Method for growing absorption-free alkali metal halide single crystals

Large ultra-pure, prism-quality essentially single crystal boules and ingots of alkali metal chlorides and alkali metal bromides are grown by the methods of Kyropoulos and Stockbarger. Optically single crystals of these alkali metal halides are clear optical bodies free of haze throughout. A typical Kyropoulos grown boule of KBr of this invention is also absorption-free, and has cleavage or crystallographic planes which deviate from parallel by 1.degree. to 3.degree. per inch. By `absorption-free` is meant freedom of absorption at 7.2 microns (.mu.) due to nitrate, at 9.5-11 .mu. due to silicate, and at between 8 and 9 .mu. due to sulfate, which are the most difficult to control but as the term implies the bodies are also free of infrared absorption for impurities such as CO.sub.3.sup.-2, PO.sub.4.sup.-3, OH.sup.-, BO.sub.2.sup.-, SH.sup.-, CNO.sup.-, HCO.sub.3.sup.-, etc., which are easily avoided by use of good commercially available growth stock. A particular process for growth for either of the foregoing methods in a silica or quartz crucible, comprises growing an ultrapure ingot of KBr from a relatively impure melt containing silicate, sulfate, nitrate and nitrite ions, and includes maintaining the sodium content of the melt during growth to less than about 20 ppm (parts per million) and the barium content less than 10 ppm, adding barium bromide to the melt if necessary, and, contacting the melt with a trace of free bromine in a covered, but not air-tight, pot.

BACKGROUND OF THE INVENTION 
Optical bodies must satisfy, simultaneously, exacting specifications with 
respect to most of their properties. These specifications include maximum 
optical homogeneity, high and uniform transmission throughout the range of 
wavelengths characteristic of the particular material, low internal 
scatter, and absence of internal stresses. It is difficult enough to grow 
small, nearly perfect crystals, but to grow large ingots of the same 
near-perfect quality in the ambience of a mass-production facility, 
demands an exceptionally compelling attention to processing techniques. 
It is known that Stockbarger growth requires generally purer raw materials. 
Thus, for economic reasons, Kyropoulos growth is preferred for mass 
production of ingots of alkali metal halides and particularly of potassium 
bromide. Moreover, Kyropoulos growth is both easier and faster than growth 
in a Stockbarger furnace. Despite the less purified growth stock used in 
Kyropoulos growth the ingot conventionally obtained is of generally high 
quality, exhibiting only slight haze, and generally acceptable levels of 
silicate absorption at 9.5-11 .mu., nitrate absorption band at 7.2 .mu. , 
and, sulfate absorption bands at between 8 and 9 .mu. . When especially 
high quality crystals are desired, for example, crystals of KBr and KCl 
such as are necessary for maximum transmittance of a beam of high energy 
radiation, the crystals must be essentially absorption-free especially at 
10.6 .mu. where the major silicate band and compensated sulfate bands 
fall. It is now possible to grow such crystals by either method. 
Briefly the Stockbarger method utilizes a dual-zone furnace having 
separately heated, individually controlled upper and lower zones. The 
zones are separated by a polished diaphragm or baffle through which a 
crucible may be controllably lowered on an elevator mechanism. The 
crucible is typically cylindrical and tapers to a point to form a conical 
bottom. Highly purified growth stock salt in the crucible is melted in the 
upper zone, the temperature being less than that required to cause 
noticeable evaporation. The crucible is then lowered slowly into the lower 
zone which is maintained at a temperature below the crystallization point, 
but not so low that the ingot will crack. A crystal is produced in the tip 
of the conical bottom, and grows upward as the crucible is lowered, until 
the entire melt forms a macrocrystal, essentially single crystal ingot. It 
is well known that a certain zone-purification effect inheres to 
Stockbarger growth and the impurities are concentrated ahead of the growth 
interface away from the cone of the crucible. With conventional highly 
purified materials it is possible to limit haze by growth rate or time 
given to allow the melt to clarify, but absorption bands from melt-soluble 
impurities, albeit at low levels, cannot be so limited, and are usually 
present throughout the ingot. 
The Kyropoulos method, as improved by numerous workers over the years, 
utilizes a large cylindrical crucible, closely fitted with resistance 
coils to control closely the temperature of the contents of the crucible. 
The crucible may be of platinum or silica, and is filled with a mass of 
finely divided salt which is heated until the mass melts. More growth 
stock salt may be added until the melt fills about three-fourths of the 
volume of the crucible. The temperature is then raised about 100.degree. F 
above the melting point. A seed crystal (a piece of a single crystal), 
held in a coolable holder, is inserted into the melt at the center, and 
rotated slowly as the temperature of the melt is reduced until the 
interface between the crystal and the melt is supercooled. This causes the 
seed to grow. 
Initially, the seed grows radially; when the diameter becomes slightly less 
than that of the crucible, the seed may be set in slow vertical motion, if 
necessary, i.e., pulled, so the crystal growth is a desirable combination 
of the rate of pulling and the rate of drop in melt level. A proper choice 
of pulling speed results in a boule or ingot of fairly regular cylindrical 
shape, with a height roughly equal to its diameter. 
Actual operation of Kyropoulos growth furnaces is somewhat more 
complicated, but it is to the improvement of the basic method outlined 
above, that this invention is directed. More specifically, there is 
currently a great emphasis on production of large, near-perfect crystals 
of alkali metal halides particularly for use as windows for transmittance 
of high power laser beams without significant absorption. The commercial 
aspects of producing such crystals dictate that an economically purified 
growth stock salt be used, despite the impurities present in it. There is 
an especial need for an economical method for producing such windows in a 
Kyropoulos furnace, but no such method was known. The process of this 
invention and the boules possessing a unique crystalline structure, 
produced therewith, are directed to filling this need. 
SUMMARY OF THE INVENTION 
It is therefore a general object of this invention to provide a new and 
improved process for the growing of essentially single ultrapure crystals 
of alkali metal halides in either a Kyropoulos furnace or a Stockbarger 
furnace, or modifications thereof. 
It is another general object of this invention to provide a new and 
improved process for growing a large boule of an alkali metal halide in a 
Kyropoulos furnace, which process includes plural steps, each of which 
steps individually contributes to an improved process, which steps 
together, unexpectedly results in an exceptionally effective process for 
growing ultrapure crystals free from absorption bands. 
It has been discovered that haze in a crystal ingot can be eliminated by 
controlling unwanted Group I and Group II metal impurities present in 
trace amounts in the melt from which a boule or ingot is grown. 
It has also been discovered that silicate absorption bands in an ingot 
grown in a siliceous crucible may be eliminated from a melt containing too 
low a level of Group II metal impurity, by the addition of a scavenger 
chosen from a divalent rare earth metal salt and an additional amount of a 
salt of the unwanted Group II metal. 
It has also been discovered that an absorption free boule of an alkali 
metal halide can be Kyropoulos grown, the boule being characterized by 
exceptionally high transmittance to a wide range of wavelengths of 
radiation. The boule, preferably grown in a silica or quartz crucible, is 
essentially free from nitrate, nitrite, silicate and sulfate absorption 
bands. The boule has a crystal structure which exhibits a progressive 
divergence of the mosaic small angle displacements, adding up to 1.degree. 
to 3.degree. per inch, unlike a crystal grown by other methods. 
It is a general object of this invention to provide a process for growing 
ingots of chlorides and bromides of alkali metals of Group IA, which 
ingots are free from silicate impurity, and are essentially haze-free. 
It is a specific object of this invention to provide a process for growing 
an ultrapure KBr single crystal ingot or boule by either the Stockbarger 
of Kyropoulos methods from a growth stock salt which is not as highly 
purified as would otherwise be required. 
It is still another specific object of the invention to provide a process 
for eliminating nitrate, nitrite and sulfate ions from a melt of an alkali 
metal bromide and/or chloride by contacting the melt with a minute amount 
of chlorine or bromine when the melt is predominantly chloride, and with 
bromine when the melt is predominantly bromide. 
It is a further specific object of this invention to provide a process for 
growing an essentially single crystal ingot from a silicate-contaminated 
melt of a chloride or bromide, or a mixture thereof, of an alkali metal of 
Group IA of the Periodic Table, comprising confining said melt in a 
siliceous crucible, contacting the melt with a metal silicate-forming 
scavenger, reacting the scavenger with silicate impurity content of the 
melt, removing the silicate impurity from the ingot grown, and removing at 
least some of the silicate impurity from the melt by depositing a silicate 
or silicate containing coating on the crucible. 
It is another specific object of this invention to provide a process for 
eliminating silicate impurity and unwanted Group II metal impurity from an 
ingot. When the growth stock melt is an alkali metal chloride and the 
unwanted Group II metal is barium or calcium, then a chloride of the 
unwanted metal is added as a scavenger in an amount in the range from 
about 5 ppm to about 10 ppm; when the melt is an alkali metal bromide then 
a bromide of the unwanted Group II metal is added as a scavenger in the 
same range specified; when the melt is a mixture of predominantly 
chloride, bromide of the unwanted Group II metal may be used. 
Alternatively, the scavenger may be a rare earth metal in melt-soluble 
form. 
It is still another specific object of this invention to provide a 
Kyropoulos method of growing essentially single crystal boules 
successively from a melt of an alkali metal bromide or chloride or a 
mixture thereof contaminated with silicate, nitrate, nitrite and sulfate 
impurities, comprising confining the melt in a siliceous crucible, 
maintaining less than 20 ppm of unwanted Group IA and less than about 10 
ppm of Group IIA metal in the melt, contacting the melt with a metal 
silicate-forming scavenger, reacting the scavenger with silicate impurity 
content of the melt, and, contacting the melt with bromine or chlorine 
when the melt is predominantly chloride, or bromine when the melt is 
predominantly bromide, and removing silicate impurity from the melt by 
depositing a silicate or silicate containing coating on the crucible, so 
as to grow a haze free boule free of infrared absorption bands due to 
silicate, nitrate, nitrite and sulfate impurities. 
These and other objects, features and advantages of this process, and the 
ingots grown therewith, will become apparent to those skilled in the art 
from the following description of the preferred forms thereof and the 
examples set forth herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Though conventional Stockbarger growth requires more highly purified 
material than Kyropoulos growth, such pure material is unnecessary in each 
embodiment of this invention. A typical, more highly purified, 
conventionally used material is represented by the following analysis on 
potassium bromide: 
TABLE I 
______________________________________ 
Potassium bromide Suprepur.RTM. 
CERTIFICATE OF GUARANTEE: 
Max. % PPM. 
______________________________________ 
Lead (Pb) 2.multidot.10.sup.-6 
0.02 
Copper (Cu) 1.multidot.10.sup.-6 
0.01 
Cobalt (Co) 1.multidot.10.sup.-6 
0.01 
Nickel (Ni) 1.multidot.10.sup.-6 
0.01 
Zinc (Zn) 1.multidot.10.sup.-6 
0.01 
Iron (Fe) 1.multidot.10.sup.-6 
0.01 
Aluminum (Al) 1.multidot.10.sup.-6 
0.01 
Manganese (Mn) 5.multidot.10.sup.-6 
0.05 
Thallium (Tl) 1.multidot.10.sup.-6 
0.01 
Barium (Ba) 5.multidot.10.sup.-4 
5 
Strontium (Sr) 3.multidot.10.sup.-4 
3 
Calcium (Ca) 5.multidot.10.sup.-5 
0.5 
Magnesium (Mg) 1.multidot.10.sup.-5 
0.1 
Sodium (Na) 5.multidot.10.sup.-4 
5 
______________________________________ 
A commercially purified acceptable material, less pure than the material 
represented hereinabove may be used in either method of growth as 
described in this invention, to produce comparable or better optical 
properties in an ingot. A typical usable material is represented by the 
following analysis: 
TABLE II 
______________________________________ 
Percent 
______________________________________ 
Barium (Ba) 0.0005 
Bromate (BrO.sub.3) To Pass Test 
(Approx. 0.001) 
Calcium, Magnesium and 
R.sub.2 O.sub.3 Ppt. 
0.005 
Chloride (Cl) 0.20 
Heavy Metals (as Pb) 
0.0005 
Insoluble Matter 0.005 
Iodide (I) 0.001 
Iron (Fe) 0.0005 
Nitrogen Compounds (as N) 
0.005 
Sodium (Na) 0.002 
Sulfate (SO.sub.4) 0.005 
______________________________________ 
For purposes of illustration KBr has been chosen, because, though KBr, KCl 
and NaCl crystals have similar physical and optical characteristics, large 
prism-quality crystals of KBr are known to be most difficult to grow. 
Furthermore, nitrate absorption bands are a more serious problem in KBr 
than in NaCl crystals. It may be possible to grow absorption free and 
haze-free macrocrystals with even poorer material than that described in 
Table II, but the precise maximum limits of impurities is not known. It 
will be readily evident, however, that super-pure material is not 
necessary to grow absorption free crystals. By `absorption-free` is meant 
freedom of absorption at 7.2 .mu. due to nitrate, at 9.5-11 .mu. due to 
silicate, and at between 8 and 9 .mu. due to sulfate, which are the most 
difficult to control but as the term implies, the bodies are also free of 
infrared absorption for impurities such as CO.sub.3.sup.-2, 
PO.sub.4.sup.-3, OH.sup.-, BO.sub.2.sup.-, SH.sup.-, CNO.sup.-, 
HCO.sub.3.sup.-, etc., which are easily avoided by use of good 
commercially available growth stock. 
In one embodiment, wherein a KBr ingot is grown in an alundum Stockbarger 
furnace, potassium bromide (as described in Table II) is used in which 
less than about 20 ppm of sodium and less than 10 ppm barium is present. 
Minor amounts of silicate, sulfate and nitrate ions are also present. The 
salt is loaded in a quartz crucible and several loadings may be required 
because of shrinkage of the salt as it melts. A platinum crucible is 
unsuitable. A conduit is also inserted through the cover and the tip of 
the conduit is placed above the surface of the melt so as to permit vapors 
of bromine contained in a reservoir outside the furnace, to contact the 
melt as it is grown into an ingot. The proper temperature gradient is 
established between the upper and lower zones of the furnace using 
recording thermocouples above and below the separating diaphragm or 
baffle. When conditions for crystal formation are obtained, the crucible 
is placed at such a level that the tip of the cone is coplanar with the 
diaphragm. A metal thin finger extending up from a gear rack shelf of an 
elevator mechanism makes contact with the tip of the crucible cone so 
that, with the rest of the metal crucible support being insulated from the 
crucible itself by a layer of alundum, heat is drawn away from the very 
tip of the crucible first, thus starting the crystallization at that 
point. After the crystal is started, the crucible is lowered at a 
predetermined rate to provide optimum temperatures and a desirable 
temperature gradient. After the crystal ingot is completely grown, it is 
separated from the crucible and allowed to cool gradually. It is found 
that the ingot grown is haze-free throughout. 
In addition to freedom from haze obtained as described hereinabove, freedom 
from nitrate and sulfate absorption bands may also be obtained, if, in 
addition to maintaining less than 20 ppm sodium and 10 ppm barium in the 
melt, a minute trace of free bromine vapors is conducted into the silica 
crucible so as to contact the melt. It is found that this injection of 
trace quantities of the same free halogen, other than fluorine, in 
elemental form as is present in the halide to be grown, unexpectedly rids 
the ingot of a nitrate absorption band of 7.2 .mu. and sulfate bands 
between 8 and 9 .mu.. The lower level of sodium impurity is not important, 
but the lower level of barium impurity may be critical depending upon the 
extent of silicate ion impurity present. For example, where the barium 
impurity is less than about 5 ppm, and the level of silicate ions is 
unacceptable, additional barium in the form of barium bromide may be added 
to maintain a level close to about 10 ppm. It is hypothesized that 
maintaining the level of barium impurity permits the formation of barium 
silicate which is extracted from the melt by the silica crucible. 
Some barium impurity will generally be present in the growth stock salt 
because of the manner in which aqueous solutions of potassium bromide are 
purified. If the existing level of barium impurity is substantially 
greater than 10 ppm, and there is insufficient silicate impurity to 
combine with the barium, a fresh batch of salt is indicated. 
The barium impurity present in the growth stock salt acts as a scavenger 
for silicate ions and is preferably added as the halide, the halogen ion 
being the same as that predominant in the melt. The same function may be 
provided by a divalent rare earth metal such as the rare earth metals 
europium, ytterbium and samarium which are added in an effective amount in 
melt-soluble form, preferably as their halides, the halogen being the same 
as that present in the halide being grown. 
In another embodiment of this invention, a commercially purified growth 
stock salt of alkali metal halide is used to grow a boule by Kyropoulos 
growth. Unlike the Stockbarger growth of an ingot, described hereinabove, 
a boule is removed from the melt before the entire contents of the 
crucible crystallize, and successive boules may be grown by maintaining a 
desired level of impurities in the melt, and leaving the impurities in the 
melt after the grown ingot is removed. Thus, where a single small boule is 
grown in a large quartz or siliceous crucible, silicate impurity is 
removed from the ingot, but may not be deposited from the melt on to the 
crucible in a sufficient quantity to be easily identified. 
The first step in obtaining a better quality Kyropoulos grown crystal is to 
maintain a sodium concentration of the melt below about 20 ppm, based on 
the weight of elemental sodium, and to maintain the barium concentration 
below about 10 ppm, also based on the weight of elemental barium. If the 
concentration of sodium exceeds 20 ppm, and the concentration of barium 
exceeds 10 ppm, a hazy boule will result. This first step of limiting the 
sodium and barium present as impurities in a melt from which is grown a 
plurality of boules, sequentially, provides an improved crystal with 
excellent clarity. Clarity, irrespective of the absorption bands of the 
crystal, is an important attribute where freedom from haze is a necessary 
criterion. 
Where silicate absorption can be tolerated, as for example, where 
absorption at 10.6 .mu. is not of critical importance, the lower level of 
concentration of sodium impurity in the melt is unimportant as is the 
lower level of barium impurity. 
In those instances where silicate absorption is critical, and silicate ion 
impurity is to be removed, barium, calcium, or any other Group II metal 
may be used as a metal silicate-forming scavenger. Where the level of 
barium impurity in the growth stock salt is substantially less than 10 
ppm, it may be deisred to add an additional amount of barium, preferably a 
barium bromide, to remove the silicate impurity. Optionally, a divalent 
rare earth metal chosen from europium, ytterbium and samarium, may be used 
in melt-soluble form as a scavenger. A small amount of the rare earth 
metal is used, preferably as the halide corresponding to the halide being 
grown. The small amount used should be sufficient to scavenge the silicate 
impurity. Irrespective of whether a Group II metal or a divalent rare 
earth metal is used, the silicate formed appears to coat the silica 
crucible with a metal silicate layer, and is thus removed from the melt. 
The ingot so grown exhibits no silicate absorption band at 9.5 to 11 .mu.. 
This second step of scavenging silicate impurity from the melt provides a 
further improvement in the quality of the crystal obtained. Thus, a 
Kyropoulos grown ingot in which the level of sodium and barium impurities 
are maintained at less than 20 ppm and 10 ppm respectively in a melt used 
for successively growing Kyropoulos boules of potassium bromide, and, if 
necessary, adding a sufficient amount of scavenger to remove silicate in a 
second step, yields a boule which is free from haze and also free of a 
silicate absorption band. Excess scavenger, more than that necessary for 
reaction with silicate impurity in the melt, reacts slowly with the quartz 
crucible which continuously removes the excess by reaction therewith. The 
quartz crucible thus has a limited useful life. 
An ingot grown as described hereinabove is still contaminated with nitrate 
and nitrite impurities which are evidenced by absorption bands in the 7.2 
.mu. and 7.9 .mu. regions respectively. Nitrite impurities are generally 
present only when the nitrate impurities are at a relatively high level. 
Both impurities are conveniently removed by the simple expedient of 
contacting the melt with free or elemental chlorine or bromine. A small 
amount of halogen suffices. For example, free bromine vapors in trace 
amounts, are introduced above the surface of the melt when a predominantly 
alkali metal bromide ingot is grown; free chlorine or bromine vapors in 
trace amounts, are introduced above the surface of the melt when a 
predominantly alkali metal chloride ingot is grown. Where a KBr ingot is 
grown, and only trace quantities of bromine vapors contact the melt, a 
brown coloration results in the melt because of diffusion of bromine 
vapors. A potassium chloride melt acquires a greenish coloration due to 
diffusion of chlorine vapors. The coloration is not invested in the ingot 
as it grows, and no trace of free halogen is found in the crystal removed. 
Also the action for removing nitrate proceeds at such low concentrations 
of free halogen that visible coloration of the melt is not a prerequisite 
for success. 
Referring now more particularly to FIG. 1 for a description of the improved 
process for Stockbarger growth, there is shown a Stockbarger furnace 
indicated generally at 10, a vertically movable platform indicated 
generally at 20, disposed within the furnace, a crucible indicated 
generally at 30, in which the optically single crystal ingot is to be 
grown, and a halogen reservoir indicated generally at 40 from which a 
compatible halogen is flowed into contact with melt 31 contained in the 
crucible 30. Typically, the furnace includes fire brick walls 11 in which 
electrical resistance windings 12 are embedded near the interior surface 
of the fire brick wall. The fire brick wall rests on a metal base 13 and a 
suitable cover 14 is provided. Thermocouples 15 and 16 are provided in the 
upper and lower zones of the furnace defined by a centrally disposed 
radiation baffle 17. 
The platform 20 is vertically movable by means of a elevator mechanism (not 
shown). The platform is adapted to hold the crucible 30 which is provided 
with a conical bottom and is supported on the platform 20 by means of an 
alundum layer 21 therebetween. Finely divided salt to be grown into a 
crystal is melted at the appropriate temperature and growth of the crystal 
begins in the conical portion of the crucible as indicated at 32. The 
crucible is conventionally provided with a lid 33. 
The halogen reservoir 40 is shown in more detail in FIG. 3. It includes a 
vessel 41 provided with an opening 42 through which halogen 50 is 
introduced into the reservoir. A tube 43 rises above the level of the 
halogen in the vessel and serves to conduct halogen vapors from the 
reservoir. The tube 43 is inserted through the cover of the furnace and is 
thereafter conducted through the lid of the crucible so as to permit 
halogen vapors to contact the melt 31. A vented stopper 44 is provided for 
the opening 42 of the reservoir. 
In operation, sufficient heat is provided to the reservoir to vaporize 
halogen contained therewithin and the halogen vapors are conducted to the 
surface of the melt 31. The vapors are absorbed within the melt and are 
distributed therewithin. When potassium bromide is to be grown, the melt 
is contained within a quartz crucible and bromine is introduced into the 
melt. The amount of bromine vapors introduced in the melt is not critical 
and it will be apparent that only a very small amount is sufficient. The 
precise amount that will serve to provide an improved optically single 
crystal ingot is not known, but only trace amounts in the order of less 
than about one part per million appears to be sufficient, the important 
point being that the melt is treated with a trace amount of free bromine 
over a long period of time, preferably over the entire period of growth. 
The diagrammatic illustration of the Stockbarger furnace described 
hereinabove is conventional except for the additional reservoir means used 
to introduce the halogen vapor. The inclusion of other substances within 
the melt of the crucible, such as for example, scavengers, is preferably 
done prior to initiation of growth of crystal. Typically, a scavenger is 
introduced before the contents of the crucible are entirely melted so as 
to ensure a relatively homogeneous distribution of scavenger within the 
melt. Growth of the Stockbarger ingot is initiated in a conventional 
manner and an optically single crystal ingot obtained by continued growth 
of the ingot in the usual way. It is found that a prism-quality optically 
single crystal ingot may be obtained with relatively impure growth 
material, for example, such as that described in Table II set forth 
hereinabove. Since the Stockbarger crucible is gradually lowered as 
crystal growth progresses, the reservoir 40 is movably disposed so as to 
permit the halogen vapor to be continuously fed to the surface of the 
melt. 
Referring now to FIG. 2, there is diagrammatically illustrated a typical 
Kyropoulos furnace including a crucible 41 exteriorly provided with 
electrical resistance windings 42 which serve to melt finely divided salt 
to be grown as a boule. Scavengers and the like are added to the salt 
prior to or during formation of the melt 48. The crucible is typically a 
silica crucible and it may be provided with a base heater indicated 
generally at 44. The crucible is provided with a cover 45 through which a 
coolable holder 46 is inserted. A seed crystal 47 is fixedly disposed in 
the end of the holder 46. 
As in the Stockbarger furnace described hereinbefore, a halogen reservoir 
indicated generally at 40, is provided to feed halogen vapor into the 
Kyropoulos growth crucible. The halogen reservoir is illustrated in FIG. 3 
and described in detail hereinbefore. In operation it provides a flow of 
halogen vapor to the surface of the melt. Again, the amount of vapor 
introduced is not critical but it will be apparent that very small amounts 
suffice. What is important is that the small amount of halogen be flowed 
to the melt over a long period of time, preferably during the entire 
growth period of the boule. It is assumed that less than one part per 
million distributed within the melt serves to provide an ultra-high 
prism-quality boule despite the fact that growth material from which the 
boule is formed may be a relatively impure and commercially purified salt. 
Where a boule is grown by Kyropoulos growth, in a silica or quartz 
crucible, by controlling unwanted Group I and Group II metal impurities 
present in trace amounts of the melt, and by contacting the melt with 
halogen vapor, as described hereinabove, the boule is essentially free 
from nitrate, nitrite, silicate and sulfate absorption bands. Optically 
single crystals obtained by the improved Kyropoulos growth of this 
invention are unlike Stockbarger grown ingots. Mosaics within Stockbarger 
ingot components have compensating plus and minus shifts in angle so the 
cleavage and crystal planes never deviate far from parallel. Typically, 
the Kyropoulos boule grown under gravity of 0.7 or more has a crystal 
structure which exhibits a progressive divergence of the mosaic small 
angle displacements, adding up to 1.degree. to 3.degree. per inch, unlike 
a crystal grown by other methods. This mosaic arrangement can be made 
visible by etching or by cleaving the body. Vapor etch that occurs at 
600.degree. C is particularly good for indicating the type of material in 
question. One can map the shape of mosaic regions and find the crystal 
direction by the angle of reflection. No conventionally grown Kyropoulos 
growth crystal exhibiting such mosaic small angle displacements are 
presently known which are free from the aforementioned absorption bands. 
Though specific reference is made hereinabove to the growth of ingots of 
alkali metal chlorides and ingots of alkali metal bromides, it may be 
desirable to grow an ingot of mixed alkali metal chloride and bromide. For 
example, if may be desired to grow an ingot of potassium chloride doped 
with potassium bromide present in an amount less than 10 mole percent. In 
such an instance, where the level of barium impurity in the growth stock 
is less than 10 ppm, addition of barium chloride as a scavenger will 
remove silicate impurity. A haze-free potassium chloride ingot doped with 
potassium bromide may be grown by contacting the melt of mixed potassium 
chloride and bromide with bromine is effective to grow a haze-free ingot 
due to sulfate, nitrate and nitrite ions irrespective of whether the ingot 
grown is a pure alkali metal bromide, a pure chloride or a mixture of the 
two. Chlorine is effective to grow a haze-free ingot free of absorption 
bands due to sulfate, nitrite and nitrite ions, when the ingot grown is a 
pure alkali metal chloride, or a mixture of alkali metal chloride and 
bromide. However, chlorine is not desirable in a metal of a pure alkali 
metal bromide or of a melt having the bromide as a major component, 
because chlorine tends to displace bromine. 
The methods of this invention are specifically directed to alkali metal 
chlorides and bromides because alkali metal fluorides do not lend 
themselves to growth in a quartz or siliceous crucible, and alkali metal 
iodide ingots are used in applications where the level of impurities 
contained in them are not significant. 
Modifications, changes and improvements to the preferred forms of the 
invention herein disclosed, described and exemplified, may occur to those 
skilled in the art who come to understand the principals and precepts 
thereof. Accordingly, the scope of the patent to be issued herein should 
not be limited to the particular embodiments of the invention set forth 
herein, but rather should be limited by the advance of which the invention 
has promoted the art.