 2 70 2 37 1

127 122 5

36 39 1 »'-ll.w

22 ^1 1 39 39 1 149 t«9 3 69 70 2 56 55 2

92 2 O'L

3« 1 US 3 32 1 105 2 109 2 20 1

66 2 109 3 58 2 99 1

17 I 93 2 115 3 109 3 70 3 130 3 10 1

<*2 1 61 2

29 1 12 I 65 2

110 3 113 3 15 1 139 3

56 2 67 I »7 I 3 97 2 9 56 1 0 1*0 3 -ISn. T 57 1 6 16 1 7 27 I

57 2 37 t 31 1 79 2

59 59 2 75 75 2 127 127 3 53 51 1 93 99 2 122 12? 3 71 70 2 Il» 115 3 10.-8*1. too 109 2

155 155 1

111 112 3

137 130 5

90 91 2 -I

1«7 3 25 I 22 1 160 9

39 1 15 I 75 2

175 129 1

16) 63 1

15 1 96 2 91 2 92 I 133 3 65 2

U« 3 55 1 137 J 99 2

109 3 32 I 123 3 1.19 3 »3 I 103 2 »1 2 113 3 15 1

119 U7

J

Table 3. Calculated and observed neutron structure

factors for Ca(H2P04)2. Columns are I, 100 F^^ 100

and 100 a {Fq) , Unobserved reflections are those with

intensity less than 2 and are marked by *.

I in lb' «

* 1» 193 r

1 lutf

J ld9

0 tif b9

1 ue I9L 7

^ 139 l>i T J is; 196 t • l»T 170 T

k 19*« ICI b9

b l»< l*i« > —

1 U.» b' i^. * 1.--" *i

2 lll> »3 »^

« 7<) B6 ID ^ <»1 62

i iiM 2t« r

* l»lT 126 7 ^ S27 312 •

« 162 S

7 2^6 2»B *

i 11? 1?) 6

3 %H 13

<* 123- 46 ^2

^ 113* *4 ^7

t 26* 261 fl 2 U6* 31 %S

1 )23> 93 t>2 « 136* 14 66

1 12«* 7A 62

^ IHI 1«1 a

J 162 13« A

I 19<«> 26 67

J 223 I4S S

1 I6S 2«9 e

2 12b* Jfa 63

i W6* SO 63

<« 209 2U3 7

•> 139 162 S

a 2«6 2>»<« S

lt2* 32 56

1 362 331 to

2 it* lb9 6

9 '9 71 9

- U3 U2 6

3 193 14? 6

6 122> 17 61

r 176 193 7

U 3b 1 332 10

1 1»4 132 6

2 300 3du 11

i 277 2/6 S

• t^i 279 8 b 111" 33 S5

« IbO 1<*S 7

7 2«i 297 A

0 Ibb 190 6

1 200 193 6

2 3bO S>»6 10

9 tbO lib 6

• 66 04 II b ^6 93 9

» ITJ 171. 6

7 127* b6 69

M.-«..3

0 2«1 25»

1 2«2 297 9

2 37, 3s5 11

3 ?T' s

<• 539 ^^^ 9 b 163 Ibf 6

12« 161* 33

62

> 156 ISl 6 221 221 7

I 13t* 114 66

a i29» 97 6«

' 33b 341 10

131

277 2«3 • l2« 15b

119*

Ub" 39 T3

122* * 61

2 12S« 41 6«

3 2«b 29* «

121*

•<.0.-9

0 296 3:6 9

1 2b3 J-iS 7 2 ?3i 7 3 357 9

t 210 J22 6

-i

3 12b* 1| 63

« 130* 102 6b

b 130* b3 6b

6 !<••.• 21 T2

IbB lb« b

1 77 HO 6

2 210 2rtb 7

3 bb* 5* 29

1 365 lAO 10

2 203 207 T

3 US* b b"*

>• 291 29b 9 b 197 19t 7 -.7. .3

1 223 22b T 2 ua« bi b»

3 119* 16 J»

6 16* 16S r

1 Itb

2 197

3 kbit

1 31

2 97

3 6)*

1 UO 90 <• 2 170 179 b

3 3b6 3<«3 IJ

!• 233 292 8

tSU 199 b

b 9t»- 21 -I

a 2'<0 226 7

7 117* 6b bl

9 U9" «2 b9

9 osl ii33 1» 1.-3.-1

3 227 2<40 h

• 71 o<* It b IbO IbT b

b 2»3 221 7

' 32S 3lb 9

a 3l>t 296 9

9 231 219 S

'«.-2i-l

0 bl b6 »

1 113 113 3

2 IIH lib 3 9 291 303 a

• 136 161 b b 29« 303 a 6 HI* b<t bb

7 107* bS 33

6 ID** *9 32 9 1*2 169 7

139* Mb 69

b 329 321 10

32 3b 0 30b 320 9 bo 34 1 196 2Q1 6

2 239 276 T •2 9 9CB 3lb 9 bb 9

1 49* 26 tb b 277 2bo 9

2 3 t> i-ft 2*2 7

J 193 191 o 7 *9 49 S

> 132 1'7 7 n i<*% 2<tB a

b I 1?" 60 37 9 77« 23 39

6 laa 203 7

3 129* wb b*

1 ISO Ibl 7

3 197 199 7

3 37, IX 5»

* 119* 97 b4

b 113 12b 9

9 163 146 7

7 153 iba 9 4,-6.-2

1 309 2*9 4

2 iJ* 230 7

I S%2 3«1 II

* 104. lOT b»

b 2C6 216 7 6 2- J 3'«4 7 7 <i4 fa« 9 238 231 •

-(.-9.-}

9 114* 44 47

1 69 93 II

3 tl3* 32 ?6

3 136 329 10

« 33b 3«0 »

b 96 nb 9 i itb* a 57

1 SOb

2 233

3 llb0 bO 60 2

1 116 113 3

2 «T 3 3 271 277 «

* 97 aa b

b 56 b4 9 6 106 190 6

7 193 19*1 7 S 113* 20 bT

1 210 206 6

2 78 7b 3

3 176 17* 3

« 143 190 6

3 375 27a 8

1 22^ ^17

2 lib* Id ; 3 6? 71

t 29b 293

H,9.-2

1 139* *« 69

19 I

0 IS! 17T 6

1 31b 3«4 a

2 b2* b9 11

11 I 276 2b3

a lb?* -.^ 73

ill 119 b

113 iia b 336 3ifl 4

377 3b* 11

70 6b 5 IIJ

1 7b« 6b 3T

2 7« 8b *

3 322 231 6

4 386 280 •

236 23<« lb ar

I'.Q 390 10 1»T IbJ T 27^ 39g 8

. 203 349 6 292 290 a 169 160 7

1 20* 112 *

2 ab 42 6

3 281 270 a 9 368 SbO 10

S 11** b3 S7 * 122* 9 61

»..b'0

0 210 20b 7

1 b3 b7 12 2 106* 16 b3

3 303 14T 6

« 109* 3b 54

b 2a9 260 a 6 Ibi 3*3 8

7 302 29* 9

a t3t* 03 66

9 126 77 11

M.bto

1 100* 3b 90

2 113 sa T

3 163 173 *

• 1'93 12* «

5 l*b 1«B T H.6.0

2 IIT Z2b T

3 67* 30 34

» I3- &9 34

393 3:6 10 90

0 331 3** 9

1 3->b 2b8 7 •2 236 230 T 63 94 b

• 96 93 b

b Ibi 132 6 6 141 17. 7

7 233 231 7 9 301 14b 7 9 12* Ut 9 H.-3'O

0 68* 27 3*

1 202 196 6

2 40 101 3 3 79 bO 5

• 2*a 2<i3 T b 7J 7i 7 u63 *32 13

86 I

1 130 156 3 2 130 l^u " 3 213 2ia 6 « 3u3 33b >4 b ivu* b3 bO

* It* |7lt 7 T 2*4 ib7 7 8 tJ7 1*9 B

1 6r« 69 3*

2 70* 2* 35

3 '7- 43 38

1 m Z08 T M.-IO.l

1 179 20* T

2 103 lib 9

3 136* 135 63

9 125* 10 82

9 127> 11 43

6 226 331 8

T 191 167 9

N.-8'l

0 120* II 60

1 196 307 7

2 27* 283 8

3 lib* 71 57

• 91 81 8

b 213 204 T 6 123* *3 61

T 213 313 8 8 187 W8 «

4 337 3»|} b iu3* 2; ' 8 tbU 279

1 Z*b 239

2 234 339

3 i>u 2!l* * 111* *3 •

0 3*7 366 to lia* 9* 99

131 102 T

2 330 336

3 67 TO 9 i»7 3«a

i.6>l

2bb 2b3 8 3 lu 119* 3 49

127a 96

3 299 30* 8

* 99 lOb 7 b 242 27b 8

6 2 3b 2*b T

7 118 II* 8

8 69 8b 19 M.-b.

0 230 218 6

1 IbO lb* 5

2 313 311 9

1 99» 3 «b

4 3*1 339 10 b 258 366 a

6 b' 33 13

31 61

160 5 % 3W 3ua

19 «0

3 W3 131 •

3 3b4 ib» 19

I9C 186 6

0 33b 32C

6 294 .•»7 «

> H7 3»* 9

a i.M* 7* M

9 149 190

1 lb* 38 b7

3 107 130 a

3 92 105 4 • 117* 3b b9

b 193 197 7 6 129* 10* 6*

T 14«* 133 73

0 2*4 2*2 7

1 3b0 331 10

2 390 296 9

3 *3S *3» 13

3 7, 71 b

* 271 373 8

b 236 2*6 7 b 7b* 39 37

7 903 244 4

« 130* 118 6b H.-3.3

0 38* 36* II

t III 116 *

2 12b ita • 3 169 17| 3 H'1'1

I 13b 160 o I 173 3bb 8

3 137 Ibl b

* b* 79 B b 3d<1 36* 11 6 9U7 3*8 9

7 I** Ibb 7

•«'2*l

1 2b r 2*9 A

; 4*3 *3b 13

387 374 11

n7«

t ll<* 12b 11 3 laa 146 b 4 333 319 ID

9 1*7 1*7 4 H.-9.2

0 136« 6 b*

b 69 43 It

0 131'- .9 I 1 Wj» ^5 I 3 3UJ t3i

3 343 303

1 *bt •21 13

3 2b* 33* 7 3 229 22b 7

2 173 176

3 80 79

* 20* 31*

3 103 b->

0 111* 69 39

1 133 It ) J 3t-i 39* 9

112 78 196 173

' 33 3 3*3

H.-b.3

0 »b* 32 22

1 *7 b7 10

2 10b 109 9

3 103 109 8

• 329 222 7 b 330 33* 9

6 130« 16 60

7 12b* bb 62

a 358 3*0 to

bb 0 131 116 b

0 1*2 163 *

1 120 133 * 2 *U3 426 U

3 205 233 6

—0.2

1 217 219 6

2 9** 1ft »2

3 139 127 b

* L '6 301 6

3 I'b 171 6

b 2*3 256

23b

116 113 9

bS

1 361 973 13

133 13* 6 9*6 92b 10

120* 93 60

111* 67 ^^

lib* '3 3*

60 f3 1*3 134 4 2*9 219 8

2 lib 121 4

3 132* 6* 66 0 : * 153 158 a 1 b 196* 14 68 2

0 lb* 1*3

1 1*7 139 7

2 69* 82 99

3 190 13a 7 0

« 212 14B 7 1 5 139* *b 6* 2 6 13* 136 9 3

5 903 242 9

b l*b 133

0 112* 16 bb

1 24M ^49 4

2 26; 339 •

9 Ibb 153 •

0 3b* 3*2 10

1 340 373 II 2 117* 11 b9

3 10* 121 8

• Ibl 161 7

5 8b 97 10 6 137. 72 63

T 209 120 a

0 Iba 1*6 6

1 113 100 7

2 *80 7b 14

9 319 3C9 7

* 39* 2*6 a

b 117. b? 54

6 184 la: 7 7 67 100 16

192 193 b

191 192 6 11** 2p 57

a 192 198 a H.-S.

0 86* 117 *i3

1 87* SH

2 179 17b b

3 2*7 2bl 7 * 277 39* 8 b 308 219 7

0 10?* I 51

1 102* 21 51

2 IB* |9i (, 3 13b 40 7

4 179 189

70

1 173 182 9

2 71 99 6

3 273 2SJ a

* bb 13 13

b 129* 194 62

6 1*6 Ibl 7 13** b2 62

I.e.

1 328 339 9

2 *0b *ia 11

3 219 333 7 * 176 173 6

5 106* 33 *3

6 112* 59 ^b -.1.3

1 i*b 1*? b

; 14. 43 50

) ?3fc 313 •

* I T6« ^3 "iJ b 106- IJ 53

1 M* 30»

3 294 3b*

* .'16 313

.•.3.3

1 112* 3 56

1 316 31> 7

» 112. 7b 5*

• t«6 1*< '

I 306 378 4

i 119- 9v' »4

3 fit 3L.3 7 I 37. 762 9

9 l»2» 7S t\

•.-a.b

0 I'j 7

0 113* 3 b6

1 115-

2 136 12: •

9 lb* 160 •

* lib* 17 a»

b 200 137 7 H.-3.»

0 120 117 7

1 105 12* 7 3 2b9 262 a

1 11** 81 37

0 2*8 3 76 7

1 16* 1*0 6

2 110* BC bb

9 90 no •

* b7« 7k a

9 156 i*b a

11

1 343 933 10

2 273 367 a

5 337 33* 9

9 100 103 6

t 102* 71 ^1

3 309 317 4

3 337 5)8 10

» 119* 93 60

b 48 lib a 6 367 363 a

7 13b* 92 »a ^.-2.4

0 131 1*1 5

1 3btt 342 10

2 3*6 56b 10

3 16* 1*3 6

* 267 196 8

b 321 3 30 7 8 123* bb 62

1 34b 307 8

3 ab I' 9

9 319 309 4

* 1'4 187

b 114* 9 60 6 29b 2b5 8

1 109* 19 5*

3 247 J96 9

9 192 19b 6

* 113* 92 56

b 114. 29 b9

. 377 3«9 6

1 211

2 1*1

3 tia

1 193 177 •

1 211 313 7 2 125* *7 ft2 3 129* 12 a«

•«.-$. 6

0 334 33..

1 203 19. 7 2 64* a 3b

5 91 13 11

• 78* 13 34

0 16* 16J 9

1 1*7 197 ft 2 lib* 3* b*

3 Ibb . 13* 7

1 114* •b 37

2 11* 118 •

9 1*1 3*3 s

1 l!0 84 6 ' 60 0 18* 187 8
C3 enviror 1^U ^ C Tl T'\ ^ ft ^ 1 o U ci 1 1 C_ L" / H or" 3 n CI 1 G A—X ay riG utx on Ca, 0(2) 9 4 T r J. Ca, 0(1) Z . J / I' ^ 0/ Ca, 0(5)

Ca, 0(7)

Ca, 0(1' ) 2.4316 (6) 2.433 (4) Ca. 0(5' ) ^

*> A D"7/1 / "7 \ • 4t> /4 V / / 0 /ID 7 /zl \ Ca, 0(4) Z • Dt>^ / \ t) ) Z9 A"7n (A \ , D /U J Ca, 0(6) Z • O^DZ \ 1 ) Z

0 0 /A ^ . b Jl J P Q Oa orouTJ P(l . 0(1) X • Dl^ -> \ O ) 1 CI /A ^ X - jlL J (-4-

] P (1 , 0(2) X • OUbl lb J X . t4 J P (11 , 0(3) Jl • D /u y lo ; X. Db3 (4) P (1) , 0(4) 1 ^1 AT) ie>.\ X • J /o (.4 J o(i' . 0(2) 9 =LR1 0(1! , 0(3) Z , JO J o(i] . 0(4) o Ann'^ ( R

^ • H\JU-^ [ O ; 0(2] . 0(3) 9 AQA Q I Q\ 0 AO.'? ( / ^ 0{2 , 0(4) ^ • OoU 14 J 0(3) , 0(4) ^9 Ac "J ^ ^ - ^J 14 J o(i] , P(l) . 0(2) X J. / . D y i J / XX / • ^ V J o(i) . P(l) , 0(3) ±X-5 . 4U \ ^ ) XXJ . J i -3 J o(i] . P(l) , 0(4) 102.07 (3) 102.5(3]

0(2] . P(l) , 0(3) 107.69(4) . 107.6 (3) 0(2] . P(l) . 0(4) 111.95 (4) 112.0(3)

0(3) , P(l) . 0(4) 103.17 (4) 103.4 (3) 0(3J , H(l) 0,82(2) A 1.030 (8) 1-016 (8) o(4: . H(2) 0.73(2) 1.000 (8) 1.011 (8) P(l] , 0(3) , a(i) 110(1) " 115,9 (4) P(l]... 0(4) . H(2) 117(1) 117.0 (3)

Tabic 4. Structural details in Ca(HjP04)2

?a. ?! P(2)0. qraUT-> Cent . distance, t or anale, dea .

E^PL2

P(2), 0(5) 1.4959 (6) A 1-499(4) I P(2), 0(6) 1.6001(6) 1.556(4)

P(2), 0(7) 1.5266 (6) 1.528 (4) P(2). 0(8) 1.5401(6) 1.540 (4) 0(5), 0 (6) 2.5292 (9) 2.530 (4) 0(5), 0(7) 2.5341 (8) 2.540(4)

0(5), 0(8) 2.5389 (9) 2.535 (4) 0(6)

,

0 (7) 2.4372(9) 2.439 (4) 0(6-), 0(8) 2.547 3(9) 2. 547 (4) 0(7), 0(8) 2.4869 (9) 2. 484 (4)

^ Neutron values for internucl ear distances

0(5

0(5

0(5

0(6

0(6

0(7

0(6

0(7

0(8

P(2

• P(2

P(2

p(2;

p(2:

P(2

p(2:

p(2;

P(2

H(3;

H(4]

H(5:

0(6:

0(7:

0(8]

environ:

0(6)

0(7)

0(8)

0(7)

0(8)

0(8)

H(3)

H(4)

H(5)

•er.ts

X-ray

109.52 (4) " 113.95(4)

113.49(4)

102.41(3)

108.42 (4) 108.38 (3 ) 0.78(2) I 1.2171 (5) 1.2116 (6) 116 a ) " 113.75(4 ) 121,59(5 )

neutron

109.6 (3) ' 114.1 (3) 113.0(3)

102.7 (3) 108.6 (3) 108.1 (3

0.957(8] i .985(8) " 1.218(3)

1.210 (3) 111. 7(5)

113.6 (2) 121.9(2)

0(1) , Ca 2,3795 (6) 2.379(4)

0 (1) , Ca' 2 .4316 (6) 2 . 433 ( 4) 0 (2) , Ca 2 ,3547 ( 5

2 . 346 ( 4) 0(2) , H(2) 1.70, 1.98(2 )± 1.710(6)

0(2) , 0(4) 2.6778 (9) * 2.677(5)

0(3) , 0(8) 2.6210 (9) * 2.620(4) * H(l) , 0(8) 1.62, 1.80(2)* 1.625( 7) 0(3) , H(l), 0(8) 177° 175 (1 ) ° * 173 .6 (6) » 0(3) H(3) 1.90,2.11(2) ii 1.922(8) I 0(3) , 0(6) 2,8101 (9) 2 . 81 7 ( 4) * 0(4) Ca 2.6647 (6) 2.671(5)

0(4) 0(2) 2,6778 (9) 2.677(5)

H(2) 0(2) 1,70, 1.98(2)* 1.710(6)

0(4) H(2), 0(2) 164, 161(2)°* 162.8(6) ' 0(5) Ca 2,4238(6) A 2.426(4) A 0(5), Ca- • 2.4874 (7) 2.483(4)

0(6) . Ca 2.8262 (7) 2.830(4)

0(6) , 0(3) 2.8101 (9)2.817(4) * n(3), 0(3) 1.90, 2.11(2)* 1.922(8)

0(6)

H(3), 0(3) 150,150(2) ° ± 154.5(6) ' 0(7), Ca 2.4284(6) I 2.42 9(4) I 0(7). 0(7' ) 2.434(1) * 2.434(5)

H(4) , 0(7' ) 1.43, 1.217 (5) ±1.217(3)

0(7), H(4), 0(7') 180, 180° * 180°

0(8) , 0(8') 2.423(1)* 2,419(6)

H(5) , 0(8' ) 1.44, 1.216 (6) *1. 210(3)

0(8) , H(5), 0(8') 167, 180' * 180°

Figures in parentheses are standard deviations in the last digit (s) and should not

be taken as ir-plying the mean bond distance is kno-<vTi to this precision.

0 - H dist5r.ce of 1.000 4. p - o - 11 angle of il-". = aspu-ed.

* first froT. calculstod hydrogen por ition, as described in text, second value froTi x results

• Riding r.odel correction (Busing and Le',-y, 1964).

* These 0...0 distances involve Hydrogen bonds.
-P

o

o

(U —

o

J-4 Q) gO

H >i E-i ^1O

Eh o

5-1

5-1 o

4JO

^1 -P

« 0) m ^

5-1

4-Jc

rH O ^ 5-1

0

iH

(Uo

rd U

CD .C

-P

o

co

•H

fd

5-1 -PW

:3

e

e

CO

0) 4-) rOC

•H

5-) 0

O

U

<U

-P

C

(DW

0)

o

-p

<4-l o

u

<u p

c

<DO^

P

5^

rO

Q)C

ui

0)w

10

n3ftC

•Ha - u

-p

u

-H

ftO !^ U

inO

CDU

CD

CO

U CD H e

ft 13 ^3 O O

Iji -P o c

H

CDu

4-> •HC

:3

CD

-P

CD M-lo

CD

(D

^ o

CDCPo -P

0)

•H
.0100-

.0300-

Orf .062$ .1350 .1175

Figure 2 - The variation of the mean-square amplitudes of

hydrogen atoms H(4) and H(5) in Ca(H2P04)2 with

displacement from the centers of symmetry.
40

20-

100

3500 3000 —T—2500 —T—2 000 1500

I 1300

400

FREQUENCY ^Ow'')

Figure 3

A - Infrared spectrum of Ca(H2P04)2 in the region 1300 to

4000 cm~l taken from a halocarbon oil mull smeared

between KBr plates. Particle sizes under 10 [jm.

B - Spectrum in the 400 to 1300 cm"-^ region taken from a Nujol mull. Spectra were run on a grating spectrophotometer using the standard slit program and a scan

speed of 33 cm~^/min. Frequencies were checked against

a standard 0.05 mm polystyrene film.
Figure 4 - A comparison of the zero point energy, E^, and

the central barrier of two combined harmonic

potential wells for an extreme (dotted lines)

case and a more probable (solid lines) case for

hydrogens H(4) and H(5) in Ca(H~PO.)^.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

311.05-3110561 December 29, 1972

NBS REPORT

10 979

Progress Report

on CLINICAL EVALUATION OF A RADIOPAQUE COMPOSITE

RESTORATIVE MATERIAL AFTER THREE AND ONE HALF YEARS

Chandler, H. H.*, DDS; Bowen, R. L.**, DDS;

Paffenbarger, G. C.***, DDS; and Mullineaux, A. L.****

* Research Associate American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234 on leave of absence from the Ohio State University College of Dentistry, Columbus,

Ohio 43210.

** Associate Director American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234. *** Senior Research Associate American Dental Association Research Unit

at the National Bureau of Standards, Washington, D. C. 20234.

****• Research Associate American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234.

This investigation was supported in part by NIH Research Grants DE02742-0i thru -05

and DE02494-02 thru -06 to the American Dental Association from the National

Institute of Dental Research and is part of the dental research program conducted by

the National Bureau of Standards in cooperation with the American Dental Association;

the Dental Research Division of the United States Army Medical Research and Development Command; the Dental Sciences Division of the School of Aerospace Medicine,

USAF; the National Institute of Dental Research; and the Veterans Administration.

NATIONAL BUREAU OF STANDARDS

re^toratiy^ m^jte ^J^^^,g^.g^,^_^ P^ig

half years

Synopsis

A clinical comparison was made of an experimental radiopaque

composite restorative material and Addent®35. The radiopaque

experimental formulation contained a novel ternary eutectic

dimethacrylate as the resin binder and fused silica and a

novel BaFs -containing glass as the reinforcing fillers.

Forty-six pairs of Class III and V restorations were placed.

Results of the evaluations indicate that the experimental material compared favorably with the commercial

material over the course of the investigation. At 3.5 years,

a significantly greater number of the restorations made

of the experimental material were rated as being better,

compared to those of Addent®35.
- 2 -

The need for radiopaque dental materials has been discussed

previously"*^ ' ^ and is based primarily on the necessity for

accurate and prompt diagnosis using radiographic methods.

Examples are: the detection of secondary caries around dental

restorations and the localization of ingested or aspirated

denture base materials.

Physical properties of one such experimental composite

material developed at the National Bureau of Standards were

reported previously.^ That report indicated that the properties measured were within ranges that would make it suitable

for at least limited clinical evaluation as an esthetic anterior restorative material. Therefore, a clinical study

was initiated using a material essentially identical to that

reported except that the silica particles in the filler had

been spheroidized. This report gives the results of the

evaluation for the first 3.5 years. The results are compared

with those obtained using Addent®35* which, although no longer

being sold, was at the time of placement (1968-69) a commercially available material.

* 3M Co., St. Paul, Minnesota
- 3 -

Materials*

With the exception of benzoyl peroxide contained in both, the

ingredients of the two composite materials were completely

different. The composition of the liquids*'^ and powders®

used to make the experimental formulations is given in Table 1.

Seventy-nine parts by weight of the reinforcing filler and 21

parts by weight of monomer were mixed on a glass slab for one

minute prior to insertion.

As with most of the commercial dental composite materials,

Addent®35 had a resin matrix of the BIS-GMA type"^'^ {i.e.,

based on 2,2-bis[4(2-hydroxy-3-methacryloxy propoxy) phenyl]

propane |r. it contained an inorganic filler of spheres of

soda-lime glass, short rods of Type E ^ss and a finely

divided lithium aluminosilicate . It was comparatively radiolucent.

To facilitate color change evaluations, only one shade

of each material was used.

All of the cavity preparations to receive the experimental

composite were treated with the NPG-GMA solution, the ingredients of which are listed under "Coupling Agent Formulation"

in Table 1.

* Certain commercial materials and equipment are identified

in this paper to specify the experimental procedure . In

no instance does such identification imply recommendation

or endorsement by the National Bureau of Standards or that

the material or equipment identified is necessarily the

best available for the purpose.
- 4 -

The solution is not a pulp protecting cavity liner in the

traditional sense but could be partially effective in this

respect if it successfully seals the interface between the

restoration and tooth. The results of a clinical test of

cavity preparations for Addent 35 were treated with the

liner supplied by the manufacturer who described it as a

synthetic vinyl copolymer designed to minimize pulp response.

The liner was removed from the cavo-surface margin as dire cted

by the manufacturer.

Clinical methods

Forty-six pairs (one of each pair, selected at random, being

the experimental material and the other, Addent®35) of Class

III and V restorations were placed in adult patients selected

on their probable availability for recall for as long as five

years. In order of/preference , the restorations in each pair

were matched using approximating surfaces on adjacent teeth,

the same surfaces on contralateral teeth, the same surfaces on

different teeth in the same quadrant or different surfaces

in the same quadrant.

this material are reported elsewhere.^ The

decre
- 5 -

An oral prophylaxis was performed on the teeth in

areas to receive the restorations. A rubber dam was always

used and cavity preparation procedures followed conventional

techniques. Where cavity depth was judged to be greater than

1 mm in dentin, a commercially available calcium hydroxide

liner or base* was applied. After washing with a stream of

tap water, the preparations for the experimental material

were treated with the solution of NPG-GMA carried on a cotton

pledget. The excess was immediately removed with a dry

pledget. Methods of insertion and finishing have been described earlier.

Evaluation methods

The restorations were evaluated by three dentists^n the

basis of sensitivity to percussion, general sensitivity or

pain, gross fracture, restoration loose or missing, secondary

caries, the need for replacement, color, surface texture,

marginal notch, marginal stain, and degree of marginal

flushness. Finally, the restorations of each pair were

evaluated to determine if one was better, the same or worse

than the other in its overall condition.

(three o f the author s

)

* Hydrex - Kerr Manufacturing Co., Romulus, Michigan 48174
- 6 -

Sensitivity was determined by questioning each patient

as to his experiences. Teeth were percussed with a mirror

handle by at least one of the evaluators at each examination,

scores were given from "0" for no sensitivity up to "10" for

severe sensitivity.

Gross fracture (as opposed to minute marginal fracture)

,

loose or missing restorations, and secondary caries were

recorded as was the need for replacement. The latter

evaluation was based solely on the evaluator's clinical

judgment.

Surface texture evaluations were made by comparing

the texture of the restoration with the texture of a series

of glass blocks. Figure 1. The blocks were of increasing

roughness and were designated "0" - polished glass, "2" -

glass ground with aluminum oxide of 20 Mm particle size,

"4" _ ground with silicon carbide, 50 |jm, "6" - ground with

silicon carbide, 100 Mm, and "8" - ground with silicon

carbide, 250 Mm. If the surface roughness of the restoration was greater than the block designated as "8," a

score up to "10" could be given. Comparisons were made by al

ternately "feeling" the restoration and the various blocks

with a dental explorer until the evaluator perceived similar
roughness. Intermediate scores could be given if the evalua

tor perceived roughness between those in the series of block

Surface staining evaluations were given from "0" for

no visible stain up to "10" for severe stain. These

evaluations were made on the facial surface where possible

and were recorded only when the stain was greater than that

on the surrounding tooth enamel. If the facial surface was

not visible, the evaluation was made on the lingual surface.

Color evaluations were made by comparing the restorations with a shade guide containing ten anterior porcelain

tooth crowns of increasing darkness numbered from "0" to '9,

Figure 2. If darkness greater than "9" was perceived, a

score of "10" could be given. Evaluations were made on tie

facial surface where possible.

Notch, stain, and flushness evaluations were made on

labial, lingual, gingival and incisal margins of Class III

restorations and mesial, distal, gingival and incisal or

occlusal margins on Class V restorations.

Notch evaluations were made by tactile senss tion with

a dental explorer. Scores could range from "0" for no notch
- 8 -

up to "10" for a notch estimated to be 1 mm wide. Although

not used in this investigation, a notch guide was

during the studydeveloped/with which the operator, using a dental explorer,

could make direct ccanparisons of the notch existing at the

p7" shaped

cavo-surface margin with that of the/^ri angu1ar) notch in the

guide block. Figure 3.

Marginal stain evaluations were made similar to those

of surface staining in that each evaluator could give

scores of "0" for no stain up to "10" for severe stain.

Marginal flushness scores were determined by ccsnparing

the lack of flushness with a step block. Figure 4. A score

of "0" was given if the restoration was flush with the tooth

and up to ^10" for a lack of flushness similar to that of

the 1 mm step on the guide block. If the tooth was highei

than the restoration, the score was preceded by the letter

"U" (up) and i£ lower, the letter "D" (down)

Finally, the better, same or worse evaluations were

based solely on clinical judgment with no limiting criteria

but were usually made on differences in margins, stain, color

or flushness.
subtle

- 9 - Because of a/difference in color between the experimental and commercial materials it was possible for the

evaluators to distinguish theitYduring the evaluation procedures. Therefore, the study was not entirely blind.

The first evaluation was made as soon as possible

after the restorations were finished. Additional evaluations were made at approximately 1, 2.5 and 3,5 years.

Results and discussion

The patients available for recall at 3.5 years had 36 pairs

of restorations or about 78% of the original sample. Eight

pairs in two patients were lost due to crown and bridge

placement and two pairs due to two patients' unavailability.

This low sample mortality (the twenty patients were workers

at the National Bureau of Standards) is very desirable as

it reduces the need for a large initial sample.

M General evaluations ; Evaluations for sensitivity, secondary

caries, and the need for replacement are listed in Table 2.

m some cases
- 10 -

None of the restorations fractured, became loose or were

missing.

Only three teeth exhibited any kind of sensitivity

at any time during the investigation. In two cases the

sensitivity had disappeared at the time of the next recall.

The other instance was noted at the 3,5 year recall. Because of the insensitive nature of the evaluation method,

no conclusion should be made concerning the overall clinical

pulpal response to these materials. One report indicates

that the pulpal response to developmental formulations of

Addent®35 was more than to silicate cement with a zinc

phosphate cement base but less than to cold curing plastic

materials with commercially available liners; the use of

the liner supplied with the material did not eliminate the

response '

;

Pulpal response to the experimental formulation used in

this study has not been determined. However, acute oral

toxicity of the monomer was evaluated by gastric intubation
- 11 -

Of 1.0, 3.16, 10.0, and 31.6 mlAg of body weight to four

pairs (each dosage to one pair) of albino rats. The 31.6

ml/kg dose was fatal to one rat. Thus, the formulation

is considered relatively harmless by this route.

Acute irritation to rabbit eyes was determined by

applying 0.1 ml of the monomer to one eye of each of three

albino rabbits. No irritation was produced in one eye and

slight irritation in two eyes. The irritation disappeared

by 72 hours. Thus, the monomer was considered to be essentially non- irritating to rabbit eyes.

There were 11 evaluations noting caries associated

with four different patients and eight different restorations. Five of the restorations were of Addent® 35 and

three were of the experimental formulation. There was only

one instance where all three evaluators noted caries involved with the same restoration. In the remainder of

cases only one evaluator noted caries at any specified

time. The differences among evaluators with respect to

caries incidence probably occurred because the caries involvement was in a very incipient stage and was difficult

to distinguish from marginal breakdown and stain.
- 12 -

Of the 72 restorations available for examination at

3,5 years, two (one of each material) were in need of replacement because of secondary caries.

^ Color and surface evaluations ; The results of the color,

surface texture and surface stain evaluations, given in

Table 3, are the average given by each investigator for

the specified material and time.

Color - All three evaluators noted a shift toward

darker colors with the passage of time but not all to the

same degree. Evaluator 1 noted less change than evaluators 2 or 3 . The most change occurred during the first

year, with less change per year noted at subsequent recalls. The darkening of Addent®35 was significantly greater

than was the discoloration of the experimental material

(p<0.02).^^

Both materials were subjected to the color stability

test as outlined in American Dental Association Specification No. 12 for Denture Base Polymers ^ There was a

slight discoloration of Addent®3 5 but no perceptible darkening of the experimental material from exposure to the sun
- 13 -

lamp. Thus, there is correlation to the extent that the

material that darkened more under the sun lamp also darkened more in clinical use.

The experimental material discolored somewhat clinically but not under the sun lamp. This may or may not be

related to color formation that appears to occur upon

combining the phthalate monomers and the amine accelerators

this slight coloration is suspected of being related to

charge transfer complex formation and is currently being

investigated.

Sujr^ace^ textu^ - The data in Table 3 indicate that

evaluators 1 and 2 perceived little difference in the

change in surface roughness of the experimental and commercial materials with the passage of time. Evaluator 3 detected considerable change toward roughening.

If a direct comparison is made of the surface texture

( evaluations between the two materials for each evaluator

and for each time interval, it may be seen that the experimental formulation was rated smoother than Addent^'35

in 10 out of 12 instances. This would not occur unless
- 14 -

there was a significant difference in smoothness

(p <0.02) The clinical significance of this smoothness

is not known but did not result in appreciably less surface

stain (Table 3) . However, smoothness in all dental restorations is desirable.

Sjjr^ice^^taj.nj^ - During this study there was no

apparent trend in the average evaluations of surface staining (Table 3) . In many instances there was more stain

detected earlier in the investigation than at later recalls

and all evaluators reported similar amounts. There were

22 evaluations greater than "0" on the test material and

26 on Addent®35. Each evaluator reported staining on

seven patients, five of whom were smokers.

In some patients it was difficult to differentiate

surface staining and discoloration of the material itself.

Several patients (most of whom were smokers) presented no

stain on the labial surface but did have stain on the lingual

surface. Evidently, the presumed better brushing that occurred

on labial surfaces was effective in removing the stain.
- 15 -

Also, more stain is deposited on lindual surfaces of

smokers than on labial surfaces.

Marginal evaluations ; Table 4 lists results of evaluations

for marginal notching, staining and flushness. Evaluations

on incisal and gingival margins of Class III restorations

were not used because their accuracy was in doubt due to

poor visibility beneath the gingival margin and because

the exact area to be evaluated on the incisal was difficult

to determine. Notch values for evaluator 3 were not used

because of a marked change in his procedure midway in the

investigation. This was due to the development and his

use of the marginal notch evaluation guide (Fig. 3) in a

separate, concurrent clinical study of other materials.

Marg;inal notch ; There was no significant difference

between the two materials with respect to the degree of

marginal notching at any given time. Neither was there a

noticeable trend of increased notching with time. In

other words, there was as much notch at the beginning or 0

year examination as there was at 3.5 years. Margins exhibiting some degree of notching averaged about 9% of the total
- 16 -

sample. The materials were about equal in this respect on

the labial margin but on the lingual margins there were

three times as many Addent^35 restorations exhibiting notch

as compared to the experimental formulation.

Marginal Stain - All evaluators noted an increase in

marginal staining with tine. The amount of change was

(R) somewhat greater with Addent^35 than with the experimental

formulation (p<0.05) .

"""^ At 3.5 years, some degree of margin

stain was noted in 45 out of 175 restorations of the test

formulation compared to 74 of 175 of Addent®35. Statistical examination reveals this difference to be significant

(p<0.01) .^^

Several instances of marginal stain were noted at the

first evaluation. This can probably be attributed to retained stained enamel since there was not yet time for the

deposition of stained material at the interface. There was

considerable difference among the investigators as to the

incidence and degree of staining. This may be in part due

to the arbitrary nature of this evaluation and the lack of

a guide to which constant referral could be made by all

three evaluators.
- 17 -

Marginal flushness - Flushness evaluations indicated

there was little difference between materials and little

change with time. In most cases the tooth margin was

slightly higher than the restoration even at the initial

examination indicating that the restorations were overfinished.

Better^ same and worse evaluation : Table 5 lists the

percentage of experimental and Addent®35 restorations that

were rated better than their matching restorations at each

time period and by each evaluator. Evaluator 1 rated

about equal percentages of both materials better at 0 and

3,5 years. Evaluator 2 rated equal numbers better at 0

years but between two and three times as many of the test

formulation better after 3.5 years. Evaluator 3 rated more

(r) Addent^35 restorations better at 0 years but at 3.5 years

rated twice as many of the test material restorations as

better.

All three evaluators recorded a decrease in the

percentage of restorations rated the same with the passage

of time. Evaluator 1 rated 85% the same at 0 years, evaluator 2, 82% and evaluator 3, 57%. These percentages had
- 18 -

changed to 53, 31 and 25, respectively after 3.5 years.

Pooled results of all investigators at 3.5 years showed that

there were 45 notations rating the experimental restorations "better"

as compared to 24 for the Addent^35. This difference is significant at a 99% confidence level, '^'^

Summary and conclusions

Forty-six pairs of Class III and V composite restorations were

placed in adult patients. One of each pair was restored with

an experimental composite material consisting of a resin binder

of a ternary eutectic dime thacrylate and a reinforcing filler

of two parts fused silica and one part radiopaque BaFs-containing glass. The other of each pair was restored with Addent^

35 a commercially available material at the time of placement

(1968-69)

The cavity preparations for the experimental material

were treated with N- (2-hydroxy-3-methacryloxypropyl) -Nphenylglycine an adhesion promoting coupling agent. The

preparations for Addent®35 were treated with the liner

supplied by the manufacturer.

The restorations were evaluated by three dentists at

0, 1, 2.5 and 3.5 years on the basis of sensitivity, gross

fracture, restoration loose or missing, secondary caries,

need for replacement, color, surface texture, surface stain.
- 19 -

marginal notch, marginal stain, marginal flushness and

whether it was better, the same or worse than its mate

in the pair.

Evaluations for color, texture and flushness were

made with the aid of newly developed evaluation guides.

Results of the evaluations indicated there was essentially no difference in the two materials with respect to

general sensitivity, sensitivity to percussion, gross

fracture, loose or missing restorations, secondary caries,

need for replacement, surface staining and marginal flushnes

There was a slight difference in the restorations in

favor of the experimental material with respect to color

change, surface texture, marginal notch and marginal stain.

A significantly greater number of the restorations made

of the experimental material were rated as being better

compared to those of Addent®3 5.

Restorations made from the material were still clinical

sound in a high percentage of cases after 3.5 years and

compared favorably with a commercial material* However,

further improvements in color stability are needed.
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-23-

Table 4 Margin evaluations

erty

rial

uator Labial (incisal on Class V s) Lingual (distal on Class V s)

a

o +j

H

(d years year s u

s. >w - 0 1 1.5 3.5 0 1 1.5 3.5

to -P

(!) g

rH

1 0.5*

(1.9) t 0.4

(1.7)

0.15

(0.67)

0.22

(0.63)

0.00

(0.00)

0.04

(0.19)

0.00

(0.00)

0.06

(0.22)

C

<ua

X

w

2

0.4

(1.8)

0.6

(1.8)

0.6

(1.9)

0.2

(1.1)

0.06

(0.24)

0.07

(0.38)

0.06

(0.25)

0.00

(0.00)

Notchj in

ro @+J 1 0.3 0.26

\ J- m J'O ) 0.18

(0 PI ^ 0.14

(KJ . / ±)

0.25

(0.84)

0.00

(0.00)

0.10

(0.24)

0.14

(0.45)

(U <

2

0.3 0.4

[± .D)

0.3

{± .yj)

0.35 0.18

(0.53)

0.07

(0.27)

0.17

(0.49)

0,20

(0.65)

P

1

0.00

(0.00)

0 .06

(0.34)

0.15

(0.80)

0.12

(0.55)

0.00

(0.00)

0.11

(0.42)

0.26

(0.73)

0.4

(1.6)

c

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0.03

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0.8

( 1 9^

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CL. w

3

0 19

(0.32)

U . 44

(0.96)

U . 3o

(0.67)

0 .41

(0.78)

0.31 0.9

(1 .4j

0.40

U . D / j 0.8

(1.3)

1 nu . nnUU

(0.00)

nU . nnUU

(0.00)

Pi "3 1 U . o±

(0.86)

U . b

(1.4)

0.00 0.00

(\ur\ r\n\ . yjij

)

0.37

I n QA ^

V U

.

j 1-0

(^ . 4;

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2)-P 2

U . Id

(0.53)

0 • / (1.3)

1 • 0

(1.2)

1.5

(2.0)

0.06

f ri A \ (U . z4;

0.6 0.42

(U . /i j 0.22(U . bU

j

(D <

3

0.28

(0.73)

0.9

(1.5)

0.8

(1.0)

1.1

(1.3)

0.37

(0.71)

0.7

(2.0)

0.7

(1.0)

1.2

(1.4)

fd

4-> 1

-0.03

(0.11)

0.00

(0.36)

0.09

(0.25)

-0.02

(0.23)

-0.06

(0.39)

-0.02

(0.49)

0.00

(0.23)

-0.18

(0.32)

1 0) •H 2

0.11

(0.33)

0.06

(0.26)

0.12

(0.29)

0.17

(0.35)

0 .09

(0.52)

0 . 13

(0.55)

0.23

(0.34)

0.28

(0.52)

less

(!) X

W

3

0.05

(0.28)

0.10

(0.33)

0.03

(0.13)

0.05

(0.18)

0.12

(0.31)

0.22

(0.38)

0.13

(0.16)

0.14

(0.24)

lushr in 1

-0.012

(0.077)

-0.03

(0.17)

0.06

(0.23)

-0.14

(0.71)

0.03

(0.12)

-0.04

(0.19)

0 .08

(0.52)

-0.18

(0.32)

©

4-> C

<D 2

0.12

(0.77)

0.03

(0.12)

0.06

(0.17)

0.21

(0.31)

0.20

(0.30)

0.19

(0.37)

0.33

(0.37)

0.50

(0.64)

-T! <

3

-0.04

(0.20)

0.06

(0.29)

0.01

(0.17)

0.08

(0.16)

0.16

(0.30)

0.27

(0.49)

0.15

(0.19)

0.19

(0.1^)
- 2 3a -

Footnotes to Table 4:

* Each entry represents the average score given by each

investigator for the indicated time and property.

Higher numbers indicate greater notching, staining

and lack of flushness. For flushness, negative

nijmbers indicate that the restorations were higher

than the tooth and positive numbers indicate the

restorations were lower than the tooth.

The number of evaluations ranges from 43 ± 3 at 0 years

to 33 ± 1 at 3.5 years for the labial margin and 32.5

±0.5 at 0 years to 25 at 3.5 years for the lingual

margin.

+ Standard deviation.
in in CN o o in • 0^ CNJ CN in rH in CN c0 .-^ •H M in cn o rH

4-1 >H

H CN

m CN

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(1) • rH CM ro <N g

<0

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H

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ft

+J <: < X < (0 H mm

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4-) M

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Q) ,Q

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4-J fd C H

(U

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fd

fd •H

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- 25 -

Bibliography

1. Chandler, H. H.; Bowen, R. L., Paffenbarger , G. C,

and Mullineaux, A, L. Clinical investigation of a radiopaque composite restorative material. JADA 81:935

Oct 1970.

2. Chandler, H. H.? Bowen, R. L., and Paf fenbarger , G. C.

The need for radiopaque denture base materials: A

review of the literature. J Biomed Materials Res 5^:245

July 1971.

3. Barton, J. A., Jr.; Burns, C. L,; Chandler, H. H., and

Bowen, R. L. An experimental radiopaque

composite material: An in vitro study. [Manuscript

submitted to J Dent Res for publication]

4. Bowen, R. L. Crystalline dime thacrylate monomers.

J dent Res 49:810 July-Aug 1970.

5. Bowen, R. L., and Argentar, H. Diminishing discoloration

in methacrylate accelerator systems. JADA 75^:918 Oct 1967.

6. Bowen, R. L., and Cleek, G. W. X-ray-opaque reinforcing

fillers for composite materials. J dent Res 48:79

Jan-Feb 1969.

7. Bowen, R. L. Properties of a silica-reinforced polymer

for dental restorations. JADA £6:57 Jan 1963.

8. Bowen, R. L. Dental filling material comprising vinyl

silane treated fused silica and a binder consisting of

the reaction product of bisphenol and glycidyl acrylate.

U. S. Patent 3,066,112 Nov 1962.
- 26 -

9. Chandler, H. H.; Bowen, R. L.; Paffenbarger , G. C,

and Mullineaux, A. L. Clinical evaluation of a toothrestoration coupling agent, [Manuscript submitted to JADA

for publication.]

10. Advertising brochure - 3M Co.

11. Langeland, Leena K. ; Guttuso, James; Jerome, David R.,

and Langeland, Kaare . Histological and clinical comparison of Addent with silicate cements and cold-curing

materials. JADA 72.-373 Feb 1966.

12. Siegel, S. Nonparametric statistics for the behavioral

sciences. New York, McGraw-Hill Book Co., Inc., 1956,

pp 36-42, 250.

13. Guide to dental materials and devices, 1972-1973, ed 6.

American Dental Association Specification No. 12 for

Denture Base Polymer. Chicago, American Dental Association, 1972, p 201.

14. Hoel, P. G. Introduction to mathematical statistics.

New York, John Wiley & Sons, Inc., 1962, pp 148-151.

15. Hoel, P. G. Introduction to mathematical statistics.

New York, John Wiley & Sons, Inc., 1962, pp 104-105.
&1 •H

tn

ro

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U

C

CO

u

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0) o

rH O Xi

rH

0) -P

to

CD +J

CO C

fd 0

inO

too

g M

o

C -P H (0

^3 -P

O

-P

rO

13 iH>

tP <1)

n3 +J

0) P

O

u

CO

CO (U 0) U fi m ^ ip tn

S-) S3 <u

:3 o ^

W S-) -P

I

01

toc

•H

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o

u

o

-H

+J

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-P

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(U O

-P Vo

<-\ o

rH O

Q) M-l O

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0) C

-P

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171

Cm

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

311.05-3110561 December 29, 1972

Progress Report

NBS REPORT

10 980

on CLINICAL EVALUATION OF A TOOTH-RESTORATION

COUPLING AGENT

Chandler, H. H.*, DDS; Bowen, R. L.**, DDS;

Paffenbarger, G. C.***, DDS; and Mullineaux, A. L.****

* Research Associate American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234 on leave of absence from the Ohio State University College of Dentistry, Columbus,

Ohio 43210.

** Associate Director American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234. *** Senior Research Associate American Dental Association Research Unit

at the National Bureau of Standards, Washington, D. C. 20234.

**** Research Associate American Dental Association Research Unit at the

National Bureau of Standards, Washington, D. C. 20234.

This investigation was supported in part by Research Grants DE02742-01 thru -05

and DE02494-C2 thru -06 to the American Dental Association from the National

Institute of Dental Research and is part of the dental research program conducted by

the National Bureau of Standards in cooperation with the American Dental Association;

the Dental Research Division of the United States Army Medical Research and Development Command; the Dental Sciences Division of the School of Aerospace Medicine,

USAF; the National Institute of Dental Research; and the Veterans Administration.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
4<
Clinical evaluation of a tooth-restoration

SYNOPSIS

The clinical effectiveness of a tooth-restoration coupling

agent (NPG-GMA) was tested by treating 54 Class III and V

cavity preparations with an acetone solution of this surfaceactive comonomer. Fifty- four additional preparations were

treated with acetone alone for comparison. The teeth

were restored with an experimental composite material.

The restorations were observed by three dentists for

3.5 years. The evaluations indicated that the restorations placed over the NPG-GMA coupling agent had signiand

ficantly better margins, /a significantly higher niomber of

the NPG-GMA associated restorations were rated better as

compared to those placed over the acetone control.
- 2 -

One of the most important advancements in restorative dentistry

would be the development of methods and materials that could

be used to restore teeth in such a way that there would be no

leakage around the restoration. Prevention of leakage could

be achieved if the filling material bonded directly to the

tooth structure or if there was an intermediary substance

that would bond to both the tooth and the filling material.

Such bonding must be sufficiently strong to overcome the stresses

that occur at the tooth -res toration interface.

Marginal leakage has been demonstrated in vitro by

penetration of the tooth-restoration interface with radioisotopes, dyes and air, by artificial caries development

and by percolation brought about by thermal cycling in water.''"

In vivo , marginal leakage may be at least partially responsible for pulp pathology, a loss of marginal integrity,

corrosion of metallic restorations, tooth staining

and secondary caries.^

The purpose of this investigation was to eo^aluate

clinically the effectiveness of a tooth-restoration coupling

agent used in combination with an experimental, composite

resin filling material.
- 3

Materials*

The composite restorative material was a crosslinked methacrylate

polymer as the resin binder and a combination of a radiopaque,

BaFg-containing glass and spheroidized fused silica as the inorganic reinforcing filler. A more complete description of

the powder and liquid formulations used to form the composite

is reported elsewhere.^

The experimental coupling agent was [N- (2 -hydroxy- 3-

methacryloxypropyl) -N-phenylglycine] (NPG-GMA) recrystallized

from acetone; this has been referred to as a "surface-active

comonomer."^ It was applied as a 5.0% solution using acetone

as the volatile carrier. The solution also contained 0.14%

hydroquinone monomethylether"*" and 0.12% di-_t-butyl sulfide as

stabilizers

.

* Certain commercial materials and equipment are identified

in this paper to specify the experimental procedure . In

no instance does such identification imply recommendation

or endorsement by the National Bureau of Standards or that

the material or equipment identified is necessarily the best

available for the purpose.

t Lee Pharmaceuticals, South El Monte, Calif.; (custom synthesis)

.

* Eastman Kodak Co., New York, N. Y.

Sk & K Laboratories, Inc., Plainview, N. J.
- 4 -

Methods

Fifty-four matched pairs of restorations were placed by one

operator in 26 adult patients. Methods for selecting the

patients and pairs and the clinical procedures were reported.^'

After cleaning the preparations with a stream of tap

water, they were dried with compressed air. At random, one

of each pair was treated with the NPG-GMA solution carried

to the cavity preparation on a cotton pledget. The other

was treated in an identical manner except that acetone alone

was used for comparison and control. Neither the operator

nor the other two evaluators knew which preparations received the NPG-GMA solution.

With methods described elsewhere,^ the restorations

were evaluated by three dentists on the basis of general

sensitivity, secondary caries, marginal notch and marginal

stain. In addition, each evaluator, using any criteria he

thought appropriate, determined if one of the restorations

in each pair was better than the other or if they were the

same in their overall condition. The evaluations
- 5 -

were carried out as soon as possible after the restorations had been finished (at least 24 hours after insertion)

and at about 1, 2.5 and 3.5 years.

Results and discussion

Of the original 26 patients, 23 were available for recall

at 3.5 years. This resulted in the loss of seven pairs of

restorations or about 13% of the original sample.

a Sensitivity ; The sensitivity and caries evaluations

are listed in Table 1.

There was no significant difference in the sensitivity

produced in the teeth that received the NPG-GMA coupling

agent and those that received the acetone control. The

sensitivity was usually in response to cold but sometimes

to heat or sweetness.

There were several instances where the tooth exhibiting

sensitivity contained two restorations, one of which had

been placed over the NPG-GMA solution and the other over

the acetone control. Therefore, it is not known which

material, if either, might have been the cause. With

one exception, the sensitivity noted at the original examination had disappeared at the first yearly recall.
- 6 -

The cause of the apparently high incidence of sensitivity

is not known, but over half (24) of the evaluations noting

sensitivity occurred on one patient who had numerous Class V

restorations and who reported that his anterior teeth were

chronically sensitive to cold.

It is doubtful that an effective tooth- res toration

coupling agent would appreciably reduce the type of postoperative sensitivity noted in this investigation. Such

sensitivity commonly occurs and is generally attributed to

traiima from cavity preparation or initial pulpal response

to the materials used. It is more likely that a toothrestoration coupling agent would help to prevent

pulpal response that occurs after a period of

time due to long-term leakage. This type of response

was not noted in this investigation either with teeth that

had received the NPG-GMA or the acetone control.

M Caries : There were 12 notations of secondary caries, 9 of

which occurred in association with the coupling agent and 3

of these

with the acetone control (Table 1) . Ten/notations were
associated with one patient who exhibited a high degree of

marginal notch and marginal stain that was difficult to

distinguish from caries. This may account for the poor

agreement among the evaluators with respect to caries incidence. For example, one investigator noted no caries at

any time, while another noted eight instances of caries

at 2.5 years but none at 3.5 years with the same restorations in the same two patients. This indicates that the

criteria for defining secondary caries need improvement.

< Marginal notch ; Marginal notch and stain evaluations

given in Table 2 show the average value of

at each

each evaluator for each material an^/time^ Evaluations

are listed for the labial and lingual margins of the

Class III restorations and incisal and distal of the

Class Vs. The notch values of evaluator 3 were not used

because of a marked change in his procedure midway in the

investigation; this was due to the development and his use

of a marginal notch evaluation guide ^ in a separate, concurrent, clinical study of other materials.

There was no apparent time trend with

the degree of marginal notching. If one compares the average
- 8 -

notch evaluation for the NPG-GMA and acetone treatment for

each evaluator, surface and time^ the average notching for

those restorations placed over the control treatment is

greater than that of the NPG-GMA treatment in 11 out of 15

instances. This difference is significant only at about a

90% confidence level.

There were 57 evaluations noting some degree of

marginal notch with those restorations associated with

the NPG-GMA solution and 97 with those restorations

receiving the acetone control. There were a total of

512 evaluations in each group. To determine if these

proportions were significantly different, a normal

approximation to the binomial distribution was made."^

Results indicated that there was a significantly lower

incidence of marginal notching with those restorations

placed in the NPG-GMA treated teeth (at a level of confidence greater than 99%)

,

Marginal stain : All evaluator s noted a general increase in marginal staining with the passage of time.

More stain developed at the lingual than at the labial

margins (Table 2)
_ 9 -

If a direct comparison is made of the amount of increased

staining between 0 and 3.5 years for the NPG-GMA test and

acetone control treatments of the labial and lingual margins

recorded by each evaluator, there was less change associated

with the NPG-GMA treatment in 5 out of 6 instances (Table 2).

This would probably occur less than 11 times out of 100 if

there were no real difference.^

There were 212 evaluations noting marginal stain on the

teeth that received the NPG-GMA solution and 237 in teeth that

received the acetone control out of a total of 914 evaluations

in each group. Statistical evaluation of these results indicated the difference was significant only at about an 80%

confidence level.

a Better, same^ worse ; Table 3 lists the percentages of

restorations pre treated with the NPG-GMA and acetone solutions that were rated better than their matching restorations

for each time period and by each evaluator.

All evaluators rated a higher percentage of the

restorations placed over the NPG-GMA solution as better

than those placed over the acetone control at all
- 10 -

time periods with one exception (evaluator 1 at 2.5 years).

This would indicate a significant difference in the materials

at a greater than 99% confidence level.

Although the restorations and teeth were

evaluated separately for sensitivity,

secondary caries, and marginal notch and stain, there were

additional criteria used for the better, same or worse

evaluation. In fact no limitation was placed on the

criteria that could be used in this rating. Therefore,

many of the "better" evaluations were based on such properties

as surface stain, color, contour, roughness or the presence

of voids. Presumably, such properties could not be influenced by

the presence or absence of the NPG-GMA coupling agent.

Therefore, the reasons for the "better" evaluations at 3.5

years were compiled to determine if the differences between

test and control restorations were caused by differences in

marginal integrity or by other factors, of 132 evaluations

where reasons were noted, the restorations associated with

the NPG-GMA were rated better 60 times. Of the 60, 45 were

based on differences in margins and 15 on other factors.

Thirty-four pairs were rated "same."
- 11 -

The restorations associated with the control treatment

were rated "better" 38 times. Of the 38, 24 were on the

basis of margins and 14 on other factors. Therefore,

the differences in the better, same or worse evaluations

were caused predominantly by differences in marginal

integrity and were significant at a 99% confidence level.®

Significant improvements were reported in bonding resinbased materials to enamel and dentin with NPG-GMA in laboratory

tests.* However, the efficacy of improved adhesive bonding

by this means under clinical conditions had not been studied

previously. Certain differences in test conditions are apparent

1) the laboratory tests were done on flat tooth surfaces under

conditions that minimized tensile stresses induced by polymerization shrinkage. During hardening of the restorative material

in clinical preparations, significant tensile stresses should

develop.'^ 2) In the laboratory tests, the temperature was

approximately constant, whereas under oral conditions it fluctuates widely.^ 3) In the adhesion tests, the stress to break

the bond was applied only once; in the mouth repeated
- 12 -

mechanical or thermal^° stressing may result in fatigue of the

tooth-restoration bond.

In view of these and other challenging conditions of

the oral environment, it is worthy of note that significant

even though modest improvement was demonstrable clinically

with the use of the NPG-GMA coupling agent.

Observations on replacement of restorations ; During this

and a similar investigation, the operator (H.H.C.) replaced

over 125 defective silicate restorations, and a number of

unreinforced direct resin fillings. After the outer surfaces

of the silicates had been removed, there was seldom any

clinical evidence of leakage. If the restoration was not

loose, it appeared to be in very close approximation to

the cavity wall, and there was no odor of decay. It was

usually necessary to remove the last vestiges of the filling

material with a bur or by prying it loose with a spoon

excavator. This last material was not stained and the

underlying tooth appeared dry and hard.

When the unreinforced direct resin fillings were removed,

it was not uncommon to observe clinically detectable moisture

beneath the restorations and occasionally an odor of decay

and a softening of the dentinal wall.
- 13 -

Although there is some shrinkage during hardening, ^

the thermal expansion-^ ^ and stiffness""" ° of silicate cement

nearly match those of the tooth; this may account for the

preceding observations and the low incidence of secondary

decay associated with this material.

In the authors' opinions bond strength and permanence

would be markedly improved if shrinkage on hardening of the

restoration could be eliminated and if its coefficient of

thermal expansion and stiffness properties could be made

to match those of the dentin or enamel.

Summary and conclusions

Fifty-four pairs of Class III and V restorations were placed

in adult patients using an experimental radiopaque composite

filling material. One of each pair of the cavity preparations

was treated with an experimental tooth-restoration coupling

agent consisting of a 5.0% solution of [N- (2 -hydroxy- 3 -me thacryloxypropyl) -N-phenylglycine] (NPG-GMA) in acetone. The

other of each pair was treated with acetone alone as a control.
- 14 -

The restorations were evaluated by three dentists at

0, 1, 2.5 and 3,5 years on the basis of general sensitivity,

secondary caries, marginal notch and stain and whether one

of each pair was better, the same or worse in its overall

condition than its mate.

Results indicated little difference in the pretreatments

with respect to general sensitivity. Lack of reproducibility

or agreement among the three evaluators in assessing the presence of secondary caries shows the need for better clinical

criteria for such determinations.

There was a significant difference in the incidence

of marginal notching in favor of the NPG-GMA pre treatment

,

however there was not a significant trend of increased

notching with time with either the pretreated or control

restorations.

The increase in marginal staining with those restorations placed over the NPG-GMA pre treatment ranked less than

the controls.

All evaluators indicated a significantly higher number

of those restorations placed over the NPG-GMA solution were

better as compared to those placed over the control liquid.

In addition, each evaluator rated a higher percentage of the
- 15 -

NPG-GMA associated restorations as being better at all

four time intervals, with one exception, at which they were

rated equal.

The results clinically demonstrate the effectiveness

of an adhesion promoting tooth-restoration coupling agent

through improved margins of composite restorations.
Table 1 B Sensitivity and caries evaluation

property 1 d. ±, \^ CL ulLlv^li la V d-L LlCL \^\J±. Years

0 1 2.5 3.5

1 6* 1 0 0

WPG—GMA <^z / 1 0 U

Sensitivity

aJ 8 1 0 U

9 0 0 0

TV /-^ -i— Ace uone Z D 0 0 U

•3 /-D 0 0 U

1 0 0 0 3

NPG-GMA 2 0 0 0 0

Caries

3 0 0 6t 0

1 0 0 0 1

Acetone 2 0 0 0 0

3 0 0 2t 0

* Each entry represents the nimber of teeth exhibiting

the indicated property.

t Since these same teeth were judged non-carious at 3.5

years (the restorations had not been replaced) , better

criteria to distinguish caries from marginal stain are

needed.
- 17 -

Table 2 tt Marginal evaluations

Property

Treatment Evaluator Labial (incisal on Class V's) Lingual (distal on Class V's)

Years Years

0 1 -2.5 3.5 0 1 2.5 ' 3.5

Notching

O

1 o

Pk

1 0.00*

(0.00)

t

0.24

(0.73

1

, 0.3

(1.5)

0.22

(0.57)

0.3

(1.9)

0.2

(1.1)

1 0.6 j 0.32

(1.6) j(0.84)

2 0.15

(0 66

)

0.4

(1 3)

0.04

(0 20)

0.14

(0 77)

0.21

(0 62

)

0.00

(0 00)

0.5 ' 0.27

(1.3) '(0.94)

0)0

-pu

1

0.6 0.5 0.24 0.35

( 0 n)

0.6

M 3

)

0.21

^ 0 62

)

0 24 i 0.18

(0.58) 1(0.66)

2

0.22

(0 92 ") 0.4

(1 3)

0.6

(1 7)

0.4

(1.1)

0.24

(0 74)

0.28

(0.70)

0.29 i 0.4

(0.83) ;(1.3)

1 o

1

0.04

t U . z /

;

0.17

V U . /Dj

0.21

I J- . u; 0.3 0.03

^ u . ly j 0.4 1.1

(2.4)

1.3

(2.4)

2

0.22

(0.57)

0.19

(0,51)

0.38

(0.68)

0.6

(1.2)

1 0.14

(0.35)

0.45

(0.98)

1 1.3 1 1.5

(1.7) |(2.5)

ling 3

0.28

(0.71)

0.32

(0.87)

0.60

(0.90)

0.40

(0.76)

0.7

(1.4)

1.2

(1.8)

2.6 i 1.6

(3.4) !(2.3)

Stall

Q)C

O

4J

<DU

<

1

0.00

(0.00)

0.07

(0.38)

0.8

(2.3)

0.5

(1.6)

0.17

(0.93)

0.6

(1,4)

0.9 ' 1.5

(1.9) i(3.1)

2

0.17

(0.42)

0.31

(0.90)

0.7

(1.2)

1.1

(1.7)

0.07

(0.26)

0.55

(0.91)

0.9

(1.6) ,

1.5

(2.4)

3

0.31

(0.61)

0.6

(1.1)

1.3

(2.0)

0.8

(1.7)

0.4

(1.0)

0.9

(1.5)

1.6

(2.2)

1.1

(2.1)

* Each entry represents the average score given by each investigator

for the indicated time and property. Higher numbers indicate greater

notching and stain.

The number of evaluations ranges from 54 at 0 years to 45 ± 2 at 3.5

years for the labial margins and 29 at 0 years to 22 at 3.5 years for

the lingual margins.

t Standard deviation.
in ! O OD * n 00 'si' <N LD oo

xU o te u —

Q) }-) (P * •r11 1 CO ID ro ro 'sT t7> •H u

+-' J-J • ID CM 03 -H

ni10 (0 P Ti

/l\ vU

C 5 C

0) •H

10

U

U K,

K> f—

O o> Oi !m (D (U ^ • rH CM fn (U fd 6 iH P. -P 5 tH

/It •H fd

IT' Q)

U

M •fs

O

O fO LT) ro CO +J

m CD rH iH CN W

et

w

Th

-P

A 0 0

'd •H • d)

es te at

g

(1) CD

u •H • to 0 i

>-i 0 -P

O o O o 0 0) -p 1 1 +J 1 -p -P 5-1 CO w 13 o Q) o o Q) fi 0) CO 0) 0) 0 CM u 0 >i 0 fd 4J iz; <: u •H CD -P -P

rti

C fd

(U S-l •H •H o

!-!

u

St tc te te rn 0 Q) fd fd •P U e

fdd *

r-lrd Ev
- 19 -

Bibliography

1, Going, R. E. Microleakage around dental restorations:

a sTjmmarizing review. JADA 84:1349 June 1972.

2. Bowen, R. L. and Mullineaux, A. L. Adhesive restorative

materials. Dent Abstracts 14:80 Feb 1969.

3. Chandler, H. H.; Bowen, R. L.; Paffenbarger , G. C., and

Mullineaux, A. L. Clinical evaluation of a radiopaque

composite restorative material after 3.5 years. [Manuscript released by Washington Editorial Review Board for

publication in J Dent Res]

4, a) Bowen, R. L. Adhesive bonding of various materials

to hard tooth tissues. I. Method of determining bond

strength. J Dent Res 44:690 July-Aug 1965.

b) Bowen, R. L, Adhesive bonding of various materials

to hard tooth tissues. II. Bonding to dentin promoted

by a surface -active comonomer. J Dent Res 44:895

Sept-Oct 1965.

c) Bowen, R. L. Adhesive bonding of various materials

to hard tooth tissues. Ill Bonding to dentin improved

by pre treatment and the use of a surface-active comonomer.

J Dent Res 44:903 Sept-Oct 1965.
- 20 -

4. d) Bowen, R. L. Adhesive bonding of various materials

to hard tooth tissues. IV. Bonding to dentin, enamel

and fluorapatite improved by the use of a surfaceactive comonomer, J Dent Res 44:906 Sept-Oct 1965.

;e,)>'v Bowen, R. L. Adhesive bonding of various materials

to hard tooth tissues, V. The effect of a surfaceactive comonomer on adhesion to diverse substrates.

J Dent Res 44:1369 Nov-Dec 1965.

:; . f) Bowen, R. L. Adhesive bonding of various materials

to hard tooth tissues. VI. Forces developing in direct

filling materials during hardening. JADA 74:439 Feb 1967.

5. Chandler, H. H. ; Bowen, R. L.; Paffenbarger , G. C, and

Mullineaux, A. L, Clinical investigation of a radiopaque

composite restorative material. JADA 8J.:935 Oct 1970.

6. Segel, S. Nonparametric statistics for the behavioral

sciences « N. Y,, McGraw-Hill Book Co., 1956 pp 36-42,

250. ,

7 . Hoe 1 , P . G . Introductions to mathematical statistics .

N. Y., John Wiley & Sons, Inc., 1962 pp 148-151.

8. Hoel, P. G. Introductions to mathem.atical statistics .

N. Y., John Wiley & Sons, Inc., 1962 pp 104-105.
- 21 -

9. Paffenbarger , G. C; Nelsen, R. J., and Sweeney, W. T.

Direct and indirect filling resins: A review of some

physical and chemical properties. JADA 47:515 Nov 1953.

10. Brown, W. S., et al. Thermal fatigue in teeth. J Dent Res

SI: 461 Mar-April 1972.

11. Smith, D. L. and Schoonover, I. C. Direct filling

resins: dimensional changes resulting from polymerization shrinkage and water sorption. JADA 46:540 May 1953.

12. Souder, W. H. and Peters, C. G. An investigation of

the physical properties of dental materials . Washington,

D. C. Govern. Printing Office, 1920 pp 9-19.

13. Bowen, R. L. and Rodriguez, M. S. Tensile strength and

modulus of elasticity of tooth structure and several

restorative materials. JADA 64:378 Mar 1962.

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-3110561 December 29, 1972 10 981

Progress Report

on Ca4(P04)20, TETRACALCIUM DIPHOSPHATE MONOXIDE;

CRYSTAL STRUCTURE AND RELATIONSHIPS TO

Ca5(P04)30H AND K3Na(S04)2

B. Dickens, W. E. Brown*, G. J. Kruger**,

and M. M. Stewart***

* Director, American Dental Association Research

Unit at the National Bureau of Standards.

* Postdoctoral fellow, Department of Computer

Science, University of Maryland, 1970-1971,

Permanent address: National Physical Research

Laboratory, South African Council for Scientific

and Industrial Research, Pretoria, Republic of

South Africa.

* Department of Chemistry, University of Maryland,

College Park, Maryland 20742.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Abstract

Ca^(P0^)20/ tetracalciiom diphosphate monoxide,

crystallizes in the monoclinic unit cell a. = 7.023 (1) A,

b = 11.986(4) A, c = 9.473(2) A, 3 = 90.90(1)° (at 25°C)

in space group P2^ with 4[ Ca^ (PO^) per cell. 3288

x-ray data were measured from a single crystal by fi-2 6 scans using MoKa radiation; 56 of these reflections were

of " unobservable" intensity. The structure was solved by

an application of direct phasing methods and subsequent

calculation of an E map. It was refined anisotropically by fullmatrix least-squares to = 0.036, R(F) = 0.037.

Allowance was made for isotropic secondary extinction but

not for anomalous scattering or absorption.

The dimensions of the unit cells of Ca^(P0^)20 and

Ca^ (PO^) 3OH (hydroxyapatite) , an idealized form of the

major inorganic phase in the human body, are simply

related. Although this 3-dimensional relationship in

the unit cell shapes is not carried over into the details

of the actual structures, Ca^(P0^)20 does contain a layer which is similar to a layer in Ca^ (PO^) ^OH, and an

epitaxial relationship between the two compounds is conceivable. Ca^(P0^)20 is also related to the K2Na(SO^)2

(glaserite) structure. In this relationship the oxide

ions are "extra" ions.

One Ca ion in Ca^(P0^)20 is coordinated weakly to a
-2-

Abstract (Cont.)

face of a PO^ group, a feature which has been previously

observed for Ca only in a disordered cation site in

^-Ca^iPO^) 2' The two crystallographically discrete oxide

ions are surrounded by tetrahedra of Ca ions, with Ca...O

distances in the range 2.136(4) A to 2.277(3) A. Thus,

the oxide ions do not lie in a channel formed by cations

in the structure and Ca^(P0^)20 cannot be considered to

be an oxy-apatite.

The positions of the P atoms and the Ca and oxide

ions lie close to those required by space group Pmcn.

This explains the appreciable twinning exhibited by

Ca^(P0^)20. It also makes the existence of a higher

symmetry modification feasible.
-3-

Introduction^

This investigation is the sequel to an earlier

investigation (Brown and Epstein, 1965) on Ca^(P0^)20,

tetracalcium diphosphate monoxide. The more usual name

for Ca^(P0^)20 is tetracalcium phosphate, from the formula

when written as 4CaO*P20^. Brown and Epstein showed

that Ca^(P0^)20 may have a structural relationship to

Ca_ (PO .) - (OH,F) (Kay, Young and Posner, 1964), the mineral

o 4 J

hydroxyapatite, which may be considered an idealized form

of the major inorganic phase in the human body. Here we

report on the determination of the crystal structure of

Ca^(P0^)20 using a small single crystal, and discuss the

Ca4(PO^)20 - Ca^(P0^)30H and Ca^(P04)20 - K3Na (SO^) 2 relationships,

pata^ collecti on^^nd tru5^J^_^,^.g.^.i^g.fjrij:gjllf^_£. The crystal used in the data collection was an

irregular colorless disc with radius ~0.33mm, thickness

'-^.llmm. This was the largest untwinned crystal found in

the sample described by Brown and Epstein (1965) . The

crystal was attached to the goniometer head in our usual

way (Dickens and Bowen, 1971a). Crystal data are as follows:

Formula (ideal): Ca^(P0^)20

cell at 25°C: monoclinic

a = 7.023 (1) A

b = 11.986 (4) A
-4-

c = 9.473(2) A

3 = 90.90 (1) "

volume = 797.3 A"^ space group cell contents 4[ Ca^ (PO^)

calculated density 3.051 g-cm ; -3 observed density 3.06 g*cm (Bucking and Linck, 1887)

The cell parameters and their standard deviations (given

in parentheses) were calculated by least-squares from 20 2

values observed on a diffractometer using MoKa radiation

filtered through 0.025 mm Nb foil. The wavelength X (MoKa) = 0.710688 A was assumed. (To make b the unique monoclinic

axis, the cell a.', b' , c_' given by Brown and Epstein has been

transformed here to jc, a_, b.

)

The x-ray data were measured using the 9-2 9 procedure

described by Alexander and Smith (1962; 1964) and as adapted

by Reimann, Mighell and Mauer (1967) . Each reflection was

measured once only. There were 3288 reflections total of

which 56 were "unobserved" [i.e.,

^y^y^j/^ ^"^hk;?,^^*

l-^(^o)

was taken to be 30.3 cm"-'-. Because of the small crystal

size, absorption corrections were neglected; the resultant

maximum error in is estimated to be 5% for the 2 9 range

0 to 80°.

The structure was determined using the sequence
-5-

DATRDN - NORMSF - SINGEN - APHASE - TANGEN - FOIJRR in

the XRAY system of Stewart et.al (1972). The quasi-normalized

structure factor statistics (Dickinson, Stewart and

Holden, 1966) were very close (see below) to those expected

for an acentric space-group. In the following list, E

is the quasi-normalized structure factor and the values

in square brackets are the theoretical values for an

acentric structure. Quantities enclosed in < > brackets

are average values. 
= 0.886 [0.886], <|e -l|> = 0.748 [0.736] , < |e^-1 | ^> = 1 . 061 [ 1 . 000] , <|e^-1|^>=

3.186 [2.000]; %E>3.0 = 0.05 [0.01], %E>2 . 5 = 0.22 [0.19],

%E>2.0 = 1.95 [1.83], %E>1.8 = 4.10 [3.92], %E>1.6 =

7.91 [7.73], %E>1.4 = 14.6 [14.1], %E>1.2 = 23.9 [23.7],

%E>1.0 = 35. 9[ 36.8].

The following phases were fixed during the tangent

refinement part of the program: [1] 4,0,0(0); [6] 4,4,0(90);

[24] 0,1,3(0); [29] 1,0,-13(0); [89] 8,0,3(0), where the

values given are [sequence number in ordered list of E

values] , h, k, (phase in degrees) . The tangent refinement was carried out in four stages, using 100, 150,

225 and then 300 E values (minimum E's were 1.887, 1.981,

1.661 and 1 . 563, respectively) . The 300 E values from the

final cycle were used to calculate an E map which contained

peaks at the Ca and P positions as its salient features.
-6-

The oxygen atoms were found in a siibsequent Fourier

synthesis phased from the Ca and P atoms.

The structure was refined isotropically using the XRAY

system program CRYLSQ to = 0.041, R = 0.050, and

then anisotropically in a series of block refinements

(varying half of the structure at any one time) to

Ry, = 0.036, R = 0.037. The y. coordinate of Ca(l) was

fixed to define the origin along [010] . The scattering

factors used were those for the neutral atoms in

International Tables for X-ray Crystallography (1962)

Before the anisotropic refinements, the weighting scheme

was changed from one based on counting statistics,

a^(F), to w(F) = (a^(F) + 0.0005 F^ )"\ m the final refinement, the average shift/error was 0.005 and the

standard deviation of an observation of unit weight was

0.34. The extinction parameter 3, (Zachariasen, 196 3;

1967, Larson, 1970) refined to 1.38(6) x

10~^

degrees (for an isotropic Gaussian distribution of mosaic blocks) . The largest correlation

coefficients are 0.64 between the extinction and scale

factors, and 0.50 to 0.59 between the ^ parameters of

the pairs of atoms [Ca(4), Ca(5)], [Ca(5), Ca(6)],

[Ca(5), Ca(8)], and [Ca(6), Ca(8)]. The three largest

peaks and two largest "holes" in a difference electron
-7-

density synthesis calculated after the final refinement

were ~0.8 electrons -A and —0.8 electrons -A , respectively. They are all attributed to the background.

The atomic parameters are given in Table 1 and the

observed and calculated structure factors in Table 2.

The structure of Ca^(P0^)20 is shown in Fig. 1.

There are eight unique Ca ions, four PO^ groups and

two oxide ions. The Ca and P atoms lie near four sheets

perpendicular to [010], two adjacent sheets being crystallographically related to the other two in the cell by the

two-fold screw axis of the space group, P2^. Each sheet

contains two cation-anion columns and one cation-cation

coliimn. These columns are [Ca(l), Ca(2)], [Ca(3), P(2)0^]

and [Ca(4), P(1)0^] in one sheet (Fig. 2) and [Ca(7), Ca(8)],

[Ca(5), P(3)0^] and [Ca(6), P (4) O^] in the other (Fig. 3).

The calcium ion environments

Ca(l) is coordinated to seven oxygen atoms (see Fig. 2),

including the PO^ edge [0(21), 0(22)] and the oxide ion 0(1),

arranged in an approximately pentagonal bipyramid (see

Table 3 for details of all ionic environments)

.

Ca(2) is coordinated to seven oxygen atoms, including

the PO^ edges [ O (21) , O (23) ] and [ O (41) , 0 (42) ] , and the
-8-

oxide ion 0(1). The coordination polyhedron of Ca(2) is

irregular

Ca(3) is coordinated to seven oxygen atoms, including

the PO^ edges [ O (12) , O (13) ] and [ O (31) , O (34) ] , and to the

oxide ion 0(2). Six of the oxygens in the coordination of

Ca(3) may be considered to be arranged in a partial

pentagonal bipyramid. The seventh member of the coordination polyhedron is 0(34), which is necessarily displaced

from the remaining equatorial position of the pentagonal

bipyramid because it is the second member of the

[ 0 (31) , O (34) ] PO^ edge. it is the oxygen with the weakest

bond to Ca ( 3 )

Ca (4) is coordinated to seven oxygen atoms, including

the PO^ edge [ O (32) , O (33) ]

, and the 0(1) oxide ion. The

coordination geometry of Ca(4) is derived from pentagonal

bipyramidal, with 0(1) occupying one of the axial positions.

Ca(5) is coordinated to seven oxygen atoms (Fig. 3),

including the PO^ edges [ O (41) , O (44) ] and [ O (12) , O (13) ]

,

and the oxide ion 0(2). The coordination of Ca(5) is derived from pentagonal bipyramidal with [ O (41) , 0 (44) ] in

the equator

Ca(6) is coordinated to eight oxygen atoms, including

the PO^ edge [ O (11) , O (14) ]

, and the oxide ion 0(1). The

coordination polyhedron of Ca(6) may be considered to
-9-

include the PO^ face [ O (32) , 0 (33) , O (34) ] although the

primary ionic bonding of all these three oxygen atoms

is to neighboring Ca ions. Coordination of Ca to a PO^

face is rare; the other known example is in the P-Ca^ (^0^)

structure (Dickens, Bowen and Brown, 1971b).

Ca(7) is coordinated to seven oxygen atoms, including

the PO^ edges [ O (11) , O (13) ] and [ O (42) , 0 (43) ]

, and the

oxide ion 0(2). The coordination geometry is irregular.

Ca(8) is coordinated to seven oxygen atoms, including

the PO^ edge [ O (31) , O (33) ] , and the oxide ion 0(2). The

coordination is pentagonal bipyramidal with the above

oxygens in equatorial positions, along with 0(11) and

0(43).

Proceeding on the assumption that any Ca...O distance

less than ~2.5 A denotes a strong Ca...O bond, we note

that the bonding of all Ca ions to the oxide ions 0(1)

and 0(2) is strong. The number of strong and less strong

bonds to all coordinated oxygens from individual Ca ions

are as follows:

Ca(l) 6,1; Ca(2) 3,4; Ca(3) 2,5; Ca(4) 5,2; Ca(5) 4,3;

Ca(6) 4,4; Ca(7) 4,3; Ca(8) 6,2.

The PO^ environments

In the P(1)0^ group (Fig. 2), each oxygen is bonded

to three Ca ions. The P(1)0>, edges [ O (11) , O (13) ] ,
-10-

[0(11) ,0(14)] and [ O (12) , O (13) ] are "shared" by coordination

to Ca ions.

In the P(2)0^ group, 0(21), 0(22) and 0(23) are each

coordinated to three Ca ions, and 0(24) is coordinated to

two. The edges [ O (21) , O (22) ] and [ O (21) , O (23) ] are "shared".

In the P(3)0^ group (Fig. 3), 0(31) and 0(32) are both

bonded to three Ca ions, while 0(33) and 0(34) may be

considered to be bonded to four. The edges [ O (31) , O (33) ]

,

[0(31) ,0(34)] and [ O (32) , O (33) ] are "shared", as is the

[0(32) ,0(33) ,0(34)] face. As judged from the O...Ca distances for the P(3)0^ group, the bonding of the P(3)0^

oxygens to Ca(8) and Ca(6) is weak, apparently because of

repulsion of these Ca ions by the other two Ca ions, Ca(5)

and Ca(4), to which the P(3)0^ oxygens are bonded more

strongly.

In the P(4)0^ group, each oxygen is bonded to three

Ca ions and the edges [ 0 (41) , O (42) ] , [ O (41) , O (44) ] and

[0(42) ,0(43)] are "shared".

In all PO^ groups, the 0-P-O angles less than the

tetrahedral angle, 109.5**, are associated with PO^ edges

which are coordinated to Ca, in accord with Pauling's rule

(Pauling, 1960)

The oxide ion environments

Oxide ion 0(1) is coordinated very strongly to the
-11-

Ca(l), Ca(2), Ca(4) and Ca(6) ions, which are arranged in

an approximate tetrahedron about 0(1). Ca(6) is in a different sheet from the other Ca ions. Oxide ion 0(2)

is similarly coordinated to Ca(5), Ca(7), Ca(8) in one

sheet and Ca(3) in another. The 0(1)... Ca and 0(2) Ca

distances (Table 3) are unusually small.

Discussion

Cruickshank (1961) has suggested that the average

of the P-0 distances in PO^ groups is constant. This

has been denied by Baur and Khan (1970) . It is commonly

observed that the length of an individual p-0 bond

apparently depends on the environment of the oxygen atom,

i.e., whether the oxygen is covalently bonded to hydrogen

or ionically bonded to one or more Ca ions. It seems

intuitively reasonable that the average P-0 bond length

will similarly depend on the total environment of the

PO^ group, because the more electropositive the environment, the more electrons will be attached onto the oxygen

atoms, affecting the amount of n bonding and the hybridization of the a bonding in each P-0 bond.
-11aThe average P-0 distance in Ca^(P0^)20 for

3-coordination of oxygen by Ca ions is 1.538(10) A,

where the standard deviation (in parentheses) is

estimated from the spread of the individual values.

The two examples of possible 4-coordination, 0(33)

and 0(34), average to 1.537(18) A, practically the

same value as for the 3-coordination. The sole

example of 2-coordination is 0(24), with P(2) - 0(24) =

1.511 A, which is appropriately less than the P-0

distances for 3-coordinated oxygens. Examination of

the P-0 bond distances along the lines described by

Baur (1970) does not explain the individual variations.

The average P-0 distances in the P(l), P(2), P(3)

and P(4) PO^ groups are 1.533(5), 1.537(19), 1.535(14)

and 1.541(8) A respectively. The standard deviations

(in parentheses) were again estimated from the spread

of the individual values. From the bond lengths in the

Ca^(P0^)20 structure, it appears that the effect (through

the valence unit concept) of coordination number alone

on P-0 bond lengths is relatively wide-spread, yet the

average values of individual PO^ groups are effectively

constant. The average for all PO^ groups is 1.537 A,

with a standard deviation of 0.003 A when only the spread

of the averages is considered, and 0.012 A when the
-12-

spread of the individual values is considered. Corresponding

values from our study of magnesium-containing P-Ca^ (PO^)

2

(Dickens, Bowen and Brown, 1971b) are 1.537(10) A and

1.533(8) A. The overall average value of 1.537 A agrees

very well with that of 1.539 A obtained by Baur and Khan

by averaging over PO^ groups in various forms, i.e.,

^2^^4 stc, in several structures found in the literature.

Therefore, even when one or more P-0 bonds are increased

to ~1.59 A by the formation of covalent 0-H bonds, the

remaining P-0 bonds apparently decrease to essentially

preserve the average value, and any effect of the

environment of the PO^ group is very small.

In the usual considerations of coordination polyhedra,

attention is given to the configuration within a

covalently bonded group or to the arrangement of

oxygen atoms around a central cation. In the case of the

-13-

0(Ca)^ groupings in the Ca^(P0^)20 structure, one must insteadconsider the arrangement of cations around an oxygen

atoms. Some light can be shed on the unusually small Ca...O

distance of 2.213 A in the Ca^O grouping by the following

reasoning: CaO (Oftedahl, 1927) has the NaCl structure

with a Ca...O distance of 2.406 A. This octahedral coordination of 0 by Ca can be compared with tetrahedral

coordination of O by Ca by assuming a tetrahedral arrangement of CaO exists with lattice energy comparable to that

of the octahedral arrangement. Then the ratio of Ca...O

distances is given by R^/R^ = [ (A^B^) / (A^B^) ] ^'^'"'''^ with

A being the Madelung constants, B the repulsion constants,

and n the exponent of the interatomic distance (n = 8 on

average for Ca^"^ and ) in the Born formulation of the

lattice energy (see Pauling, 1960, p. 508, p. 509, pp. 537-

538). Substitution of R = 2.406 A, A = 1.7476, A. = 1.640

o . o t

(the average of the Madelung constants for the sphalerite and

wurtzite structures) B = 6B ' , B . = 4B', where both B and

o ' t o

B^ are considered to be proportional to the number of

nearest neighbors, and n = ^ gives = 2.305 A. In the

O (Ca) ^ groupings in Ca^(P0^)20 there is actually increased

attraction over that calculated using a Madelung constant

for tetrahedral coordination because the other oxygen

2- neighbors of the Ca ions tetrahedrally bonded to O are
-14-

further away than the O is. Thus, the groupings will be

further contracted, and allowance for this factor would

bring the calculated distance close to that observed.

In the coordination of Ca ions to PO^ ions in

Ca. (PO^) ^O, (i) each oxygen is usually coordinated to

three Ca ions; and (ii) the Ca...O bond tends to be in

the plane 0-P-O' where 0' is one of the other three

oxygens in the PO^ group. Thus, the ideal coordination

of a PO^ group appears to be when it shares each of its

six edges with a Ca ion (Pauling's third rule not withstanding) , making a total PO^ coordination of six Ca ions.

would, of course.

Apex coordination of a PO^ group/require twice as many

and the effects of Ca. . .Ca repulsion would be expected to be

surrounding Ca ions as edge coordination does/ important. Because

Ca bonded to a PO^ edge forms twice as many bonds

to that PO^ group as does Ca bonded only to a PO^

apex, the former are bonded more strongly than

the latter and probably govern the arrangement of Ca

ions around the PO^ group through Ca...Ca repulsions.

In general, the Ca arrangements in Ca^(P0^)20 are close

to those expected when all Ca ions are coordinated to

edges. From the point of view of minimizing Ca...Ca

repulsions, face sharing would be preferred. This is

more undesirable than edge sharing however, because of

increased Ca...P repulsions, as Pauling's third rule

states. Therefore, edge sharing where possible appears
-15-

to be a reasonable compromise in the coordination of Ca

this case,

to a PO^ group. In/the three Ca ions and one P atom

coordinated to each O atom in the PO^ group are arranged

in approximately tetrahedral directions about the oxygen

atom

.

Although the structure of Ca^(P0^)20 is actually

monoclinic and acentric (P2^) , and the distribution of

the quasi-normalized structure factor compares well with

that expected for an acentric structure (see above) , the

positions of the Ca, PO^ and oxide ions approximately

correspond to the requirements of the centric space group

p2^/m, 2^/c, 2^/n, (Pmcn-D2^) . All symmetry elements in

except for the 2^-axis parallel to [010] are pseudo-symmetry

elements (designated with primes in the following discussion)

.

These pseudo- symmetry elements are placed in Ca^(P0^)20 as

follows

:

(i) mirror planes, m', perpendicular to [100] pass

through the P atoms, the Ca(3), Ca(4), Ca(5), and Ca(6)

ions and the oxide ions 0(1) and 0(2).

(ii) a c '-glide plane perpendicular to [010] is at

y_ ~0.4 with the present placement of the origin along [010]

.

(Because P2^ is a polar space-group, placement of the origin

along [010] is arbitrary in the actual structure.)

(iii) an n '-glide perpendicular to [001] is at z^=0.5.
-16-

Thus, the pseudo- symmetry in the crystal structure is

sufficiently pronounced for the existence of a higher

temperature orthorhombic modification to be feasible.

Brov/n and Epstein (195 5) noticed appreciable pseudo-orthorhombic symmetry in their x-ray photographs of Ca^(P0^)20

and also concluded that an orthorhombic modification may

exist. That the Ca^(P0^)20 structure is capable of absorbing

a large amount of energy is revealed by the fact that the

melting point of Ca^(P0^)20 is very high ('-ITIO'C, Tromel

and Fix, 1961). It is possible that the Ca^(P0^)20 sample

studied by Tromel and Zaminer (1959) is an orthorhombic

form, stabilized perhaps by some impurity. On the other

hand, their sample may have looked orthorhombic because of

extensive twinning.

There are eight general positions in space-group Pmcn.

Eight Ca ions [Ca(l), Ca(2), Ca(7), Ca(8) and their counterparts generated by the 2^-axis of P2^] would be crystallographically equivalent and lie in a general position in

Pmcn. The remaining atoms under consideration, the equivalent quadruple sets 2[P (1) ,P(3)] , 2[P (2) ,P (4)] , 2[Ca (3) ,Ca(6)] , 2[Ca(4) ,Ca(5)] and 2[ O (1) , O (2) ], would all be in special

positions on mirror planes in the orthorhombic structure.

Two oxygens of each PO^ group would lie on the mirror plane,

and the remaining two would be related by the mirror plane.

The c^'-glide relationship between P(1)0^ and P(3)0^ groups

is such that the equivalency relationships between their

oxygen atoms are O (13)<>0 (31) , O ( ll>o= O (34) , and 0(13),

0(12)<5=0(32) ,0(33) . The relationship between P(2)0. and
-17-

P(4)0^ is less clear, but it appears (Fig. 1) that a rotation of P (4) 0^ by about 90° around a. (so that it

would satisfy the requirements of the mirror plane m')

would bring the two PO^ groups into a c^-glide relationship. Their oxygen atoms would then be related as

follows: 0(21)00(42), 0(24)^0(43), and 0 (23) , O (22)o

0(41) ,0(44) . The pseudo-symmetry in Ca^(P0^)20 is also of interest

because of possible twinning modes. Tennier and Richard

(1895) state that (001) and (100) (in the present notation)

are the lamellar twin planes. This was corroborated by

Brown and Epstein (196 5) , who state that the composition

planes for normal twins they observed would be (001) and

(100) . Twinning in which the (100) and (001) planes are

composition planes are the only ones involving pinacoids

that could be detected optically. The possible existence

of a high- temperature orthorhombic modification with the

symmetry elements 2^/m and 2^/n associated with the a. and

_c axes, respectively, lends strong support for twinning

by any of these four pseudo- symmetry operations. Of the

remaining symmetry operations, 2^/c^, which are associated

with the b axis, only the c^-glide could produce twinning

in monoclinic Ca, (PO.)-O because the 2 axis is present
-18-

as a true symmetry element. Twins produced by the -glide

operation, however, would be "optical twins" and would

not be detected with the polarizing microscope. Note

that the plane of the c^-glide lies at the center of the

apatite-like layer in Ca^(P0^)20 described below. The

existence of a similar layer in Ca^ (PO^) ^ (OH) , hydroxyapatite, suggests that the operations corresponding to

2^/c could produce twinning in Ca^(P0^)20H. Although

these operations have not been suggested previously for

Ca^CPO^)^©!! twins of this type (i.e., with (100)

the twin plane) have been reported (Palache, Berman and

Frondel, 1951) . One may speculate that a twinning

mechanism of this type (which would also produce optical

in that Ca5(P04)3CH is an idealization of the major inorganic

twins) may have some biological significance/phases in the body.

For example,

/electron micrographs of the apatitic crystals of dental

enamel frequently show a plane of demarkation parallel

to (100) which could be the composition plane of a twin.

Relationship of Ca^(P0^)20 to other structures

As was outlined by Dickens and Brown (1972) , calcium

phosphates typically crystallize in five structural types:

(i) structures related to Ca^ (PO^) ^ (OH, F) , hydroxyapatite,

(ii) structures containing CaPO^ chains and sheets, (iii)

structures related to K2Na(SO^)2/ glaserite, (iv) structures
-19-

related to Ba2(PO^)2» and (v) structures related to

MgNH^PO^*6H20, struvite.

Only types (i) and (iii) contain both cation-cation

and cation-anion columns. The Ca:P ratio of 2:1 in the

formula of Ca^(P0^)20 suggests that it may have a structural

relationship to glaserite, K^Na (SO^) 2 (Gossner, 1928). The

presence of the oxide ions means, however, that the glaseritetype structure must be modified to accommodate extra, but

small, anions. In other structures in which the cation: anion

ratio is not 2:1, e.g., Ca^ (PO^) 2SiO^ (Dickens and Brown,

1971c) and Ca^Mg^ (Ca, Mg) 2 (PO^) ^2 (Dickens and Brown, 1971d),

the semblance of the glaserite structure is retained by

resorting to crystallographically systematic cation vacancies

in the cation-anion columns. The relationship between the

structures of K^Na (80^)2 (glaserite) (Fig. 4) and

Ca^(P0^)20 (shown in an a. projection in Fig. 5) can be seen

most readily in Fig. 6. The latter is derived from Fig. 5

by shifting the layer bounded by the broken lines at A and A'

about 2 A to the left. (Each layer consists of two sheets of

[Ca,Ca] and [Ca,PO^] columns.) It can be seen that the

highly distorted diamond- shaped cells of Fig. 5 have become

fairly regular in Fig. 5. Each cell has [Ca,Ca] coliamns

at the corners and two [Ca,PO^] columns on the major

diagonal, much as they are in Fig. 4. The unit cell of
-20-

Ca^(P0^)20 corresponds to four glaserite cells.

With the idealization of the structure of Ca^(P0^)20,

it is now possible to see why the Ifiyers in Ca^(P0^)20 are

shifted. In the structure of Fig. 6, 0(2) would have

coordinated to only three calcium atoms Ca(7), Ca(8) and Ca(5)

at three apexes of a tetrahedron; Ca(4) is too far away

along a, to coordinate to 0(2). Similarly, 0(1) would

coordinate to Ca(l), Ca(2) and Ca(4) but not to Ca(5).

When the layer A-A' shifts to its actual position, as shown

in Fig. 5, Ca(3) comes into coordination with 0(2) and

Ca(6) with 0(1) so that each oxygen can have full tetrahedral coordination.

The idealized structure approximates orthohexagonal

symmetry, suggesting that the orthorhombic, high- temperature

form postulated above may become orthohexagonal

if 0(1) and 0(2) become mobilized at

higher temperatures.

The lanthanide elements form compounds of the type

M2SiO^O, which for purposes of comparison with Ca^(P0^)20

may be written (SiO^) 2O2 • These compounds

contain the same cation-tetrahedr al anion ratio as

Ca^ (PO^) 2O and glaserite, but they contain twice as many

oxide oxygens as Ca^(P0^)20. The lanthanide oxysilicates

actually form two structurally different series with the
-21-

demarkation occurring at Tb (Felsche, 1971) . In both series

of compounds (Smolin and Tkachev, 1969; Smolin, 1970) , there

are sheets each of which contains two cation-anion columns

for every one cation-cation column, and the oxide ions have

tetrahedral coordination of cations as they do in Ca^(P0^)20,

but the juxtapositions

of these sheets differ significantly from those

in Ca^(P0^)20 and glaserite. Thus, there is only a somewhat

tenuous relationship between the structures of Ca^(P0^)20

and the lanthanide oxysilicates

.

The Ca^(P0^)20 structure also bears a surprising

resemblance to part of the Ca^(P0^)30H structure (Kay,

de s cr ibed above

.

Young and Posner, 1964), i.e., to structural type (i)/

The atoms inside the BB ' layer in Fig. 7, which

depicts six unit cells of Ca^ (PO^) ^OH, are compared in

Fig. 8 with atoms in the "apatitic layer" AA' of Fig. 5.

Although there are some significant differences in

orientation of PO^ groups, both structures contain

similarly positioned [Ca,Ca] and [Ca,PO^] columns comprising

two sheets in each layer. Thus, in the relationship of these

"apatite layers" in Ca^(P0^)20 and Ca^ (P0_^) ^OH, the cationcation columns [ Ca (1) , Ca (2) ] and [ Ca (7) ,Ca (8) ] in Ca^ (PO^) 2O

correspond to the calcium colinnns Ca(I) in Ca^(P0^)20H. Each

of the pairs [ Ca (3) , Ca (4) ] and [ Ca (5) , Ca (6) ] in Ca^lPO^)^©
-22-

corresponds to one side of a Ca(II) "hexagon" in Ca^(P0^)20H.

In this relationship, tins locations of the oxide ions 0(1)

and 0(2) in Ca^(P0^)20 correspond to the locations of the OH

ions in Ca^ 3OH. Because they are actually located in

the neighboring apatitic layer in the complete Ca^(P0^)20

structure whereas the OH ions are not in an apatitic layer

in Ca_(P0.)^0H, the oxide ions account for much of the

misorientation of the PO^ groups in Ca^(P0^)20 as compared

with Ca5(PO^)20H.

The salt CagH2 (PO^) ^

* 5H2O is also known to contain an

apatitic layer (Brown, 1962; 1966) . In that case, however,

the layer is thicker than the one in Ca^(P0^)20 (approximating

the shaded area in Fig. 7) and the atomic positions are

closer to those of Ca_ (PO . ) -,0H.

A more complete relationship between Ca^(P0^)20 and

Ca^ (PO^) ^OH can be derived from the idealized glaserite

structures given in Fig. 4 and Fig. 6. If all [ Ca (7) , Ca (8)

]

colTimns in the next layer below (towards the bottom

of figure 6) ' replaced by [ OH, OH] columns

and the oxide ions 0(1) and 0(2) are removed, one obtains

the approximation for the Ca^ structure first

derived from glaserite by Wondratschek (196 3) . The unit

cell for Ca^(P0^)20H, defined by lines of open squares in Fig. 6, is

hexagonal and in this projection consists of three glaserite

unit cells. Since, as noted above, the Ca^ (PO^) 2O unit cell
-23-

contains four glaserite units, the Ca^ (PO^) (PO^) ^OH

unit-cell relationship is 4/3, in contrast with

the ratio of the volumes of the two cells which is very

close to 3/2. The difference is consistent with the fact

that each glaserite-like unit cell contains [Ca^(P0^)2O] in

Ca^iPO^) 2^ and only [ Ca^ ^3 (^0^) 2^'^^^ 0 67-' °^ average

in Ca^ (PO^) ^OH.

The coordination of the Ca(ll) ion in Ca_(PO.)_OH is

D 4 3

to nine oxygens comprising three PO^ apexes and three

PO^ edges. In Ca^ (PO^) 2© the coordinations of the

corresponding Ca(l), Ca(2), Ca(7), Ca(8) ions are all

seven, including at the most [for Ca(2) and Ca(7)] two

PO^ edges. Each of these cations is also coordinated

to an oxide ion.) Seven-fold coordination of Ca in

calcium phosphates is far more common than 9- fold, the

only clear example of which in calcium phosphates is the

Ca ion on the 3-fold axis in Ca^(P0^)20H. Note that the 9-fold

coordination of Ca in the aragonite phase of CaCO^ is a

consequence of (i) the overall packing giving a higher

density for aragonite (the high pressure phase) over

that of calcite, and (ii) the smaller O. . .0

separation in the edge of the CO^ group compared to that

in the PO^ group allowing more oxygens to enter a given

size of coordination polyhedron. Since apatites are among
-24-

the most stable calcium phosphates, the 9-fold coordination

of Ca(ii)in Ca (PO ) _0H maybe a concomitant of efficient

packing of the other ions. This probably accounts for the

observation that Ca is on the borderline of apatite stability

in terms of its radius.

The sequence Ca(ii). . .0(3) -P-0(3) . . .Ca(li) in

Ca^iPO^) ^OH is reproduced quite well in Ca^(P0^)20 in the

sequences Ca (3) . . .0 (22) -P (2) -0(23) . . .Ca (3) ; Ca (4) . . . O (12)

P(l)-0(13) ...Ca(4)7 Ca(5) . . .0 (33) -P (3) -0 (32) ...Ca(5); Ca(6) ...

0 (41) -P (4) -0(44) . . .Ca(6) . The O. . .Ca 0 columns involving

Ca(4), Ca(5) and Ca(6) are fairly straight, but the one

involving Ca(3) and the oxygens of the P(2)0^ group is

more distorted than the others.

Epitaxy, the ordered growth of one substance on another,

is often thought of as being governed chiefly by the metric

fits of networks based on the unit cell translations. While

epitaxy is certainly not rare, especially if the conditions

under which the second phase is forced to grow on the first

are sufficiently drastic, in practice only a few orientations are found. Under the mild conditions found in vivo , good chemical fits are obviously required. The combination

of good metric and chemical fits between Ca^ (P^^) 3OH and

CagH2 (PO^) g 5H2O makes epitaxy between these two compounds

very important (Brown et _al . , 1962; Brown, 1966). The unit

cells of Ca^(P0^)20H and Ca^(P0^)20 (Table 4) suggest that

there will be many good metric fits of networks between these
-25-

two compounds. Because of the structural similarities

between Ca^(P0^)20H and Ca^(P0^)20, epitaxy between

them may have considerable importance. Undetected

epitaxy between Ca^ (PO^) ^OH and Ca^(P0^)20 would increase

the Ca:P ratio of an apparent apatite above 1.55 7 and on

a fine scale would resemble solid solution. It would

also result in an increase in the apparent c unit cell

dimension and probably a reduction in the apparent

dimension relative to the values for pure Ca^(P0^)20H,

(because 3/2 (d^go^ Ca^ (PO^) ^OH is less than b of

Ca^ (PO^) 2©, while the two a. dimensions are approximately

equal), Ca^(P0^)20H is therefore an unusual salt in that

it has two related salts with which it may enter epitactical relationships. One salt, CagH2 (PO^) ^ • 5H2O, is

more acidic, and the other, Ca^(P0^)20, is more basic.

Although production of Ca^(P0^)20 itself in an aqueous

environment would be difficult because of the oxide ion,

a carbonate-containing product may form more easily.

Ca^(P0^)20 known to react with CO^ at elevated temperatures, although the reaction products have not yet been

identified. Epitaxy between carbonation products of Ca^(po^)20

and apatites may be important in the problem of carbonate

apatites.
-26-

Acknowledqement

J. S. Bowen and P. B. Kingsbury provided technical

help. This investigation was supported in part by research

grant DE-00572 to the American Dental Association and by

contract to the National Bureau of Standards from the

National Institute of Dental Research. Much of the

computer time was obtained through the facilities of the

Computer Science Center, University of Maryland, College

Park, Maryland 20742.

is
-27-

References

Alexander, L. E. and Smith, G. (1962). Acta Cryst., 15 ,

983-1004.

Alexander, L. E. and Smith. G. (1964). Acta Cryst., 17 , 1195-1201.

Baur, W. H. (1970) . Transactions of American Crystallographic Association, 6^, 129-155.

Baur, W. H. and Khan, A. A. (1970). Acta Cryst., B26 , 1584-1596.

Brown, W. E. (1962). Nature, 196, 1048 -1050.

Brown, W. E. and Epstein, E. F. (1965) . J. Res. NBS,

69A, 547-551.

Brown, W. E. (1966) . Clinical Orthopedics, 44, 205-220.

Brown, W. E. , Smith, J. P., Lehr, J. R. and Frazier, A. W.

(1962). Nature, 196 , 1050-1055.

Bucking, H. and Linck, G. (1887) . Stahl und Eisen.

245-249.

Cruickshank, D. W. J. (1961). J. Chem. Soc, 5486-5504.

Dickens, B. and Bowen, J. S. (1971a). Acta Cryst., B27 , 2247-2255.

Dickens, B, Bowen, J. S. and Brown, W. E. (1971b).

Abstract G3, American Crystallographic Association Winter

Meeting, Columbia, S. C.

Dickens, B. and Brown, W. E. (1971c) . Tschermaks

Mineralogische und Petrographische, jU5, 1-27.

Dickens, B. and Brown, W. E. (1971d) . Tschermaks

Mineralogische und Petrographische, jL6, 79-104.
-28-

Dickens, B. and Brown, W. E. (1972). Acta Cryst., in press.

Dickinson, C, Stewart, J. M. and Holden, J. R. (1966).

Acta Cryst., 21, 663-670.

Felsche, J. (1971). Naturwissen, 58^, 565-566.

Gossner, B. (1928) . N. Jb. Min. Geol . 57A, 89_ii6,

International Tables for X-ray Crystallography , (1962)

The Kynock Press: Birmingham, England, Vol. Ill, p. 202.

Kay, M. I., Young, R. A. and Posner, A. S. (1964).

Nature, 204 , 1050-1052.

Larson, A. C. (1970). In; " Crystallographic Computing",

F. R. Ahmed, Editor, Munksgaard: Copenhagen, Denmark,

Oftedahl,I. (1927). Zeitsch. F. Phy. Chem. 128 , 154-158.

Palache, C, Berman, H. and Frondel, C. (1951). The

System of Mineralogy of J. D. Dana and E. S. Dana,

7th Edition, 2, J. Wiley and Sons, New York.

Pauling, L. (1960) . The Nature of the Chemical Bond , 3rd Edition, p. 559. Ithaca: Cornell University Press.

Reimann, C. W. , Mighell, A. D. and Mauer, F. A. (1967) . Acta Cryst. 23 » 135-141.

Smolin, Y. I. and Tkachev, S. P. (1969). Soviet Physics -

cryst. 14 > 14-16.

Smolin, Y. I. (1970). Soviet Physics - cryst. 14, 854-858.

Stewart, J. M. , Kruger, G. J., Ammon, H. L., Dickinson, C. and

Hall, S. R. (1972) . X-ray system of crystallographic

programs, version of June, 1972.

Technical Report TR-192, The Computer Science Center, University

of Maryland, College Park, Maryland 20742.
-29-

Termier, M. P. and Richard, M. (1895) . Bull. Soc. France

Min. 18. 391-395.

Tromel, G. and Fix, W. (1961) . Arch. Eisenhuttenw. 32 , 209-212.

Tromel, G. and Zaminer, C. (1959) . Arch. Eisenhuttenw. 30 , 205-209.

Wondratschek, H. (1963) . N. Jh. Miner. Abh, 99/ 113-160.

Zachariasen, W. H. (1963) . Acta Cryst. 16, 1139-1144,

Zachariasen, W. H. (1967) . Acta Cryst. 23, 558-564.

Figure legends

Figure 1. - A stereoscopic view of the Ca^(P0^)20 structure

viewed along [ 100] . The origin of the crystallographic

coordinate system is at the lower right front corner

of the unit cell. Only a unique set of atoms is labelled.

Figure 2. - The environments of the Ca ( 1) , Ca (2) , Ca ( 3) , Ca (4)

,

the same sheet parallel to (010)

.

P(1)0^ and P(2)0^ ions in Ca^(P0^)20. These ions lie in/

Figure 3. - The environments of the Ca ( 5) , Ca (6 )

, Ca ( 7) , Ca (8)

,

the same sheet parallel to (010)

.

P(3)0^ and P(4)0^ ions in Ca^(P0^)20. These ions lie in /

Figure 4. - A schematic drawing of the structure of K-^Na (50^)2/

glaserite (Gossner, 1928). One S,0,K column in the unit

cell points "up" and the other points "down"

.

Figure 5. - The relationship of Ca^(P0^)20 to K^Na (50^)2

(glaserite) and Ca^ (PO^) ^OH (hydroxyapatite) . The glaseritetype pseudo-cell in Ca^(P0^)20 is shown by type lines.

The actual cell of Ca^(P0^)20 is shown by light lines. The

layer AA' is the " apatitic" layer in Ca^(P0^)20 and

encompasses two sheets of [Ca,Ca] and [Ca,PO^] columns

(see text). The dotted lines show the vestiges in Ca^(P0^)20

of the Ca "hexagons" in Ca^ (PO^) ^OH.

Figure 6. - An idealization of the Ca^(P0^)20 structure.

The diagram in Figure 5 has been cut along the lines A and

A', and the AA' layer moved to the left by ~2 A. In this

idealization the structure is hexagonal if the oxide ions

0(1) and 0(2) are either ignored or placed on the nearest

[Ca,Ca] column. The K^Na (SO^) 2-type cell is represented
-2-

by A hexagonal-type cell is shown by dotted lines.

A Ca^ (PO^) ^OH-type cell is shown by lines of open squares.

Figure 7. - The structure of Ca^CPO^)^©!! (Kay, Young and

Posner, 1964) . The OH site may be seen in the center of

the Ca hexagon in the center of each unit cell. The

shaded region is common to the structures of Ca^(P0^)20H

and CagH^ (PO^) g

• 5H2O (Brown, 1962). The layer BB ' is

common to the structures of Ca^(P0^)20H and Ca^(P0^)20.

Figure 8. - A comparison of the apatitic layers in Ca^(P0^)20

(AA') and Ca^(PO^)^OH (BB').
0.0311

O.S320<

0.76541 o.zees

0.7409

0.2501 0.0099

0.51461 0.7&6S

0.2S93 0,2376

0.76411

0.73191

0.61131 0.9541

0.7702

0.28671

0 . 0960

0.45571 0.33091 0.2165

0.0766

0.4194 0.2444

0 . SB90 I 0.83e4

0.71641 0.9190

0,2642

0.76S6I

Tabl« 1. AtoDlc pum

B?441 (9) 69733 (9) 5299(1] 25229(9)

25600(9)

01201(9] 63310(9)

59216(9) 2140 C 6240(

31811:

2667 2860

2936

74B0

6226 6594

4843

4737

2626

3023

2262

9992 B407

7862 0085

0163

4905

•TB In Ca^'PO^)

.

lo'o,, lo'o,.

Thermal parajE«t«r8 have th» forn «3cp[-2tt" (Unh"^"" +

+ 20i .hta'g* + 20,J

Piguraa tn p&renthaaas ara atandArd dcviatLona in last slgnlf.

10 'O,,

2Uiah!<a*b*
Table 2. Calculated and observed structure factors for

Ca.(P04)pO. Columns are i. 10 F^, 10 F and 10 ct(F_) . Unobserved reflections are those with intensity less

than 2 a (I) and are marked by *.

0'0< 2b 1*9

631 19 SOb 23

10 387 390 US

11 11*^ 33t> 12 19t> 20b Ub

15 196 171

2U12B - 193 66 S2b ttl • 235 57 307 40

eu 63

10 9b

95 3U 29 316 27

171 33

90 39

1*09 21

32 231 It

' 26 73

91 39 316 2D 392 22 172 31 30^ 26

7it7 29

721 30 559 31

51 99 257 36

1*2 63 ' 151 37 53195

5 66 315 29

39"* 30 226 31* 550 32 56U 33 270 37 202 m« 151 53

luia** 16 63

236 38 236 55

201 188 Sb 252 222 Ub 65 25113

187 176 «8

165 163 52

122 131 6tt 108 109 61 lOUO 1021 35 258 268 35

165 162 56

29it ae9 itO mu ItO 56

502 502 32 567 5=>8 31 242 26" 35 620 637 26 335 332 25

570 5^3 2t 616 616 22

1914 192 18 100 69 36

H*l 156 26

15* W6 77

251 236 22 919 911 27

80 62 U9

661 659 27

351 35t 27

133 137 U2 388 391 31 210 216 t*! 310 3tO 36

23ct 33

12 317 308 ' 13 39« U02 US -10 206

15 130 125 67 -6

326 320 «1

165 167 92

62 67106

205 201 "2 522 323 33

121 ii32 31 806 620 33

79!1 609 33

3«1 380 31 590 565 31 335 329 33 56 51 91 505 197 33 135 130 31 210 202 15

362 351 57 396 392 38 3°5 397 Ul 100 106 89 193 166 60 '3 76119

IIT 7i 176 61 202 56

551 11 1051 02 217 16

182 36 202 36 993 37 219 10 215 52 282 36

110 57

530 10 285 11

75 69 56121

3S -11 101 -13 258 256 '

-6

11 132 121

12 208 191 16 -11 136 116 76

13 132 130 12 -10 101 360 11 11 77 67 91 -9 103 99 82

15 281 271 36 -8 175 157 10 2'OtL -7 281 282 16

15 279 267 51 -6 200 1B1 16

11 65 99117 -5 176 161 16

13 131 135 12 -1 312 315 26

12 217 227 19 -3 170 173 16

11 61 51109 -2 385 391 31

10 in 156 52 -1 59 19 71 -9 61 61 72 0 90» 20151

-6 127 117 17 1 57» 12 99

-7 361 103 29 2 262 271 5b -6 121 120 11 3 309 300 56

-5 250 259 26 5 223 205 12

-1 1837 2005 16 6 259 235 11 170 26

1 615

5 312

-2 91 105

-1 17. 15 S7

0 li7b 1161 57

1 129 123 51

2 520 522 22

3 737 b97 26 • 1 1655 1667 16 • 5 137 125 36

6 115 112 11

7 220 231 31

6 273 272 31

9 216 225 10

10 160 171 3b

11 238 21b 12

12 115 111 b3

13 328 316 11

128 117 78 557 I 228 17

90 77 293 37

121 62 569 32 626 31 39b 31 77 16 159 10

7 81 106 IS

62 16

' 110 97

< 26 61 111 21 676 27

160 26

99 11 356 36 311 32 251 35 51103

71 63 135 26

159 57

331 32 215 55

239 17 13U2

186 62 261 52

151 65

92 90 250 51 311 15 227 50 211 17 81108 70103

275 11

771 36 19b 13 16106

531 37

371 38 157 57 531 39 353 13 36107

75129

27130

119 78 137 71

501 17 373 15 •

296 12

71 82 170 31 359 31 165 51 331 30

'6 86 81 5b

5 99 95 16

1 301 310 25

3 207 202 27

2 1383 1161 35

1 917 971 £7

0 233 232 23

1 767 7ie 21

2 1396 1370 36

3 180 172 28 599 26

315 26

351 28 595 30 386 32 126 35

191 12 119 63

307 11 379 13 25126

91117

106 "3

lib 69

261 12 367 37

182 16 288 31 158 31 620 29 825 30

17 59

63b 807

616

618

295

139

696

113

660 . 832 25

617 25

336 23

353 23

191 21 630 26

280 27

152 27 666 30 135 31 351 33 629 55

10 125 120

-5 276 211 I 106 72 265 31 15117

59 65 1151

156 61 101 59

151 11 227 51 107 62 137 62 323 17

206 70

-11 112 391 16 -13 125 10b 62 -12 93 113 93

-11 68 81117

-10 237 226 12

-9 172 166 55

-6 565 579 37

-7 93 117 be

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80 96 90 -12 380 369 "" 0 29" 301 9U -12 l23 112 66 2 75 b9l29 C 269 270 36 535 357 36 6.10*1. 8 16" 187 60 2 Z>*-r 237 59 -U 50 67136 " 9^ o7 269 273 39 -II 233 229 i*9 1 lOfl 100 72 -11 263 260 50 3 75 58130 1 110 97 56 -5 190 18S "2 -7 156 150 69 9 257 263 "9 5 16? 169 uu -10 360 151 "6 5 138 151 's 192 195 "6 -10 157 152 57 2 216 21" "8 -10 156 126 67 0'9«L 2 166 I75 •*6 -" "55 ""6 33 -6 182 169 62 10 131 200 61 " 501 50?i je -9 2"0 735 5J o i"5 2bl 5a 365 366 3o -9 278 27" "i 5 15l 157 G6 -9 l?a 131 65 1 201 188 35 3 236 2'»7 39 -3 35ft 359 52 -6 167 150 63 It 198 189 62 5 561 372 3». -8 f-^ 596 "i b'l3.L

18" 179 "a -e 279 289 "0 " 238 253 "8 -8 "57 "55 38 2 596 390 21 " 951 "65 3b -2 256 25" 25 -" 25" 299 50 5'11»L 6 155 169 5! -7 3»9 "02 "2 -5 81 =9119

389 37" 38 -7 397 "IS 35 5 198 201 55 -7 171 1 1 •! 63 3 551 5"3 18 5 382 37u 38 -1 91? 9ii 3" -3 282 272 "b -10 275 266 52 7 109 132 ''h -6 210 216 "7 -2 16- I79 h4

502 29" "1 -6 591 59" 33 6 357 352 H5 -b I61* ibO "9 " 138 133 "b 6 65 81112 0 8"" 836 33 -2 199 1"Q 65 -9 66 82103 S 9" ntt 6^ -5 331 321 39 -1 l5t> 15- 80 283 £72 "* -5 550 5"fl 31 7 230 235 56 -5 96 81 72 5 i6b 18" "2 7 205 210 "9 1 668 658 32 -I 369 39" it" -s 1 7e 192 62 9 l"n 1"6 6" -" 2"7 ?56 "2 0 13o 15b To 52" 329 96 -9 172 17" 26 6 307 311 55 -9 512 32" 26 6 295 3o0 26 9 202 216 52 2 588 57« 32 0 188 20" 57 -7 85 92103 10 2"0 260 53 -3 l77 1711 "7 1 122 llt> do

129 12" 81 -3 238 231 32 9 108 109109 -3 133 133 "9 7 309 299 37 9 "98 "55 "2 3 172 16" "1 1 71 81122 -6 227 22" "9 11 23« 255 53 -2 "8fl "Pi Jb 2 195 207 (,1 191 187 60 -2 67" 678 30 9-7iL -2 265 265 3" fl 468 "72 56 10 72 5912" " 328 3I6 3" 2 303 502 "b -5 250 25" "b 5.12.L -1 60 21 73 3 loO 6d

7" ""128 -1 569 568 26 -7 83 I0"120 -1 382 1*06 32 9 l23 llS 66 11 101 113102 5 270 260 36 3 209 205 55 -" 23J 235 52 -11 523 52" 50 0 15(1 159 59 U»l".l.

6. b»U 0 831 835 51 -6 231 213 53 0 578 5flO 51 10 "26 "25 90 fc.9»L 6 391 "00 35 9 316 521 "7 -5 Bb 64 76 -10 93 100100 1 2'*6 2"3 "8 J <:1" ZOh "2 189 163 61 1 397 3"3 28 -5 263 25" 50 I 3"6 35" 32 11 299 300 53 -10 2"8 291 55 7 2"7 2bfl "1 5 2"1 250 55 -2 983 "7Q 37 -9 "59 t*29 "2 ? "0" 591 3b i <.'9o 277 5u 302 298 98 2 939 9"2 33 -" 161 153 6" 2 992 506 31 12 199 192 68 -9 3"9 391 "6 8 ""1 955 37 (, 131 136 6" -1 322 32"' 39 -8 1"7 IU9 6= 3 259 ?"6 "0 2 57d 373 36 78 65106 3 956 999 29 -3 170 166 61 3 519 537 32 15 IbU 19" bZ -6 77 70111 9 196 20" 50 7 77 53132 0 309 507 36 -7 119 i?o 60 " "77 "?i. 37 3 Ibl l-ti 55 105 9" 69 " "91 523 30 -2 220 21" 52 " "17 "12 3^ 1-9. L -7 199 197 52 10 3"3 5f.l "9 9'10»U 1 167 161 "9 -6 «bb "51 3B 5 1°5 ?r9 "9 4 "52 M"L 57 103 109 82 5 2bl 272 35 -1 161 178 60 5 58» 25100 -15 330 31b 9U -fa "71 "73 59 11 7b 7"113 -5 135 105 77 2 "82 "66 5b -5 19« 179 U7 & 3^6 33? "0 3 199 197

328 323 30 6 538 569 33 0 l7i 161 58 6 7o 71 89 -12 .:57 236 51 -5 65 "8112 12 205 209 59 -" 7«« 36128 3 387 398 38 -" 37" 378 57 7 lufi 150 65 « m c.^ 't> ""0 "38 2S 7 170 170 95 I 212 215 53 7 1I6 116 63 -11 l7l lob 56 -u 521 330 29 5.I0.L -3 1S5 i7S 62 " 65 58109 -5 "17 "12 36 fl l^S 705 5" 7 137 135 7^ 163 176 9J 8 297 295 39 2 99u "97 "2 8 20? 210 53 -IJ 38b 373 90 -3 313 325 58 -12 3"i 53ft 98 -2 508 5li "5 5 320 330 "3 -2 3I-' 309 57 9 5"3 U5 3 12d 107 72 231 256 9^ 9 106 122 72 3 157 m? 65 9 65 51113 -9 300 298 "1 -2 79 67 S" -U 175 IR? 63 -1 120 ll* 93 6 116 115 68 -I l"" 1^9 "3 10 113 Q9 j ^lu J96 556 370 "0 10 106 11" 78 " 300 280 97 10 1?9 120 69 -a 960 "59 3b -1 219 237 "5 -10 91 105 97 0 199 191 59 7 2"2 263 96 0 353 559 37 n 173 173 (.7 |«q 7t5 63 12109 11 75 56107 5 329 310 "7 H "59 "77 "3 -7 b7o 568 35 0 213 21" 95 -9 "13 "09 "0 1 73 62126 8 599 "05 "" 1 7> 63 88 2'13.U l-l-.L

217 221 "7 12 196 195 66 6 92 6610" 12 1?2 112 82 -b 518 510 35 1 25l 273 "2 -8 171 159 55 2 luO 130 76 9 3"! 352 "6 2 "2" "19 35 -II 7u '^9i?7 -lo 212 22t> oi, 72 71103 15 135 130 78 7 79 85136 5,fl,L -b 957 mb 32 2 231 2U7 92 -7 160 12" 52 3 516 519 50 10 176 179 65 5 31" 522 S"! -IC '9 7R127 -4 159 150 ni> 258 269 "6 9.7iL 10. 7. L -12 "3 102111/ -h "72 "60 31 3 193 197 "5 -fa 66 lOS 8" " 271 278 52 fa-lLU " 23f- 272 "1 -° 229 2f"i 51 f<^ 30o 29" "o

"35 "28 92 -13 73 ""126 -U 1"7 1"3 75 -11 250 223 51 -3 iSb 150 "0 9 6" 18110 -5 195 192 "5 0-11. L -9 2b0 ZtZ 51 5 "67 "71 37 -a i-ii 93 fli, -7 jei 37e <»2 69 30U9 -12 73 66125 -3 121 115 83 -10 17S 16fl 60 -2 597 523 50 S 198 20" 50 -" "7" "69 25 1 778 77^ 27 -B 2?9 229 5" 6 fa5« 63ll? -7 2°? ?fl7 hu -b ^98 29C .5 76 75130 -11 379 377 "5 -2 223 2?" 56 -9 83 9810" -1 306 296 50 6 2"7 2"0 "6 -5 7" 55 79 2 36O 5bb 17 -7 230 23^ 51 7 18" 16Q 52 -6 3^9 '60 "0 —j 'jbb 57-* 3-* 366 5bb "7 -10 102 109 80 -1 77 91132 -6 206 299 "3 0 291 239 51 7 221 218 50 -2 57 uH 69 3 350 3m 33 -t 65 22119 fl 123 139 76 -5 6? initi6 c7i 2t,i. ^3

76 29131 -9 65 61112 0 196 I89 61 -7 5"7 357 39 1 Ibi/ 16? b3 8 292 259 52 -1 277 285 55 " i7" 154 "6 -5 169 169 58 9 7i "5122 -" 1'" 127 56 -J ^:1" tl" -3 9'6»L -8 199 127 53 1 2"5 2"2 55 -6 170 170 50 2 "12 "11 29 9 193 195 60 0 101 R6 59 5 227 22o 91 -" 155 157 60 10 "17 «i«9 "5 -3 3«" 372 57 -2 61 7c 311 289 50 -7 "63 "71 55 2 236 251 57 -5 2"5 2"8 "0 5 b6" 6"7 31 10 18" 161 63 1 628 629 32 6 210 20b "" -3 203 217 U9 11 236 2"9 "0 -2 l^l IS" "5 -I ^2o 212 232 221 53 -6 63 5" 95 3 161 177 72 -9 2^7 289 56 9 529 550 31 7.9«L 2 297 794 3" 7 507 "99 36 -2 201 209 35 ">I?.L -1 2''" 19? "2 j ^o'J ?5o -5 71« 13125 -5 186 190 "2 " 225 22" 57 -5 72 77 85 5 918 "19 32 -9 90 6"106 5 "03 "09 3" 8 2"9 255 "5 -1 329 55« "0 -10 252 239 55 P 161 153 "3 1 s92 533 37

70 39120 -" 558 565 2" 0'8»L -2 1<»1 197 no a 261 279 37 -8 292 291 51 " 167 172 "" 9 ^1*^ 2"9 59 0 1 13 111 72 -» 72» ?212" I 167 172 "9 2 »" 1" 209 196 55 -3 293 256 3" " 627 553 21 -1 "93 521 33 7 79 67 76 -7 112 92 80 5 "75 "81 3" 10 151 15b 73 1 23^ 239 "5 -8 185 202 61 2 l73 16« 50 5 ^"9 25i -J

332 315 "5 -2 101 93 5" 1 "0" 398 20 0 29" 306 35 3 217 2u9 U6 -6 218 226 51 6 27i 2fl« 39 n i96 202 65 2 207 212 "9 -7 126 127 7i 3 351 12° 38 - t05 W" -a 510 50" "1 -1 351 356 30 2 306 3C1 15 1 657 682 33 9 39b 920 38 -5 2b5 255 "6 7 598 37" 38 J2 61 "7 bb 3 519 5"5 39 -6 72 87109 " 1"6 i"0 "i" S 4:79 -5 70» 7b 85 0 233 2"5 33 3 556 3"5 21 2 68 86 89 10 95 85 fl" -" 165 160 55 fl 90 120 H8 I'lLL " 319 316 92 -5 19n 202 50 5 1^6« 165 53 o ..^l 268 275 "8 1 "10 "25 30 9 699 6"5 51 3 129 199 52 11 1"9 1"5 65 -3 38" 576 90 9 116 1 15 70 -12 153 135 95 5 220 2"1 S3 -" 151 136 63 6 ^--J 260 "5 7 jlo «i- <7 69 51116 2 393 909 30 5 159 155 "2 " lOfl 9" 65 12 326 338 33 -2 280 262 30 ID 3b5 386 "3 -n 316 31b "9 6 11 3 117 91 -3 21'- 211 "" 7 2'*9 ?"5 "7 3 <l35« 2jf *- 255 255 "9 3 915 "38 31 6 220 220 27 5 9I5 "0" 36 15 107 95 68 -1 3"fl 351 "0 II 252 256 51 -10 196 199 56 7 275 28" 99 -2 278 272 29 * l9<i 195 Cb 9 4*c ^sb

26" 267 "7 u 182 176 "1 7 13u 111 55 6 526 509 36 2r9tU 0 "29 •*"7 58 12 122 120 65 -9 10" 125 95 6 l"d 12" 71 -1 266 275 "1 9 226 '25 52 lU 1"1 l-' 292 261 "6 5 "98 "55 35 8 315 307 "3 7 117 120 72 -13 i78 163 66 1 172 192 55 ".lO.L -8 11" 12o 71 9 285 26" 52 0 371 379 38 10 135 115 75 2.1-.w

Ufa 65 81 6 3"6 3"6 3" 9 115 73 66 6 26" 2"fl "" -12 313 359 "9 2 65 "1109 -12 77 50152 -7 216 2Vb "" 7.I1.L 1 "80 "73 5fe 3'13.L -U' 72 --l?*

96 107107 7 928 """ 36 10 117 62 69 9 l"S 138 6" -11 l5" 155 63 5 101 100 76 -11 387 368 "6 -6 375 372 3b -p 95 95102 2 274 295 "1 -10 '9 7ii2i -9 olUJ

231 227 58 6 127 125 59 11 "61 i«6" "0 10 132 155 76 -10 258 256 "7 " 137 11*5 bb -10 lb7 lb? 62 -5 i27 131 58 -7 16" 155 66 3 212 210 "5 -9 l7(t i7n (.3 -j, ia2 59 10t6tL 9 163 165 51 12 85 77iO" 11 77 79125 -9 290 286 92 5 236 2"3 51 -9 2o7 199 5" -u i*07 i*09 3" -6 379 37" "S " 209 207 "7 -9 2U3 2'- "9 -7 7i* slll2

68 77106 to 301 309 9" 13 157 161 70 12 162 160 75 -a 160 180 "6 6 250 259 50 -fl 31" 315 92 -3 295 285 25 -S "29 "32 93 5 316 317 ui -7 1O2 176 55 -o 225 2-2 -lO 215 196 56 11 22b 225 53 l.ftiL 6.ftfL -7 2"3 237 59 7 279 255 "9 -7 250 256 "3 -2 670 659 26 -" "0" 397 i*3 6 172 19" 5" -6 5^0 39 -5 121 12- 73 155 13" 66 12 1"8 1"7 73 -13 "5" 425 95 -11 162 170 72 -6 56" 579 39 B 59" 585 "6 -6 208 208 "5 -1 300 297 5" -3 122 10" 71 7 219 2"5 51 -5 m 125 6" -u 5I0 5^3 29

179 171 61 13 77 61132 -12 136 1"5 7" -10 157 151 7o -5 68 65 86 9 136 190 63 -5 180 179 "8 0 267 269 35 -2 278 278 "b 8 16" 175 65 -" 5?2 322 29 -5 189 1^1 5t' 71 3" 87 5»7.L -11 189 192 5" -9 179 177 58 -" "36 "28 31 B.9.L -9 "58 "65 2b i 299 293 "0 -1 278 502 "b 9 2"=. 25" 51 -5 2"" 718 "7 3ow 39 111 127 92 -12 79 98130 -10 315 307 "1 -6 b8 "5117 -3 56" 358 51 -6 183 167 66 -3 507 517 3" 2 595 56d 32 0 87 66 89 10 79 67122 -? S'l 375 58 -1 J39 35* is 270 269 52 -11 225 217 5" -9 162 151 50 -7 5"7 3"5 *»1 -2 9"9 ""7 22 -7 103 105 97 -2 2"9 250 27 3 534 523 33 j 103 88 5'l?tL -1 1*" 18" "5 J "7|, U67 57 168 189 61 -10 129 121 66 -8 169 178 "9 -6 2''1 292 "2 -1 266 285 51 -b 231 216 5" -1 """ ""7 33 it 320 3"9 35 2 159 191 6" -If, 2"5 225 53 0 "9 97 79 1 18" 17o »b

163 1"9 66 -9 32" 317 «2 -7 255 25" 37 -5 121 130 65 0 185 178 36 -5 3"2 529 "5 0 263 270 37 5 "00 55 3 129 119 71 -9 l5l 125 79 1 188 179 <*b Z 3"3 35" -J 37" 57" "8 -6 212 217 t.7 -6 329 31" 55 -" 253 261 "0 I S55 550 30 -" 26" 250 "7 1 2"1 231 37 b 320 325 57 i* 3bO 570 «4 -8 ittH 19" 5-3 2 "67 1,71* 57 5 b7 361''- 76 7115" -7 63* 36106 -5 561 5«8 51 -3 135 13" 55 2 967 "62 30 -5 180 190 56 2 571 569 3" 7 "24 929 57 5 395 "11 "b -7 "15 "05 "2 3 1"? 1"1 57 it 209 2yf «7 0-7»L -6 330 355 37 -" 502 SO" 29 -2 2«7 30? 37 3 "62 "5" 5U -2 ZbS 267 "6 3 299 308 57 g 391 39^ 39 ^ 300 303 50 -6 lOO 9u 93 1, 93 pu 78 5 70 tIUIU

602 586 17 -5 228 231 39 -3 1"8 1"0 39 -1 132 199 56 4 105 106 58 -1 155 131139 " 575 575 55 9 i^u i5tj 6b 7 62 5"12" -5 207 20" 51 5 228 72^ "5 o l<ib Wb 52 286 276 1" -9 138 137 *9 -2 "16 "07 21 0 187 189 "3 5 209 212 38 0 2"6 2"5 "9 5 209 218 "6 10 160 169 75 8 9i 6"120 -" 22" 217 "9 b '59 2?a k6 7 <;23 2 U' in "8" "7" 16 -3 295 260 37 -1 512 506 28 1 2^5 259 "0 6 "95 523 54 1 66 67UH 6 265 501 "0 11 87 67102 6'11«L -3 165 172 56 7 i?9 329 u5 K i5B 1.^ "18 "10 21 -2 989 "99 52 0 370 352 27 2 107 71 7 313 329 3b 2 10" 119.66 7 99 _103 77 12 385 565 "6 -5 268 260 5" -2 157 ibl 57 t 56" 589 "2 9 7\i* 19i?i

391 392 30 -1 515 321 33 1 710 685 29 3 6" 91103 6 265 272 "0 5 302 307 "9 8 202 21" 50 2. ILL -" 1"5 12" 69 -1 211 210 "7 9 11" 117 si lu 9t> ^5:05

115 100 "9 0 332 329 32 2 393 385 29 " 189 199 "7 9 137 136 59 5 l"6 l"a 69 9 169 178 60 -12 8" 115122 -3 71 3" 8b 0 127 ISi 6" 10 lft9 701 6" J«1".L

5"0 591 3" 1 107 96 55 3 3"3 332 29 5 319 511 39 10 152 163 62 6 185 183 65 10 55? 5"ft "7 -11 175 i7b 62 -2 81 82X10 1 395 59o 59 9'15'L "13 -20 ib 98 80 63 2 688 750 33 " 217 216 39 6 27" 261 "3 n 2"2 259 35 7 305 307 50 11 196 216 61 -10 296 2"b 50 -1 "09 "06 "3 2 179 164 "o -10 239 22"' 112 lib 89

161 170 "5 5 281 287 35 5 396 "00 51 7 227 235 "9 12 291 302 35 8 285 27" 55 S-lOtL -9 508 505 "1 0 232 252 59 3 128 l"l b5 -o '?• "3124 -7 217 221 53

248 296 "9 4 242 231 38 6 793 775 35 6 18" 191 58 13 186 200 "6 9.9»U -11 lfl7 195 bb -8 215 204 i»b 1 177 182 62 9 77 76 95 -8 215 219 ^5 -h 87 7b 9^

392 380 42 5 235 231 41 7 163 166 50 9 155 158 69 3»9»L -6 192 141 76 -10 131 105 81 -7 319 312 39 2 112 154 89 5 225 214 "9 -7 "5 up, Ob <;09 201 bl

124 128 79 6 256 256 "0 8 267 261 39 10 97 103106 -12 27m 2b" 51 -t5 410 399 96 -9 70 "5120 -6 108 99 63 3 190 186 56 6 995 501 41 -6 172 159 58 -4 2"" 232 "5

93 9"116 7 329 538 90 9 261 290 39 11 195 195 95 -11 l7g 173 59 -" 432 "19 32 -S 382 577 42 -5 365 3bO 55 " 190 19" 62 7 I5b 125 7o -5 237 336 "8 -5 eW 21e

60 44137 8 410 416 40 10 263 285 42 7,b,i. -10 69 56119 -5 2B0 265 50 -7 374 562 41 -u 59» 52102 5 93 67119 6 7i« "8122 -u 27fl 271 Si -2 t6b 257 43

I.7.L 9 260 262 47 U 201 20=; 51 -10 14" 1"1 77 -9 i7o 171 56 -2 1"3 146 7i -6 126 121 6b -3 356 551 3" 6 155 161 7b 9 l"b 135 70 -3 2^0 190 "8 -i "6" 4b7 30 2"6 234 55 10 325 521 "" 12 91 74 9" -9 378 37u "7 -8 256 262 "2 -1 l7i 186 66 -5 390 563 39 -2 266 299 25 9.11'L 6"12'L -2 l°2 1^2 "7 0 i2b 128 ob

69 66107 11 116 122 86 15 175 168 69 -6 5*^1 399 "6 -7 61 3"106 0 24u 257 57 150 152 3ft -1 590 395 33 -3 8" 85115 -9 9S B^m -1 i53 135 45 1 32" 321 Hu

171 166 59 12 209 203 61 2.etL -7 25l 248 "9 -6 455 "36 35 1 196 203 62 -5 433 443 36 0 86 84 7i -2 91 60115 -7 52 5 523 "9 n 65 5«10B 2 229 23" "7

236 23" "7 6<7.L -13 530 309 "8 -6 7o 65120 -5 397 389 33 2 170 160 65 -2 "52 "61 26 1 255 256 36 -1 199 202 60 -b l7l 16" 66 1 139 (36 57 5 1O5 100 73 149 152 57 -11 158 195 69 -12 217 205 5" -5 262 257 45 -9 76 67 52 3 72 52125 -I 197 205 "3 2 29S 290 55 0 222 221 56 -5 115 155 8C 2 17^ 171 a 199 192 52

. 308 298 36 -10 267 255 49 -11 178 168 56 -" 140 129 59 -3 bSO 550 32 4 275 276 52 0 215 2P9 48 3 532 520 55 1 177 193 65 -" 107 91 77 3 z7<* ?7i -u 5 b79 592 "0 259 251 35 -9 110 109 61 -10 135 127 64 -5 5S9 576 59 -2 493 501 25 5 130 128 68 1 212 220 45 " 160 151 "6 2 305 520 51 -3 "63 458 "0 " 219 71t si o 21" IB' 5ii

. 620 610 51 -6 199 185 51 -9 251 252 93 -2 296 301 29 -I 590 338 31 ;0.9»L 2 16" 172 "fl 5 246 2"" "0 3 169 185 69 -2 557 3"4 42 5 139 l"7 f,^ 7 3b5 "b

71 56 74 -7 243 227 U" -9 95 85 73 -1 2"8 263 "2 0 525 330 52 -I 112 116105 3 296 29; 38 6 86 80 7b 0.12>L -1 155 159 59 6 132 lu:* --o a 73 t>3l2b 386 365 28 -6 161 165 57 -7 299 2"9 39 0 251 259 "2 I 17" l7l "0 0 225 221 59 " 273 2«" "1 7 53" 332 38 0 237 248 45 0 159 155 60 7 i^Q 165 -^i 9 147 175 77

I 309 295 20 -5 359 172 36 -6 290 267 55 1 100 107 7B 2 565 550 31 O'lO.L ' 5 229 216 49 9 147 16b 60 1 491 486 27 1 360 365 91 8 1»>5 177 m u.i".L

920 90" 26 -4 74 61 97 -5 267 266 59 2 323 33" "0 3 263 252 55 0 568 997 21 6 "60 985 59 9 "14 423 "I 2 239 233 19 3 205 190 50 « «" 751^2 -9 ibd 171 7o 125 121 90 -5 2"0 2"9 39 -" 262 263 33 5 "9i 500 58 4 309 315 59 1 366 359 33 7 181 194 54 10 162 160 62 5 2"1 226 56 5 157 i29 66 5'13'L

I 362 356 26 -2 286 296 36 -3 212 219 3" " 137 137 63 5 305 306 35 2 97a 465 50 8 199 207 54 11 273 284 50 4 317 311 40 " 155 157 6? -9 2^*" lO"" 61 369 356 28 -1 162 l76 97 -2 515 511 22 5 57" 566 "2 b 210 232 42 3 955 457 17 9 217 239 56 12 192 195 63 5 308 302 37 5 32^ 3"0 Ub -9 2*^9 2S7 «! 899 00" 31 0 134 152 52 -1 161 166 37 6 196 206 57 7 39i "27 56 4 845 836 28 10 219 230 58 3-11. L 6 1"" 125 60 b 7fc 72i52 -7 255 262 52

I 599 599 26 1 266 292 39 0 231 250 30 ? 221 220 56 8 "78 515 37 5 238 246 98 11 296 259 57 -11 96 124104 7 298 300 "2 7 7ft 9fli3(i -b '1 59119

I 119 95 99 2 260 267 30 1 370 365 28 0 135 193 82 9 l"b 150 61 6 222 233 91 -10 01 93106 ft 192 185 39 8 169 183 69 -5 205 509 47

Table 3. Distances and angles in the Ca^

distance, 1 distance, J atoms pr anc;,;e, deg. atoms or angle, deg . Cation environments Ca(8), 0(2) 2 217 3) Ca(l) 0(1) 2 214 (3) Ca(8), 0(43) 2 394 3) Ca(l) 0(42) 2 .384(3) Ca(8), 0(23) 2 414 4) Ca(l) 0(21) 2 412 (3) Ca(8), 0(31) 2 428 3) Ca(l) 0(14) 2 415(3) Ca(8), 0(24) 2 440 4) Ca(l) 0(44) 2 416 (4) Ca(8), 0(11) 2 488 3) Ca(l) 0(34) 2 474(4) Ca(8), 0(33) 2 892 4) Ca{l) 0(22) 2 788 (4) Ca(8), 0(12) 3 092 (4) Ca(2) 0(1) 2 251(3) PO^ groups

Ca(2), 0(14) 2 360(3) P(l), 0(11) 1 531 4) Ca(2) 0(23) 2 403(3) P(l), 0(12) 1 527 4) Ca(2)

,

0(34) 2 534(4) P(l), 0(13) 1 539 (4) Ca(2)

,

0(41) 2 550(4) P(l), 0(14) 1 534 3) Ca(2)

,

0(21) 2 567 (3) 0(11), 0(12) 2 533 (5) Ca(2)

,

0(42) 2 613(4) 0(11), 0(13) 2 471 5) Ca(3)

,

0(2) 2 136 (4) 0(11), 0(14) 2 461 5) Ca(3). 0(31) 2. 399(4) 0(12), 0(13) 2 459 5) Ca(3)

,

0(22) 2. 515(4) 0(12), 0(14) 2 512 6) Ca(3)

,

0(12) 2. 546 (3) 0(13), 0(14) 2 579 5) Ca(3)

,

0(23) 2.615(4) 0(11), P(l), 0(12) 111.8 (2) Ca(3) 0(13) 2 .721(3) 0(11), P(l), 0(13) 107.2 2) Ca(3) 0(34) 2 .755(4) 0(11), P(l), 0(14) 106.8 2) Ca(4) 0(1) 2.277(3) 0(12), P(l), 0(13) 106.6 2) Ca(4) 0(13) 2 .383(4) 0(12), P(l), 0(14) 110.3 2) Ca(4) , 0(43) 2 .389(4) 0(13), P(l), 0(14) 114.1 2) Ca(4) 0(24) 2 429(3) P(2), 0(21) 1 547 4) Ca(4) 0(12) 2.441(3) P(2), 0(22) 1 537 4) Ca(4) 0(33) 2 .570(4) P(2), 0(23) 1 553 (4) Ca{4) 0(32) 2.608(4) P(2), 0(24) 1 511 4) Ca(5) 0(2) 2 238(3) 0(21), 0(22) 2 478 5) Ca(5) 0(33) 2 307(4) 0(21), 0(23) 2 462 (5) Ca(5) 0(32) 2 359(3) 0(21), 0(24) 2 542 (4) Ca(5) 0(21) 2 376 (4) 0(22), 0(23) 2 557 5) Ca(5) 0(41) 2 639(3) 0(22), 0(24) 2 533 (5) Ca(5) 0(12) 2 774(4) 0(23), 0(24) 2 479 5) Ca(5) 0(44) 2 829(3) 0(21), P(2), 0(22) 107.0 2) Ca(6) 0(1) 2 193(4) 0(21), P(2), 0(23) 105.2 2) Ca(6) 0(11) 2 381(3) 0(21), P(2), 0(24) 112.5 (2) Ca(6) 0(44) 2 404(3) 0(22), P(2), 0(23) 111.7 2) Ca(6) 0(41) 2 404(3) 0(22), P(2), 0(24) 112.4 2) Ca(e) 0(14) 2.652 (4) 0(23), P(2), 0(24) 108.0 (2) ca (6) 0(34) 2 706 (4) P(3), 0(31) 1 543 3) Ca (6) 0(32) 2 713(3) P(3), 0(32) 1 521 (4) Ca(6) 0(33) 2 996 (4) P(3), 0(33) . 1 524 (4) Ca(7)

,

0(2) 2 174(3) P(3), 0(34) 1 550 4) Ca(7)

,

0(31) 2 345(3) 0(31), 0(32) 2 572 (5) Ca(7)

,

0(11) 2 425(3) 0(31), 0(33) 2 506 5) Ca(7) 0(22) 2 446 (4) 0(31), 0(34) 2 489 5) Ca(7) 0(13) 2 563 (4) 0(32), 0(33) 2.431 5) Ca(7) 0(43) 2 576(3) 0(32), 0(34) 2 498 5) Ca{7) 0(42) 2.639(4) 0(33), 0(34) 2 535 (5)

'^) 2^ structure

distance, J distance, Jl atoms or angle, deg . ^^oms or angle, deg . 0(31) , P(3) . 0(32) 114.1(2) > 0(13) 0(31) , P(3), 0(33) 109.6 (2)

, Ca(4) 2 .383(4) 0(31) , P(3) , 0(34) 107.2 (2)

, Ca ( 7

)

2 .563 (4) 0(32) , P(3) , 0(33) 106 .0 (2) 0(13) , Ca(3) 2 .721(3) 0(32) , P(3) , 0(34) 108.9(2) 0 (14) 1 Ca(2) 2 .360(3) 0(33), P(3) , 0(34) 111.1(2) 0 ( 14) » Ca ( 1

)

2 .415(3)

P(4) , 0(41) 1.537(3) I 0 (14) , Ca(6) 2 .652 (4) P(4) , 0(42) 1.547(4)

0(21) , Ca(5) 2 .376 (4) P(4), 0(43) 1.549(3)

0(21) > Ca(l) 2 .412(3)

P(4) , 0(44) 1.532 (4) 0(21) Ca (2) 2 ,567 (3) 0(41) , 0(42) 2.513(5)

0 (22

)

Ca ( 7

)

2 .446 (4) 0(41) , 0(43) 2. 567 (5) 0(22) Ca ( 3

)

2 .515(4) 0(41), 0(44) 2.491(5)

0 (22

)

Ca ( 1

)

2 .788(4) 0(42) , 0(43) 2.462 (5) 0 (23) Ca(2) 2 .403 (3) 0(42) , 0(44) 2.515(5) 0(23) Ca (8) 2 .414(4) 0(43), 0(44) 2.550(5) 0(23) Ca(3) 2 615(4)

0(41), P(4), 0(42) 109.1(2)

0 (24) Ca(4) 2 429(3)

0(41) , P(4) , 0(43) 112.6 (2) 0(24) Ca (8) 2 440(4)

0(41), P(4), 0(44) 108.5(2) 0(31), Ca(7) 2 345(3) 0(42), P(4), 0(43) 105.3(2) 0(31) Ca ( 3) 2 399(4) 0(42) , P(4), 0(44) 109.5(2) 0(31)

.

Ca(8) 2 428(3)

0(43) , P(4) , 0(44) 111.7(2) 0 (32)

,

Ca ( 5) 2 359 (3) Oxygen atom environments 0(32) , Ca (4) 2 608(4)

(i) oxide ions 0(32) , Ca (6

)

2 713(3)

0(1), Ca(6) 2.173(4) I 0(33) , Ca (5) 2 307 (4) 0(1), Ca(l) 2.214(3) 0(33) , Ca(4) 2 570(4) 0(1), Ca(2) 2.251(3) 0(33) , Ca (8) 2 892 (4) 0(1), Ca(4) 2.277(3) 0(33) , Ca(6) 2

.

996 (4) 0(2), Ca(3) 2.136 (4) 0 (34)

,

Ca (1) 2 474(4)

0(2), Ca(7) 2.174(3) 0(34) , Ca (2) 2 . 534 (4) 0(2) , Ca(8) 2.217(3) 0(34) , Ca (6) 2 . 706 (4) 0(2), Ca(5) 2.238(3) 0(34) , Ca(3) 2 . 755(4)

Ca(6) 0(1) 0(41) , Ca(6) 2 . 404 (3) Ca(l) 113.4(1) > Ca(6) 0(1) 0(41) , Ca(2) 2 . 550(4)

Ca(2) 103.0(1)

Ca(6) 0(1) , Ca(4) 0(41) , Ca(5) 2 . 639(3)

102.3(1)

0 (42) , Ca(l) 0(1) , Ca(l) 2 . 384(3) Ca(2) 105.6 (1) Ca(l) , 0(1) , 0(42) , Ca(2) 2 . 613 (4) Ca(4) 115.1(1)

Ca(2) , 0(1) , 0(42) , Ca(7) 2 . 639(4)

Ca(4) 117.0 (1) 0(43) , Ca(3) , 0(2) , Ca(7) Ca(4) 2 . 389(4) 102.1(1)

Ca(3) , 0(2) , 0(43) , Ca(8) 2 . 394(3) Ca(8) 109.0 (1) 0(43) , Ca(3) , 0(2) , Ca(7) 2. 576 (3) Ca(5) 106.6 (1) 0 (44)

,

Ca(7) , 0(2) , Ca(6) 2. 404 (3) Ca(8) 100.0 (1) 0(44) , Ca(7) , 0(2) , Ca(l) 2 . 416 (4) Ca(5) 128.6 (2) Ca(8) , 0(2) , 0(44) , Ca(5) 2 . 829(3)

Ca(5) 109.5 (1) (ii) POx oxygens

0(11), Ca(6) 2.381(3) 5 0(11), Ca(7) 2.425(3)

0(11), Ca(B) 2.488(3)

0(12), Ca(4) 2.441(3)

0(12), Ca(3) 2. 546(3) 0(12), Ca(5) 2.774(4)

The figures in parentheses are the standard deviations in the last significant digit.
Table 4

A Comparison of the Unit Cells of

Ca^(P0^)30H(I) and Ca^ (PO^) 20(11)

I II

a 9.432 A 9.462 A

b 9.432 , 11.965

c 6.881 7.023

Y

120O 90.8"'

vol. 530 A^ 795 A"^ space group P6^/m P2.

d^QQ(I) X 1.5 = 12.27 A

530 P X 1.5 = 795 A"^
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THE REACTION OF DICALCIUM PHOSPHATE DIHYDRATE

WITH FLUORIDE

By

L. C. Chow and W. E. Brown

INTRODUCTION

The reaction of hydroxyapatite (OHAp) , Cag (PO4) 3OH, with a fluoride solution to form fluorapatite (FAp) , Ca5(P04,)3F,

is used extensively to reduce dental caries, but it proceeds

slowly with poor yields. As a result, the amount of fluoride

permanently taken up by enamel after a topical treatment is

low; furthermore, the depth of fluoride penetration into

enamel is small. Recent work''' has shown that tooth enamel

acquired larger amounts of fluoride when pretreated with

dilute phosphoric acid before being exposed to the fluoridecontaining solution; the deposited fluoride in this process

seemed to be retained more permanently. The mechanism behind

this effect has not been reported. As described below, it is

believed that the intermediate formation of an acid calcium

phosphate and its subsequent conversion to FAp and/or CaFg

may be a major factor in this improvement.

One would expect from the phase diagram for the system

Ca (OH) s-HaPO^-HsO^ • ^ that dicalcium phosphate dihydrate

(DCPD) , CaHP04*2H20, or dicalcium phosphate anhydrous (DCPA)

,

CaHPO^, would form when tooth enamel is treated with

saturated DCPD solutions or with phosphoric
-2-

acid more concentrated than approximately 3.4 x 10" ''^M and

less concentrated than about 3 M. Of these two dicalcium

phosphates, DCPA is the more stable and would be the final

equilibrium product in the acid range of concentrations.

However, DCPD is known to be capable of forming more rapidly

than DCPA and to persist for relatively long periods at

ambient temperatures. Thus, DCPD is expected to form on

tooth enamel when the acidic solution is applied topically.

In support of this, a number of studies have shown that DCPD

is produced by treating OHAp or tooth enamel with acids.

Gray and Francis'^ reported the existence of DCPD on the

surface of enamel samples equilibrated with weak acid

buffers; McCann and Brudevold^ showed that DCPD is formed

by shaking OHAp in a 0.5 M phosphoric acid solution. More

recently, Patel and Brown'^ found DCPD crystals when powdered

tooth enamel was equilibrated with 0.02 M phosphoric acid;

LeGeros and Klein'' reported the growth of DCPD crystals on

teeth immersed in phosphate solutions at pH range of 3 to 7.

In view of these findings, it appears that DCPD is likely to

form when tooth enamel receives the presently used 0.05 M

phosphoric acid pretreatment during the topical application.
-3-

The reaction of OHAp with fluoride to form FAp takes

place with virtually no change in crystal volume because

the two compounds are isostructural . As a result, the OHAp

crystal may become covered with a thin layer of FAp coating.

Further reaction would thus require the diffusion of F~ and

OH" ions through the solid. This would properly be described

as a solid-state reaction and one would expect it to take

place much more slowly than diffusion in a liquid. For this

reason the conversion of the interior of OHAp crystals into

FAp seems far too slow to occur to the desired extent in a

clinical situation. In contrast to OHAp, the reaction of

fluoride with DCPD proceeds quite rapidly because the loss

of water and phosphate ions inherent to the reaction results

in a shrinkage in the volume of the crystal. The F~ ions

may, therefore, penetrate into the crystal through the openings

produced by the shrinkage.

Since DCPD can easily be formed in

enamel and it reacts rapidly with fluoride, we propose

that FAp may be formed more efficiently from OHAp in

tooth enamel through the path,

OHAp 1 DCPD > FAp . ^ pre treatment ^ ^

Accordingly, OHAp would first be partially converted to DCPD

by an acidic pretreatment which then reacts with fluoride to

form FAp.
_4-

In preliminary experiments® designed to study the

feasibility of this proposed path, extracted human tooth

enamel was pretreated with saturated DCPD solutions (to

produce some DCPD on enamel) before being exposed to the

fluoride solution. The F deposition in this process was

found to be consistently greater than that from sodium

fluoride or acidulated phosphate-fluoride treatments

alone. A quantitative study on the formation of DCPD from

enamel will be reported in a subsequent paper. The present

work deals with the second step in the proposed path, i.e.,

the reaction of DCPD with fluoride.

Although the reaction of DCPD with fluoride is known

to proceed rapidly, there is uncertainty as to the products

formed. McCann and Brudevold^ reported that OHAp was

formed when DCPD was equilibrated for ten weeks at 34 °C

in 0.02 M iminazol buffers (pH = 6.2) containing 0.01 to

0.5 mM of F~ ions. lizuka and Ohashi^ , on the other hand,

concluded that DCPD appeared to be unchanged under similar

conditions when F~ concentrations were below 10 ppm

(0.526 mM) . Duff'" , in a more recent work, calculated

the free energies of formation of FAp, OHAp and DCPD at
-5-

25°C assiuning that complete conversion of DCPD to FAp

was attainable by equilibrating DCPD with 0.1 M KF

solution. In addition to apatite, CaPg is also likely

to be a product of this reaction particularly under

high F" concentrations.

In order to obtain quantitative information concerning the products, the reactions of DCPD with solutions

containing systematically varied amounts of F~ and P04^"

ions were studied. In view of the activity products of

the two most probable products, i.e., Ca5(P04,)3F and

CaFs/ the activities of the F~ and P04^~ ions are

thought to be the major factors governing the choice

between the possible reaction paths. If the effectiveness of a topical agent is proportional to its ability

to incorporate fluoride as fluorapatite in enamel''" '" , the

results obtained in this study may provide information

needed in searching for the most appropriate fluoride

treatment when some DCPD has been produced by a prior

treatment in the topical application.
-5aEXPERIMENTAL

DCPD was prepared by the method described by Moreno,

Brown and Osborn''" ^ . Chemical analysis of this salt

yielded (vs. theoretical) Ca, 23.1% (23.3) and P, 18.0%

(18.0) . Petrographically the material appeared as pure,

homogeneous DCPD. The sodium fluoride and trisodium

phosphate reagents were prepared from high purity commercial products; they were used without further purification.

Solutions containing various concentrations of NaF and

NasPO^ were prepared by either adding appropriate amounts

of the solid materials to or diluting the stock solutions

of NaF and Na3P04 with COg-free distilled water. Figure 1

shows the compositions of the solutions prepared.

50 ml aliquots of the solutions were transferred to

12 5 ml polypropylene Erlenmyer flasks; the flasks were

capped with '
-6-

polyethylene stoppers and then placed in a water bath kept

at 2 5 ± 0.1°C. After the solutions reached bath temperature,

0.5 g portions of DCPD were added to each flask. The samples

were agitated continuously by a shaking mechanism. The pH

of each solution was measured twice daily and the equilibration process was continued until the change was less than

0.02 pH units per day. The solid and liquid portions of the

samples were then separated by filtration. The filtrates

were subjected to final pH measurement and chemical analyses

for P and F. Ca analysis was not done because many of the

solutions were extremely low in Ca (less than 10"^ M) and

were highly concentrated with respect to P and F which interfere with the Ca determination. The solid portions of the

samples were washed briefly in distilled water and in acetone

and were then air dried. They were then characterized by

powder X-ray diffractometry and petrographic examination and

analyzed for Ca, P and F.

It was observed, in several cases, that there is an

appreciable change in the solution pH between the initial

and final states of the reaction. For these cases, a duplicate reaction was carried out in which the pH was monitored

continuously. A representative recording of pH versus time

(for sample No. 24) is shown in Figure 3. An interpretation

of the pH change as related to the progress of reaction is

given in the discussion section.

-7-

The pH measurements were made to within 0.01 pH unit

with a combination electrode and a pH meter with a readability of 0.001 pH unit. Ca was determined by atomic

absorption^ ^ with an estimated relative error of 0.02 .

P analyses (relative error 0.015) were carried out using

the vanado-molybdate reagent of Brabson, et al."""*. F was

determined (relative error 0.02) by the method of McCann^^.

Petrographic examinations were made with a standard polarizing microscope. X-ray diffraction was carried out on a diffractometer with CuKa radiation filtered through Ni foil.

RESULTS

Table I shows the initial and final conditions of the liquids.

These include the pH, P and F concentrations. The Ca, P and

F contents of the solids are listed in Table II; also shown

in this table are the Ca/P and Ca/F molar ratios and the

major solid phases of the samples as identified .by X-ray

diffraction and/or petrographic examination. Solids precipitated from solutions that were high in sodium concentration

and, as a result, highly alkaline (e.g., Nos. 13 and 19) were

found to contain small amounts of sodium (less than 3 wt.%)

.

Infrared spectra of these samples also showed detectable

carbonate peaks at 1440 and 670 cm"''" . It appears that CO2

still entered the system under highly alkaline conditions
-8-

despite the use of COs-free distilled water and closed reaction vessels. (The plastic used in these vessels is not

totally impervious to CO2.) The manner of incorporation of

sodium and carbonate in the solids is not known, and their

presence is neglected in the subsequent discussion.

It is seen from Table II that fluoride was incorporated

into the solids in all cases. Within each group of constant

initial F~ concentration (e.g., Nos. 2, 3, 4, 5 and 6), the

amount of fluoride in the solids increased with decreasing

initial Na3P04 concentration in the solution. The phosphate

content of the solids, on the other hand, decreased under

the same circumstances. Within each group of samples with

a given initial NasPO* concentration (e.g., Nos. 2, 8, 14

and 20) , the amount of fluoride incorporated decreased with

decreasing F~ concentration in the solution; the phosphate

content again showed a trend opposite to that of fluoride.

DISCUSSION

Since none of the samples was found to possess Ca/P and Ca/F

molar ratios close to those of CaF^t FAp or OHAp, they

appeared to be mixtures of compounds rather than pure substances. While the major phases of the samples were

qualitatively identified as shown in Table II, the amounts

present were estimated quantitatively from the Ca/P and Ca/F

values. Ca/P and Ca/F ratios, representing two independent
_9-

pieces of information on sample composition, permitted the

construction of two simultaneous mass balance equations.

Therefore, it was necessary to make an assumption that no

more than three solid phases were present in any given sample

.

DCPD is a possible phase in all cases because complete

'conversion of DCPD is not expected.

For the remaining two phases the following combinations are

possible: (i) FAp and CaFs and (ii) FAp and OHAp. The solubility behaviors of OHAp and CaF2 are such that they are not

likely to precipitate from the same solution either simultaneously or subsequently. Therefore, the possibility of having

both OHAp and CaFg present can be eliminated. In the

first case where the solids are assumed to consist of FAp,

CaFs and DCPD the mass balance of Ca, P and F can be expressed

by the following two equations:

X , 5Y 1-X-Y

+ r;^-r; +

^ ^-"-CaF. ^-"-FAp ^-"OCPD

P 3Y ^ 1-X-Y

FAp DCPD

X , 5Y , 1-X-Y

+ ~^ 7^ V CaFs FAp DCPD — = 2x ~ Y

F W F W

''•''•CaFs * FAp

where X, Y and 1-X-Y, respectively, are the numbers of grams
-10-

of CaFg/ FAp and DCPD present in one gram, of sample;

^•"•CaF. = ^8.08, F.W.p^p = 504.4 and F.W.j^^^ = 136.06

are the formular weights of the respective compounds.

In the case where OHAp, FAp and DCPD are present, the

following two equations can be set up using previously defined symbols except that X and ^-^'opiAp ~ 502.4 now denote

the corresponding quantities for OHAp:

5X 5Y 1-X-Y

PW PW PW

^•'^•OHAp ^-^TAp ^-^-DCPD

"p" 3X 3Y . 1-X-Y

FW FW FW ^•^•OHAp ^•'^•FAp ^-^-DCPD

5X . 5Y . 1-X-Y

+

F W ' F W F W

^

''•''•OHAp '^•''•FAp ^•'^•DCPD

^

FAp

The Ca/P and Ca/F values of the samples were substituted into

both sets of the two simultaneous equations to solve for X

and Y. As expected, only one of the two sets of equations

was found to be applicable to any given sample, i.e., to

yield positive values for the quantities X, Y and 1-X-Y. The

computed percentage compositions of the samples are shown in

the last four columns in Table II. A graphical presentation

showing these estimated sample compositions and the initial

experimental condition, as listed in Table I, is given in

Figure 2
-11-

The principal conclusions of this study are made

evident by inspection of Figure 2. When PO*^" ions are not

added to the system, the formation of FAp over CaFg is

favored only at low F~ concentrations (10^ ppm or less)

.

CaFg is the predominant product at higher F~ concentrations;

this observation is in harmony with the findings of several

investigations^ ^ ^ that CaFs was the ma,jor product formed

from the reaction of tooth enamel or dentin with APF* or

2% NaF solutions. The introduction of P04^~ ions stabilizes

the formation of apatite and thus suppresses CaFg precipitation. As a result, in the presence of Na3P04 (at the

concentration of 0.05 M or higher), FAp can be produced from

more concentrated (10* ppm) F~ solutions. Under -similar

Na3P04 concentrations, partially fluoridated hydroxyapatite

is formed from solutions with lower F~ concentrations. The

apatite materials formed in the more concentrated P04^~ and

F~ solutions are generally in poorer crystalline form than

are the materials formed from dilute solutions- This is

probably because the former were precipitated under more

alkaline conditions.

I

* 1.23% fluoride from NaF and HF in a 0.1 M phosphoric acid

solution (Acidulated Phosphate Fluoride)

.
-12-

The calculations based on the chemical analyses suggest

that small amounts of DCPD are present in most of the samples.

The residual DCPD could be due to the incompleteness of conversion, but the possible substitution of sodium for calcium

in the solids, producing lower Ca/P values, may account for

a portion of the calculated DCPD presence. The presence of

DCPD in the samples does not seem to mask the clear trends of

FAp and CaFg formation discussed in the preceeding section.

In some cases (as shown in Table I) there is an appreciable change in the solution pH between the initial and final

states of the reaction. In these cases, the change in pH may

therefore provide some information on the progress of reaction.

Shown in Figure 3 (curve a) is the pH change versus time

for sample No. 24. The product formed in this reaction was

predominantly FAp as indicated in Table II and Figure 2.

Since FAp is a more basic salt than DCPD, the drop of pH (from

7 to 4.3) is expected. There is an initial surge of pH from

7 to 7.5 which then falls back to 7.3. This is believed to

be due to the rapid dissolution of DCPD before the precipitation of FAp begins. Following this initial change the pH

remained at 7.3 for more than 1.5 hours before going down

steadily toward the final reading of 4.3. In a separate run

of a similar experiment (0.5 g of DCPD was placed in 500 ml
-13-

of solution containing 500 ppm F~) the solid portion was

separated from the solution at the time when pH was at the

plateau. Petrographic examination of the solid indicated

that CaFs was present in the form of small granuals deposited on essentially unaltered DCPD crystals. The x-ray

diffraction pattern showed that this material was predominantly DCPD with detectable CaFs peaks at 28.3° and 47° 29.

No FAp appeared to have formed at this stage. These

observations suggest that there is a rather long

induction period during which nuclei of FAp are formed with

CaF2 precipitating at the same time.

Duff"""*^ carried out a similar experiment in which DCPD

was added to a 0.1 M KF solution. It can be seen in Table I of Reference ID that a pattern of pH changes similar to the

one reported here was observed; the pH increased initially

to 7.30 and remained at this value for one hour followed

by a steady decrease to the final value of 3.14. An examination of the data given in this table suggested that calcium

fluoride may have formed initially as indicated by the

relative amounts of fluoride and calcium ions removed from

the solution. The ratio of F/Ca removed from the solution

is 1.89 during the first stage (when pH was kept constant at

7.3) of the reaction suggesting the predominance of CaFp

formation. The ratio decreased with decreasing pH and

finally reached 0.251.
-14-

(It may be adjusted to 0.248 if one assumes that all the

potassium ions lost from the solution went on to the calcium

sites in apatite.) Since the formation of pure FAp would

result in a ratio of 0.200, the final product appears (as

estimated from the analytical data for the liquid) to be a

mixture of predominantly FAp with some CaFg. The first order

fluoride uptake and the second order phosphate release reported in Reference 10 may possibly result from the initial

formation of CaFsAfter the induction period (Fig. 3) , FAp seemed to

precipitate out rapidly (as indicated by the steady falling

of pH) . In a separate run of a similar experiment, a small

quantity of OHAp was mixed with DCPD before the addition of

the solid into the solution. The pH change of this reaction

(Fig. 3, curve b) showed no apparent induction period

suggesting that OHAp acts as a template for the growth of

FAp crystals.

FAp is very possibly a more desirable reservoir of

fluoride than CaFg in the oral environment because the latter

is thought to be more soluble in saliva and too quickly lost

by dissolution to provide lasting benefits. Although this

view remains unproven, it appears reasonable and seems to be

widely accepted. The findings reported here suggest that the

formation of CaFg in topical treatments can be reduced sub-
-15-

stantially if a solution containing appropriate amounts of

F~ and P04^~ ions, as indicated in Figure 2, is used. When

some DCPD has been formed by a pretreatment, a dilute (10^ ppm

or less) fluoride solution should produce FAp on tooth enamel

without forming appreciable amounts of CaFg. However, it

appears advantageous to use more concentrated fluoridephosphate solutions, which still produce FAp instead of CaFs,

because they may additionally provide greater fluoride penetration. In some cases, CaFg may form initially partly

because at that time the F~ concentration is high. FAp,

which is the more stable product under the equilibrium

situation, would precipitate after the induction period is

over; this causes the redissolution of CaFg. The initial

precipitation of CaFg is reduced by the presence of apatite

crystals which seem to shorten the induction period and to

act as templates for the formation of FAp.. This effect

parallels that of excess P04^~ ions in solution which

stabilize the formation of apatite over CaFg.

References

1. Aasenden, R-, Brudevold, F. and McCann, H. G. : The

Response of Intact and Experimentally Altered Human

Enamel to Topical Fluoride, Arch Oral Biol 13 : 543-

552, 1968.

2. Moreno, E. C, Gregory, T. M. and Brown, W. E.:

Preparation and Solubility of Hydroxyapatite , J Res NBS,

72A:773-782, 1968.

3. Farr, T. D. , Tarbutton, G. and Lewis, Jr., H. T. : System

CaO-PsOs-HF-HgO : Equilibrium at 2 5 and 50°, J Phys Chem

. 66:318-321, 1962.

4. Gray, J. A. and Francis, M. D, : Physical Chemistry of

Enamel Dissolution, in Sognnaes, R, F. (ed.) : Mechanisms

of Hard Tissue Destruction , Washington, D. C: American

Association for the Advancement of Science, 1963,

pp. 253-257.

5. McCann, H. G. and Brudevold, F.: The Mechanism of the

Caries-Inhibiting Effect of Fluoride, in Kreshover, S. J.

and McClure, F. J. (eds.) : Environmental Variables in

Oral Disease , Washington, D. C: American Association

for the Advancement of Science, 1966, pp. 121-12 3.

6. Patel, P. R. and Brown, W. E. : Solubility Behavior of

Powdered Human Tooth Enamel at 2 5°C, abstracted, lADR

Program and Abstracts of Papers, No. 801, 1971.

7. LeGeros, R. Z. and Klein, E.: Brushite Crystals Grown by

Diffusion, abstracted, lADR Program and Abstracts of

Papers, No. 466, 1972.

8. Chow, L. C. and Brown, W. E. : Incorporation of Fluoride

into Tooth Enamel, abstracted, lADR Program and Abstracts

of Papers, No. 65, 1971.

9. lizuka, Y. and Ohashi, K.: Experiment with Apatites and

the Effect of Fluoride - II, Bull Tokyo Dent Coll .

, 11

1-9, 1970.

10. Duff, E. J.: Orthophosphates, Part II. The Transformations

Brushite Fluorapatite and Monotite Fluorapatite in

Aqueous Potassium Fluoride Solution, J chem Soc (A) , 33-38,

1971.
-2-

11. Brudevold, F., McCann, H. G., Nilsson, R., Richardson, B.

and Coklica, V. : The Chemistry of Caries Inhibition

Problems and Challenges in Topical Treatments, J Dent Res,

46:37-45, 1967.

12. Moreno, E. C, Brown, W. E. and Osborn, G.: Solubility

of Dicalcium Phosphate Dihydrate in Aqueous Systems,

Soil Sci Soc Am Proc, 2_4: 94-98, 1960.

13. Willis, J. B. : The Determinations of Metals in Blood

Serum by Atomic Absorption Spectroscopy-I Calcium,

Spectrochim Acta, 16:2 59-262, 1960.

14. Brabson, J. A., Dunn, R. L. , Epps, E.A., Hoffman, W.M.

and Jacob, K. D. : Report on Phosphorus in Fertilizers:

Photometric Determination of Total Phosphorus, JAOAC,

4]^:517-524, 1958.

15. McCann, H. G. : Determination of Fluoride in Mineralized

Tissue Using the Fluoride Ion Electrode, Arch Oral Biol,.

13:475-477, 1968.

16. Wei, S. H. Y. and Forbes, W. C: X-ray Diffraction

Analyses of the Reactions Between Intact and Powdered

Enamel and Several Fluoride Solutions, J Dent Res, 47 : 471-477, 1968.

17. Wei, S. H. Y. and Forbes, W. C. : Reactions of Powdered

Sound Dentin with Several Fluoride Solutions, J Dent Res,

48:149-152, 1969.
Figure 1

\ F~

0 526m 0.263 0 .0526 0 .0253

10*ppm 5 X 10^ 10^ 5 X 10^

0 .5M * * #13 #19

0.1 #2 #8 #14 #20

0.05 #3 #9 #15 #21

0.01 #4 #10 #16 #22

0.001 #5 #11 #17 #2 3

0. #6 #12 #18 #24

F~ source is NaF; PO^,^" source is NasPO^

* Not prepared due to solubility limitations.

Fig. 1 - Compositions of the Reactant Fluoride-Phosphate

Solutions (sample numbers are shown in the chart)

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After the Reaction with. CaHP04 -21120

Sample PH F~ (ppm) Total Phosphate (M)

Number Initial Final T v^ -1 •! -^1 initial Final Initial Final

z 11 . yb 11 1 o 11 . lo lU1 A* Q C A 1 A 3

y . ouxiu 0.1 0.120

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5 10.47 7.95 10* 7.77x10^ 0.001 0.0494

6 9.20 7.82 10* 7.34x10° 0. 0.0488

8 12.01 11.24 5x10^ 4.91x10° 0.1 0.116

9 11.86 10.65 5x10^ 4.83x10° 0.05 0.0697

10 11.55 8.74 5x10^ 3.27x10° 0.01 0.0541

11 10.69 7.83 5x10^ 3.00x10° 0.001 0.0491

12 7.69 7.80 5x10^ 2.92x10° 0. 0.0500

13 12 .37 11.92 10^ 8.71x10^ 0.5 0.487*

14 12 .02 11.32 10^ 8.57x10^ 0.1 0.116

15 11.89 10.80 10^ 8.28x10^ 0.05 0.0679

16 11.66 6.65 10^ 7.24x10^ 0.01 0.0320

17 10.65 6.32 10^ 3.22x10^ 0.001 0.0301

18 7.08 6.30 10^ 2 .92x10^ 0. 0.02 94

19 12 .38 11.92 5x10^ 3.88x10^ 0.5 0.495*

20 12.05 11.32 5x10^ 3.62x10^ 0.1 0.112

21 11.93 10.90 5x10^ 3.53x10^ 0.05 0.0671

22 11.66 6.54 5x10^ 2 .74x10^ 0.01 0.0312

23 10.75 4.75 5x10^ 33.5 0.001 0.0268

24 6 .96 4.26 5x10^ 19.8 0. 0.0263

Some phosphate has precipitated at the time of analysis.
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-3110561 December 29, 1972 10 983

Progress Report

on EXPERIMENTAL STUDY OF THE SURFACE AND LATTICE

EFFECTS ON THE SOLUBILITY OF HYDROXYAPATITE

Robert Chuong*

* Summer Student of the American Dental Association Research

Unit at the National Bureau of Standards, Washington, D. C.

20234.

This investigation was supported in part by research grants

DE00572 and T 1 DE 162-08 to the American Dental Association from

the National Institute for Dental Research and is part of the

dental research program conducted by the National Bureau of Standards

in cooperation with the American Dental Association; the United States

Army Medical Research and Development Command; the Dental Sciences

Division of the School of Aerospace Medicine, USAF; the National

Institute of Dental Research; and the Veterans Administration.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Experimental Study of Surface and Lattice Effects

on the Solubility of Hydroxyapatite

R . Chuong

Introduction

The solubility of hydroxyapatite (OHAp) , Cas (PO4) 3OH,

has been the subject of many investigations. It has been an

area of sustained interest because the solution behavior of

OHAp is fundamental to processes of physiological adaptation

in bone and teeth. However, experimental studies on the

solubility of OHAp have often culminated in confusing results.

The responsibility for much of this confusion lies with the

use of variable and nonrigorous definitions of "solubility"

in the framework of a particular study and with the failure

to consider the effect of surface on the dissolution process.

Other factor s""" that may have caused difficulties include (i) the

sensitivity of the apparent solubility of OHAp to impurities;

(ii) the difficulty involved in the preparation of pure OHAp,

and (iii) the slow growth of OHAp crystals in solution, often

resulting in the failure to achieve equilibrium with the lattice.

Many of these difficulties can be avoided if an internally

consistent thermodynamic treatment for the solubility of OHAp

is formulated. This has been done by Brown, Moreno and

Gregory.^ Their treatment reveals the importance of separating

"lattice" and "surface" reactions in the study of the solubility

of sparingly soluble salts. It is perhaps useful to briefly

explain this distinction.
"Lattice reaction" refers to the process where in a local

region of a crystal the stoichiometry of the dissolution

reaction is fixed by the composition of that portion of the

crystal lattice being dissolved or reformed. "Surface reaction"

refers to the reaction of liquid with the interfacial region

(solid-liquid interface) which need have no fixed stoichiometry.

l*ho distinction between these reactions is most readily seen

in terms of the number of degrees of freedom, f, for their

equilibria; in the ternary system Ca (OH) 2-H3 PO4-H2O at constant

temperature and pressure, the equilibrium between the lattice

and liquid has f = 1, and for the equilibrium between interface

and liquid, _f = 2.^ The consequence of two degrees of freedom

for the equilibrium with the interfacial region is that the

composition of the liquid is not confined to an isotherm. When

the two equilibria are considered simultaneously the system is

confined by the equilibrium with the fewest degrees of freedom

so that f = 1; therefore, the solution compositions are

restricted to the isotherm for the lattice-solution equilibrium.

Thus, the surface of the solid substrate should not affect the

thermodynamic solubility, a quantity expressed by the solubility

product and the phase diagram. The surface should, however,

affect the stoichiometry of the overall reaction and the composition of the equilibrium solution; that is, the apparent

solubility of the substrate depends in part on the surface.
-3-

Brown, et al^ have also indicated that the constancy of the

solubility product is a measure of a stoichiometric reaction

for the lattice, and that surface characteristics such as

charge should not affect the solubility product even though

they may affect apparent solubility.

Avnimelech et al '^ have shown that the separation of the

lattice and surface effects is valid for the Ca (OH) s-HaPO^-HsO

system by showing that although the overall dissolution of OHAp

may be nonstoichiometric (as measured by the molar calcium-tophosphorus ratio) , the OHAp behaves as a compound with definite

thermodynamic properties as indicated by the constancy of the

solubility product. Kg = (Ca^"") (PO4®") ^ (OH") and by the excellent agreement between the Ca/P ratio of the equilibrating

solid (determined from the potential diagram) and the theoretical

ratio. The present study concerns the investigation of the

solubility of OHAp in the quaternary system of Ca (OH) 2-H3PO4-

HgO-HCl. The purpose of this study is to show that the effect

of the solid-to-solution ratio on the calcium-to-phosphorus ratio

in solutions of OHAp can be explained by a consideration of both

lattice and surface reactions.

Methods and Results

Different solid-to-solution ratios (grams/100 ml) of OHAp

in 7 X 10"* M, 1 X 10"® M, or 1 x 10"^ M HCl were prepared by

placing 0.2, 0.4, 0.5, 0.8, 1.0 and 2.0 grams of solid into
_4-

Erlenmeyer flasks with 100 ml of solution. In total, 18 solutions were prepared, as summarized in Table I. The flasks

were sealed with parafilm paper and placed for five days in a constant temperature shaker bath at 25°C. The rate of oscillation of the shaker was maintained at a level sufficient to

keep the solid in suspension continuously. At the end of the

five-day period all flasks contained residual solid and the

solutions were filtered through millipore filters of 0.22 |j

pore diameter. Their pH's were determined at 25°C using the

Radiometer digital pH meter, PHM 52* standardized with the

NBS standard buffer of pH 4.008. Calciimi was determined with

an atomic absorption spectrophotometer. Strontium chloride

was used to suppress phosphorus interference. Phosphorus was

determined using the colorimetric method of Brabson et al .

The ion activity products and the electroneutrality unbalances

were calculated as described by Gregory et a_l* for each solution.

, " Discussion

From Table I, the following generalizations can be made:

(i) Marked deviations from the theoretical Ca/P

ratio of 1.67 occur in the dissolved calcium

and phosphorus

* Certain commerical equipment, instrijments , or materials are

identified in this paper in order adequately to specify the

experimental procedure. In no case does such identification

imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the

purpose
-5-

(ii) The deviations are greater, the higher

the solid-to-solution ratio,

(iii) The deviations are greater, the lower the

initial HCl concentration; the series with

the highest initial HCl concentration shows no significant change in the Ca/P ratio of the dissolved

material, and this ratio was somewhat higher

than theoretical,

(iv) Ion activity products are consistently on the

order of the solubility product of hydroxyapatite

(K_. = 3 X 10"^^) within experimental error.

S , OHAp

The variation of Ca/P with solid-to-solution ratio

indicates that surface effects contribute to the stoichiometry

of the overall reaction, but the constancy of the solubility

product indicates a stoichiometric lattice equilibrium. The

observation that as the concentration of HCl is higher, i.e.,

series B relative to A, the Ca/P ratio is closer to the

theoretical value of 1.67, indicates that the effect of the

surface is reduced as the concentration of HCl is greater.

This is expected since the lattice equilibrium is described

by an isotherm and as the acidity of the solution is greater

(lower value of pU(-) , where U(-) is the electroneutrality

balance^ ^) the relative contribution to the concentrations

of calciiam and phosphorus from the lattice will be greater.
-5-

The values of Ca/P for series C vary only slightly with

changes in the solid-to-solution ratio. This fact also shows

that as the acid concentration increases, the contribution

of the surface to the overall dissolution is diminished

relative to that of the lattice. The reason for the greaterthan-theoretical values of Ca/P for the solutions of series C

is not iininediately apparent. One possibility is the presence

of a calcium-containing impurity that is held in the OHAp

crystal structure but is released at the higher HCl concentrations. This is not an attractive explanation since it introduces new variables into the system, but is consistent with the

observation that the apparent Ca/P ratio of the initial solid,

1.69, was slightly greater than theoretical. An alternative

is to attribute the Ca/P values to the formation of CaHP04

The pH's of the equilibrated solutions in series C are all

fairly close to the pH of the expected singular point for

CaHP04 and OHAp. However, the calculated solubility products

for CaHP04 of these solutions, (6.0 ± 0.2) x 10"® are only

about one-half the value, 1.26 x

10""''', reported by McDowell

et al,® thereby detracting from this explanation for the

high values of the Ca/P ratio of the dissolved material. A

third explanation--that the surface became more enriched in

phosphorus in the solutions with the lowest pH' s-—is consistent

with the observations of Avnimelech et al,^ but there should
-7-

have been some observable effect due to changes in solid-tosolution ratio. Thus, there appears to be no entirely

consistent single explanation for the slightly high Ca/P

ratios in series C.

Otherwise, the results of this study are adequately

explained by the theory of Brown et aj^.^ The results are an

example of the fact that although the overall dissolution of

hydroxyapatite may be nonstoichiometric, the free energy of

the dissolution-precipitation reaction at equilibrium is

governed by the stoichiometric reaction for the lattice.

Earlier investigations into the solubility of OHAp

encountered difficulties because of the failure to explicitly

account for contributions from the surface to the reaction

in a manner that would not relegate the lattice to a subordinate

role, and also to the failure of workers to achieve equilibrium

with the lattice due to insufficient allotments of time for

the complete precipitation of OHAp in the approach to equilibrium. Earlier workers have also invoked the possibility of

complexes on the solid surface which control the solubility,

but inherent in this approach is the dubious assumption that

the chemical potential of the complex is independent of its

concentration on the surface. "' Confusion has also arisen from

the use of solvents of insufficient acidity in preparing solutions

for equilibration with OHAp, thus allowing relatively large

contributions of the surface to their compositions.
-8-

The difficulties and confusions in the past concerning

the solubility of OPiAp arose in general, from a failure to

explicitly consider the distinction between lattice and

surface phenomena. Once this distinction is realized and

observed consistently, the applicability of standard

solubility product principles to OHAp and other sparingly

soluble salts which show apparently aberrant behavior is

seen to be valid and that there is no need to invoke new

forms for the ion activity product of OHAp to describe its

solution behavior.

Acknowledgment

The guidance and encouragement of W. E. Brown and

P. R. Patel are gratefully acknowledged. Financial support

for carrying out this research was received under Grants

DE-00572 and T 1 DE 162-08.
References

1. Brown, W. E.; Moreno, E. C; and Gregory, T. M.

Thermodynamics of Hydroxyapatite Solubility. (In press.)

2. Avnimelech, Y.; Moreno, E. C. ; and Brown, W. E.

Solubility and Surface Properties of Finely Divided

Hydroxyapatite. (In press.)

3. Brabson, J. A.; Dunn, R. L. ; Epps, Jr., E. A.?

Hoffman, W. M. ; and Jacob, K. D. J. A. O. A. C .41 : 517,

1958.

4. Gregory, T. M. ; Moreno, E. C. ; and Brown, W. E.

J. Res NBS 74A:461, 1970.

5. Brown, W. E. ; and Wallace, B. M. Ann NY Acad

Sci 121:690, 1965.

6. McDowell, H. M. ; Brown, W. E, ; and Sutter, J. R.

Inorg chem 10:1638, 1971.
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U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. ^. Gov't Accession

NBSIR 73-130

3. Recipient's Accession No.

4. TITLE .\N"D SUBTITLE

Report on Dental Research at the National Bureau of Standards

July 1 - December 31, 1972

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Dr. James M. Cassel, et al

8. Performing Organization

MR c; TP 7 "^ — 1 n

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3110561

11. Contract/Grant No.

12. .Sponsoring Organization Name and Address American Dental Association, Chicago , Illinois

National Institute of Dental Research, Bethesda, Md.

Veterans Administration, Washington, DC

U.S. Army Medical Research & Development Command, Washington

13. Type of Report & Period Covered

Semi-annual

July 1 - Dec. 31, 1972

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

A copy of this report is also kept at the ADA Headquarters Library, Chicago, but

it is not publicized in any manner.

1 16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significar

j bibliography or literature survey, mention it here.)

Semi-annual Progress Report

July 1 - December 31, 1972

17. KEY UORDS (Alphabetical order, separated by semicolons)

18. AVAILABILITY STATEMENT

i J UNLIMITED.

: Xi FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Pric

USCOMv<-DC 5 82**-= 7;