bluuebunny/crossref_metadata_2025 huggingface dataset:
A database of incommensurate phases
Authors: Caracas Razvan
Published: July 2002

A database of incommensurate phases is currently available at http://www.mapr.ucl.ac.be/~crystal/index.html. The present database offers a fast direct retrieval system for structural, physical and bibliographical data of incommensurate phases. The database contains data about inorganic, non-composite, non-magnetic and non-superconducting incommensurate phases only. Classification is according to the physical mechanisms responsible for the incommensurate phase transition. The main classes of incommensurate phases thus obtained are: theA2BX4dielectrics family, zone-centre lock-in transition phases, cooperative Jahn–Teller incommensurates, tetragonal tungsten bronzes, charge-density wave systems and miscellaneous incommensurate phases. The latter class, because of the lack of available data, is classified on a chemical basis in several subclasses: silicates, perovskites, Mn-bearing oxides, other oxides, group VI compounds, intermetallics and other compounds. The database contains a brief description of the main physical, chemical and structural features of each phase, as stated in the literature. This description is very material- and bibliography-dependent and it is preceded by the phase transition sequence.
The non-additivity and curvature corrections in the variance method of X-ray line-broadening analysis
Authors: Edwards H. J. , Toman K.
Published: July 2002

A previous paper by Edwards & Toman on the subject of the variance non-additivity and curvature corrections is revised and extended. The formula for the curvature correction is given in a form which facilitates evaluation for cases of crystallite size broadening by crystallites of known shape; the formula involves the shape parameter M which is evaluated for the shapes of a sphere, a rectangular parallelepiped, a tetrahedron and an octahedron.
Minimizing thermal gradient problems during precession photography of protein crystals
Authors: Thaxton C. , Lattman E.
Published: July 2002

An air-stream cooling device and a new crystal mounting technique minimize thermal-gradient problems during precession photography of proteins.
Voronoi cells: An interesting and potentially useful cell model for interpreting the small-angle scattering of catalysts
Authors: Brumberger H. , Goodisman J.
Published: July 2002

A structural model, based on Voronoi polyhedral cells partly filled with support and metallic catalyst, is proposed for interpreting the small-angle X-ray scattering of heterogeneous catalysts. The model's properties are described, and the model is applied to porous Al2O3 and Pt/porous Al2O3. Surface areas calculated from the X-ray data by means of the statistical geometry of Voronoi tesselations are found in good agreement with those determined by BET (Brunauer, Emmett & Teller) adsorption methods.
Correction for escape in X-ray spectra measured using a Ge detector
Authors: Steenstrup S.
Published: July 2002

X-ray spectra measured with a germanium detector can be corrected for escape. A numerical procedure to perform the correction has been implemented. The method employs a point by point subtraction and allows for energy-dependent detector resolution.
Use of kinematical diffraction in X-ray topography
Authors: Alexandropoulos N. G. , Kotsis K. T.
Published: July 2002

A new X-ray topographic method is proposed that makes use of the kinematical diffraction component, which is significantly more sensitive to the crystal-surface imperfections than dynamical Laue–Bragg diffraction. The sensitivity of the method has been studied using a triple crystal diffractometer to separate kinematical and dynamical components of the diffraction intensity curve when the Bragg condition is nearly satisfied.
Average Rayleigh-wave velocity of a computer-simulated crystallographic plane
Authors: Crean G. M. , Waintal A.
Published: July 2002

A model is proposed for calculating the Rayleigh-wave velocity as obtained by an acoustic lens, taking into account the anisotropy of the material under study and the profile of the miniature acoustic probe. The results obtained by this model on different `cuts' of single-crystal silicon agree with both those published in the literature and our own experimental results. In addition, the model has been applied to the non-destructive thickness measurement of opaque thin films deposited on different anisotropic substrates. More significantly, using the model and experimental acoustic material signature (AMS) curves obtained with this acoustic measurement system, the thickness of tungsten thin films deposited on silicon (100) have been measured. The results are in good agreement with measurements made using a Rutherford back-scattering technique.
Auto-indexing oscillation photographs
Authors: Kim S.
Published: July 2002

An automatic procedure has been developed for indexing reflections on oscillation photographs produced by crystals with very large unit cells (cell dimensions larger than about 100 Å). The procedure works in a similar fashion to the auto-indexing used in four-circle diffractometry. Features include ab initio cell parameter and orientation matrix determination, reduced cell calculation and transformation of the reduced cell to one with higher symmetry.
Polytypism in a decagonal quasicrystalline Al–Co phase
Authors: Menon J. , Suryanarayana C. , Singh G.
Published: July 2002

A decagonal quasicrystalline phase has been produced in rapidly solidified binary Al–14 to 25 at.% Co and ternary Al–20 at.% Co–5 at.% Si alloys. Observation of periodicity along one of the directions in the electron diffraction patterns has confirmed the decagonal nature of the phase. Variations in periodicity ranging from 4 to 16 Å suggest that a polytypism-like phenomenon occurs in these phases. By in situ hot-stage electron microscopy, it can be shown that one polytype transforms into another. Reasons for the formation and transformation of polytypes in decagonal quasicrystalline phases are discussed.
Grazing-incidence small-angle X-ray scattering: new tool for studying thin film growth
Authors: Levine J. R. , Cohen J. B. , Chung Y. W. , Georgopoulos P.
Published: July 2002

Grazing-incidence small-angle X-ray scattering (GISAXS) is introduced as a method of studying discontinuous thin films. In this method, the incident beam is totally externally reflected from the substrate followed by small-angle scattering of the refracted beam by the thin film. The experiment described establishes the ability of GISAXS to provide size information for islands formed in the initial stages of thin film growth. The data presented are for gold films of 7 and 15 Å average thicknesses on Corning 7059 glass substrates. The advantages of this technique are that it is non-destructive, can be done in situ, provides excellent sampling statistics, does not necessarily require a synchrotron source, and is not limited to thin or conducting substrates.
XTL-SIZE: a computer program for crystal-size-distribution calculation from X-ray diffraction line broadening
Authors: Bonetto R. D. , Viturro H. R. , Alvarez A. G.
Published: July 2002

A computer program has been written for the determination of crystal-size distribution in a direction perpendicular to an (hkl) crystal plane, from a digitized XRD peak. It implements an information theory approach devised by Guérin, Alvarez, Rebollo Neira, Plastino & Bonetto [Acta Cryst. (1986), A42, 30–35] and Alvarez, Bonetto, Guérin, Plastino & Rebollo Neira [Powder Diffr. (1987), 2, 220–224]. The program has a fully automated operation mode and owing to the very restricted amount of input data this program is specially suited to users with a limited knowledge of crystal-size-determination methods.
A fitting method for the determination of crystallinity by means of X-ray diffraction
Authors: Polizzi S. , Fagherazzi G. , Benedetti A. , Battagliarin M. , Asano T.
Published: July 2002

A best-fitting version of the X-ray diffraction method of Gehrke & Zachmann [Makromol. Chem. (1981). 182, 627–635] for crystallinity determination, which is a modification of the method developed by Ruland [Acta Cryst. (1961). 14, 1180–1185], is presented. The data, corrected and normalized to electron units (e.u.), are plotted as I(s)s
2
vs s and fitted by pseudo-Voigt functions for the crystalline peaks added to a background scattering IB
(s)s
2, with IB
(s) = (1 − Xc
)I
am(s) + Xc
〈f(s)2〉[1 − exp(−ks
2)], where I
am is the experimental intensity of a completely amorphous sample (also corrected and normalized to e.u.), 〈f(s)2〉 is the mean square atomic scattering factor in the material, Xc
is the degree of crystallinity and k is a factor which includes either thermal or lattice disorder, where s = 2(sin θ)/λ. The use of the scattering of the amorphous sample in this non-integral form of the Ruland equations overcomes the problem, encountered with other procedures, of locating the continuous (background) scattering with accuracy. The degree of crystallinity and the disorder factor are supplied directly by the optimization process. Furthermore, the line broadening analysis which allows the determination of crystallite size is automatically obtained as a by product. Samples of polyethylene terephthalate (PET) with different degrees of crystallinity are investigated. The results are compared with those obtained by other methods which do not use fitting techniques.
Complete determination of polarization state in the hard X-ray region
Authors: Ishikawa T. , Hirano K. , Kikuta S.
Published: July 2002

A new method for complete determination of polarization state in the hard X-ray region is described. The system consists of a perfect-crystal phase retarder and a linear polarization analyzer. This method gives not only the amplitude ratio of mutually perpendicular electric vector components and the phase shift between them but also the proportion of unpolarized radiation.
On the determination of accurate intensities from powder diffraction data. I. Whole-pattern fitting with a least-squares procedure
Authors: Jansen J. , Peschar R. , Schenk H.
Published: July 2002

A new algorithm, direct intensity fitting, has been developed which performs a whole-pattern fitting for powder data. The algorithm is related to the fitting technique of Pawley [J. Appl. Cryst.
14, 357–361] but does not require start values for the intensities of the individual reflections. Comparative tests show that direct intensity fitting extracts more accurate intensities from a powder diffractogram and reduces the minimum peak distance for accurate data to half of the half-widths. The structure of the superconductor Ba2YCu3O8−x
can be solved in a default direct-methods run with direct-intensity-fitting intensities extracted from a normal laboratory powder diffractogram.
A new algorithm for incommensurate structure refinement
Authors: Paciorek W. A. , Chapuis G.
Published: July 2002

Numerical integration over an internal coordinate, which occurs in incommensurate structure refinement, has been reconsidered. An algorithm based on the fast Fourier transform has been implemented and tested. This new procedure is faster and more accurate, especially for highly anharmonic structures. In addition, the computing time is less sensitive with respect to the range of harmonics and accuracy.
An optimal strategy for X-ray data collection on macromolecular crystals with position-sensitive detectors
Authors: Vicković I. , Kalk K. H. , Drenth J. , Dijkstra B. W.
Published: July 2002

X-ray data collection on macromolecular crystals is preferably done with minimum exposure time and high completeness. A Fortran procedure – DCS – has been written in the environment of the MADNES program to predict the completeness of data before the start of actual data collection. In addition, the program can check and refine the data-collection strategy and suggest optimal settings and rotation ranges for one or more crystals in different orientations that give highest completeness in minimum exposure time, thus extending the life of the crystal. The method has been tested on previously collected data as well as on new structures. Since the program has been in full use in this laboratory, the completeness of collected data has improved significantly.
Cu(OH)2: a New Ferroelectric
Authors: Abrahams S. C. , Ravez J. , Simon A. , Reller A. , Oswald H. R.
Published: July 2002

The crystal structure of Cu(OH)2, recently redetermined at room temperature by Oswald, Reller, Schmalle & Dubler [Acta Cryst. (1990), C46, 2279–2284], is shown to satisfy the structural criteria for ferroelectricity. The Cu2+ ion at 295 K is displaced 0.131 Å from the most likely paraelectric atomic arrangement, allowing the prediction to be made that the ferroelectric Curie temperature is Tc
= 343 K. This prediction is in excellent agreement with an earlier report of a reversible structural transition at 325–335 K in Cu(OH)2. The presence of a λ-shaped anomaly in the heat capacity at about 335 K has been confirmed, and a dielectric anomaly is also observed at similar temperatures over a wide frequency range; the frequency dependence is dispersive, owing at least partly to ionic conductivity. The prediction is hence verified, and Cu(OH)2 is demonstrated experimentally to be a new ferroelectric.
Decomposition Kinetics of Bulk Amorphous Zr41Ti14Cu12.5Ni10Be22.5 Alloys Studied by Computer Simulation and Small-Angle Neutron Scattering
Authors: Uebele P. , Wiedenmann A. , Hermann H. , Wetzig K.
Published: July 2002

The decomposition behaviour during annealing in the undercooled liquid regime of Zr41Ti14Cu12.5Ni10Be22.5 is studied by means of small-angle neutron scattering (SANS). In this temperature range, a microstructure consisting of precipitates of an amorphous phase is formed. The scattering curves show a characteristic maximum which becomes more and more intense with increasing annealing time until a saturation state is reached. All scattering curves at different annealing times obey a universal scaling law by scaling the intensity and the q axis by the intensity and the position of the maximum, respectively. In order to interpret the scattering curves, the scattering behaviour of a system of interacting particles and its time evolution are simulated. The centres of the precipitates are modelled (either analytically or numerically) by a hard-core point field, i.e. they have a minimum distance from each other (`excluded volume'). At each of these points, a nucleus is created according to a given nucleation rate, either instantaneously (all nuclei start growing at the same time) or continuously (in each time interval, a constant number of nuclei starts growing). Variation of the parameters of the hard-core point field, the nucleation mechanism and the growth law for the precipitates and comparison of the calculated and measured small-angle scattering curves allow the following interpretation: The amorphous precipitates are distributed according to a hard-core structure and the nucleation is continuous. The growth of the precipitates is diffusion controlled (a ∝ t
1/2).
Shape Determination from Solution Scattering of Biopolymers
Authors: Svergun D. I. , Volkov V. V. , Kozin M. B. , Stuhrmann H. B. , Barberato C. , Koch M. H. J.
Published: July 2002

Practical aspects of low-resolution shape determination in small-angle scattering studies of biological macromolecules in solution are considered. The shape restoration method using spherical harmonics [Svergun, Volkov, Kozin & Stuhrmann (1996). Acta Cryst. A52, 419–426] is extended to account for deviations from the idealized model and to work directly on raw experimental data sets. An algorithm to restore the structure of homodimeric particles in terms of the shape of the monomer and the separation between the monomers is implemented. Applications of the program to the shape restoration of several proteins, with known and unknown crystal structures, from X-ray solution-scattering data are presented.
Comparison of Small-Angle Neutron Scattering of δ' Precipitation in an Al–Li Alloy at a High-Flux Reactor and at a Pulsed-Neutron Source
Authors: Albertini G. , Carsughi F. , Coppola R. , Heenan R. K. , Stefanon M.
Published: July 2002

Two different small-angle neutron scattering (SANS) facilities, the D11 camera at the Institut Laue–Langevin (ILL, Grenoble, France) and the LOQ time-of-flight diffractometer at the Rutherford Appleton Laboratory (RAL, Didcot, Oxon, England), were used in the investigations of δ′-Al3Li precipitation at 463 K in Al–Li 3% alloy. The results obtained from the steady-state reactor and from the pulsed source by using two different data-acquisition techniques and two different procedures for data analysis are compared. The SANS curves for the same set of samples investigated using the two different instruments are in good agreement within the experimental uncertainties. A check was also made on the metallurgically relevant quantities, namely the average size and the size-distribution function of the δ′ precipitates at the various stages of the ageing process, obtained from the two sets of SANS curves by applying the same numerical method. Good agreement was found between the results from the two data sets.
Étude par Diffraction X des Modes d'Empilement de la Nacrite Hydratée et Deshydratée
Authors: Ben Haj Amara A. , Ben Brahim J. , Plançon A. , Ben Rhaiem H.
Published: July 2002

X-ray diffraction based on the comparison of experimental and calculated powder profiles enabled the determination of the structural characteristics of hydrated and dehydrated Tunisian nacrite. Using the concept describing the structure of natural nacrite, the stacking mode of the layers in the hydrated and dehydrated nacrite has been determined. The hydrate is characterized by an 8.42 Å basal distance; one water molecule per Si2Al2O5(OH)4is intercalated in the interlamellar space, located above the vacant octahedral site of the layer atz= 6.5 Å and inserted inside the ditrigonal cavity of the tetrahedral sheet of the upper layer. The dehydrated nacrite obtained by heating of the hydrate at 423 K has the same interlayer shiftt= −0.35aas the natural nacrite. Coherence domain sizes alongc^{\ast} and in theabplane are the same as those in the hydrate but different from those of the natural mineral. After dehydration, 5% of the layers had an interlayer shift similar to that obtained from the hydrate.
High-Order Incommensurate Modulations and Incommensurate Superstructures in Transparent Sr0.6Ba0.4Nb2O6(SBN40) Ceramics
Authors: Lee Han-Young , Freer R.
Published: July 2002

Ceramics of strontium barium niobate (Sr0.6Ba0.4Nb2O6, SBN40), having the tetragonal tungsten bronze (TTB) structure, were prepared by a conventional processing route; sintering at temperatures in the range 1573–1723 K yielded products with three types of microstructure. Incommensurate superstructures were investigated by transmission electron microscopy (TEM). Modulation of the incommensurate superstructure (ICS) depended on microstructural development. The lowest-order ICS modulation was a common feature in all the SBN40 ceramics, regardless of microstructure. (h+1/2,h+1/2,l) reflections and unusually low δ (incommensurability parameter) were observed in SBN40 specimens having abnormally large grains and thus a certain degree ofA-site ordering was expected. Transparent SBN40 ceramics which exhibited large homogeneous grains were characterized by a shortenedcaxis, intense [001] X-ray diffraction reflections, a multiplication of (110) and (001) cell spacings and the appearance of higher-order ICS reflections. Planar defects, having the thickness of one d_{110} spacing, were observed aligned along \{110\}. The development of a higher-order structural modulation and of ICS reflections is believed to reduce the structural energy; the development of planar defects appears to stabilize the incommensurate phase.
Geometric distortion corrections for fiber-optic tapers in X-ray charge-coupled-device detectors
Authors: Paciorek W. A. , Meyer M. , Chapuis G.
Published: July 2002

A simple algorithm is proposed for quantifying and correcting geometric distortions introduced by fiber-optic tapers in an X-ray charge-coupled-device detector with direct taper coupling.
A tangent formula derived from Patterson-function arguments. VI. structure solution from powder patterns with systematic overlap
Authors: Rius Jordi , Miravitlles Carles , Gies Hermann , Amigó Josep M.
Published: July 2002

Besides accidental peak overlap, systematic overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data. Unlike accidental overlap which affects all types of structures, systematic overlap is restricted to high-symmetry structures (e.g.65% of the space groups compatible with a hexagonal lattice). In this work, the direct-methods sum function is adapted to cope with data extracted from patterns containing systematic overlap. Preliminary results indicate that at least for moderate-size inorganic structures, systematic overlap should not represent a serious drawback for the application of direct methods. In contrast to the usual two-stage procedures employed for solving structures with accidental overlap, here both multiplet decomposition and phase refinement are carried out simultaneously. This procedure is illustrated using two examples: the dominant crystalline phase of a hydrated high-alumina cement and the fibrous alumino-silicate `aerinite'.
Zeolite structure determination from powder diffraction data: applications of theFOCUSmethod
Authors: Grosse-Kunstleve R. W. , McCusker L. B. , Baerlocher Ch.
Published: July 2002

TheFOCUSapproach to zeolite structure determination from powder diffraction data has been applied to data from four different zeolitic materials. The solutions of the structures of two aluminophosphate molecular sieves, YUL-89 (AWO topology) and YUL-90 (ZON topology), are used to demonstrate routine applications of the procedure. The high-silica zeolite ZSM-5 (MFI topology), which has 12 Si atoms (38 framework atoms) in the asymmetric unit, and the gallophosphate cloverite (-CLO topology), the framework of which is not fully fourfold connected, provide examples of extreme cases, which challenge the limits of theFOCUSalgorithm. Taken together, the four examples give an overview of the practical aspects of theFOCUSmethod and illustrate its potential and its limitations.
The coherent electron microdiffraction of a single translation domain boundary in β-Ni3Nb phase
Authors: Pan H. Y. , Du K. , Wang Y. M. , Zhu Jing , Li S. Y. , Ye H. Q.
Published: July 2002

The microdiffraction from a single translation domain boundary (TDB) in β-Ni3Nb has been recorded by using a Hitachi HF-2000 field emission gun transmission electron microscope, while high-resolution transmission electron microscopy images were used to assist the analysis. The diffraction splitting effect of a TDB is well explained by the kinematics and dynamic calculations, and complex diffraction characteristics have been studied by using dynamics simulation.
Small-angle neutron scattering study of nanophase zirconia in a reverse micelle synthesis
Authors: Li X. , Loong C.-K. , Thiyagarajan P. , Lager G.A. , Miranda R.
Published: July 2002

Nanophase zirconia particles are synthesized by mixing two reverse micelle solutions containing the precursor salt ZrOCl2 and the base NH4OH. The primary reverse micelle solution consists of AOT, toluene and water. The reverse micelle structure of four solutions as a function of w (the molar ratio of [water]/[AOT]) is characterized by small-angle neutron scattering at room temperature. With the addition of ZrOCl2 and NH4OH into the water pools, the reverse micelle sizes decrease as compared to those in the D2O/AOT/C6D5CD3 primary system. The mixed microemulsions contain reverse micelle sizes that fall between the values of the premixed solutions. All the reverse micelles exhibit uniform size and spherical shape. The effect of concentration of inorganic salt precursor in the water pools on the reverse micelle structure is also studied.
Accurate structure-factor measurements using high-energy synchrotron radiation: a test on cuprite, Cu2O
Authors: Lippmann Thomas , Schneider Jochen R.
Published: July 2002

Using high-energy synchrotron radiation, structure factors were measured on the triple-axis diffractometer at the HASYLAB beamline BW5. A data set for cuprite, Cu2O, was recorded, and the excellent data quality [internal consistencyRint(F2) = 0.0055] allowed full multipole refinements using the programVALRAY. Extinction was still significant, but small. Using data measured at 75, 100, 125 and 150 keV photon energies, it was possible to determine and separate extinction prior to the refinements. After evaluation and comparison of extinction-affected and extinction-free data, significant differences in the electron densities were found.
Two-dimensional space groups with sevenfold symmetry
Authors: Mackay A. L.
Published: July 2002

Fivefold and sevenfold symmetry operations are, of course, incompatible with repetition by a lattice but, with the appearance of structures involving curved sheets, they and other non-crystallographic operations must now be taken into consideration as possibilities of non-Euclidean crystallography develop. Here are described the symmetry groups which might be called 732 and 73m and which may be found in two-dimensional manifolds.
The program SAPI and its applications. III. Determination of structures having pseudo-translational symmetry
Authors: Fan H.-F. , Qian J.-Z. , Zheng C.-D. , Gu Y.-X. , Ke H.-M. , Huang S.-H.
Published: July 2002

Optimization has been made of the program SAPI for automatic solution of structures having pseudo-translational symmetry. This includes: (1) floating default cutoff of E's; (2) automatic and user-adjustable balance between the number of systematically strong reflections and that of the systematically weak ones; (3) R factors for reflection subsets corresponding to the 'strong' group and the 'weak' groups. Examples on solving structures with pseudo-translational symmetry are discussed in detail. They include known as well as unknown structures and centrosymmetric as well as non-centrosymmetric structures, all of which can be solved starting from a default run of SAPI.
Properties of molecular crystals by means of theory
Authors: Panas I.
Published: July 2002

A novel quantum-chemical approach to the properties of molecular crystals has recently been developed. This Hartree–Fock-based self-consistent-crystal-field method is outlined and applied to the intramolecular torsion angle in solid hydrogen peroxide, the cold-phase structure of solid dinitrogen and the hot-phase structure of solid acetylene. The potential-energy surfaces are calculated with the assumption of perfectly ordered crystals and coherent molecular potential-energy surfaces are produced. Agreement with experiment is obtained for hydrogen peroxide. It is suggested that discrepancies between calculated and observed structures of N2(s) and C2H2(s) result from static and dynamic disorder, respectively. This interpretation is consistent with the experimentally observed intramolecular-bond shortenings in these two solids. Lattice energies in good agreement with experiment are obtained in all three cases.
Evaluation of reflection intensities for the components of multiple Laue diffraction spots. II. Using the wavelength-normalization curve
Authors: Campbell J. W. , Hao Q.
Published: July 2002

In a Laue diffraction pattern, 10–20% of the spots result from the exact superposition of two or more reflections that are 'harmonics', e.g. hkl, 2h,2k,21
etc. The use of only the 80–90% of the reflections measurable as singles may not always be sufficient and evaluation of the intensities of the components of the multiple spots is therefore important. A procedure for this deconvolution is given, based on the varying nature of the wavelength-normalization curve. A feasibility trial has been carded out using a single Laue diffraction image of tetragonal hen egg white lysozyme (HEWL) recorded on an image plate. This allowed the intensities of 103 reflections to be evaluated from multiple spots. For these reflections, their agreement with monochromatic diffractometer data gave an R-factor of 0.157 for 96 common reflections. An earlier paper described another procedure based on direct methods, which addressed the same problem.
The probability distribution function of structure factors with non-integral indices. III. The joint probability distribution in the P1¯ case
Authors: Giacovazzo Carmelo , Siliqi Dritan , Altomare Angela , Cascarano Giovanni Luca , Rizzi Rosanna , Spagna Riccardo
Published: July 2002

The joint probability distribution function method has been developed in P1¯ for reflections with rational indices. The positional atomic parameters are considered to be the primitive random variables, uniformly distributed in the interval (0, 1), while the reflection indices are kept fixed. Owing to the rationality of the indices, distributions like P(F

p
1
, F

p
2
) are found to be useful for phasing purposes, where p
1 and p
2 are any pair of vectorial indices. A variety of conditional distributions like P(|F

p
1
| | |F

p
2
|), P(|F

p
1
| |F

p
2
), P(\varphi_{{\bf p}_1}|\,|F_{{\bf p}_1}|, F_{{\bf p}_2}) are derived, which are able to estimate the modulus and phase of F

p
1
given the modulus and/or phase of F

p
2
. The method has been generalized to handle the joint probability distribution of any set of structure factors, i.e. the distributions P(F
1, F
2,…, F

n+1), P(|F
1| |F
2,…, F

n+1) and P(\varphi1| |F|1, F
2,…, F_{n+1}) have been obtained. Some practical tests prove the efficiency of the method.
About new applications of parametric X-radiation for crystallography
Authors: Feranchuk I. D. , Ulyanenkov A.
Published: July 2002

A theoretical analysis of the parametric X-radiation (PXR) produced by the nonrelativistic electrons in an X-ray tube with a monocrystalline anode is presented. It is shown that PXR can be considered as a source of quasi-monochromatic X-radiation for laboratory diffraction experiments. The intensity of the PXR is comparable with the intensity of the characteristic radiation, but the frequency of the former can be varied in a wide range. Possible applications of the phenomenon for crystallography are discussed.
Report of a Subcommittee on the Nomenclature of n-Dimensional Crystallography. I. Symbols for point-group transformations, families, systems and geometric crystal classes
Authors: Janssen T. , Birman J. L. , Koptsik V. A. , Senechal M. , Weigel D. , Yamamoto A. , Abrahams S. C. , Hahn Th.
Published: July 2002

The notation of crystallography in arbitrary dimensions is considered. Recommended symbols for point-group transformations, geometric crystal classes, families and systems are presented.
The phase transitions and crystal structures of Ba3
RM
2O7.5 complex oxides (R = rare-earth elements, M = Al, Ga)
Authors: Abakumov A. M. , Shpanchenko R. V. , Lebedev O. I. , Van Tendeloo G. , Amelinckx S. , Antipov E. V.
Published: July 2002

The structures of α-Ba3
RAl2O7.5 and β-Ba3
RM
2O7.5 complex oxides (R = rare-earth elements, M = Al, Ga) have been studied by a combination of X-ray diffraction, electron diffraction (ED) and high-resolution electron microscopy (HREM). The α and β forms have cell parameters related to the perovskite subcell: a = 2a
per, b = a
per(2)1/2, c = 3a
per(2)1/2, however, the α form has an orthorhombic unit cell whereas the β form adopts monoclinic symmetry. The crystal structure of monoclinic Ba3ErGa2O7.5 was refined from X-ray powder data (space group P2/c, a = 7.93617 (9), b = 5.96390 (7), c = 18.4416 (2) Å, β = 91.325 (1)°, R

I
= 0.023, R

P
= 0.053), the structure of the α form (space group Cmc21) was deduced from ED and HREM data. The important feature of the α and β structures is the presence of slabs containing strings of vertex-sharing tetrahedral Al2O7 pairs. Two almost equivalent oxygen positions within the strings can be occupied either in an ordered manner leading to the low-temperature β phase or randomly resulting in the high-temperature α structure. The critical temperature of this order–disorder phase transition was determined by high-temperature X-ray diffraction and by differential thermal analysis (DTA). In situ ED and HREM observations of the second-order phase transition confirmed the symmetry changes and revealed numerous defects (twins and antiphase boundaries) formed during the phase transformation.
Parameterization of the temperature dependence of the Debye–Waller factors
Authors: Gao H. X. , Peng L.-M.
Published: July 2002

Parameterization has been made for the temperature dependence of the Debye–Waller factors of 68 elemental crystals and 17 compounds with the zinc-blende structure. The Debye–Waller factors of 46 elemental crystals were calculated based on the available phonon density of states, and those of the remaining 22 elemental crystals were estimated using the Debye approximation for the phonon density of states. The Debye–Waller factors of the zinc-blende compounds were obtained from the 14-parameter shell-model calculations of Reid [Acta Cryst. (1983), A39, 1–13].
Crystallographic and molecular mechanics investigation of an order–disorder transition and dimorphism in 5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione
Authors: Chohan Zahid H. , Harrison William T. A. , Howie R. Alan , Milne Bruce F. , Wardell James L.
Published: July 2002

Single-crystal X-ray structures are presented for three forms of 5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione. The α (at 150 K) and α′ (at ambient) forms are very similar and differ only in the presence of crystallographic m symmetry in the molecules of α′, which is absent in the case of α. This pair is related by an order–disorder transition. The β phase (also determined at 150 K) has a different structure in terms of the molecular packing from either of the other two and therefore constitutes a true polymorph. Molecular mechanics calculations indicated that the most stable CHCl3-solvated conformations for the title compound were a pair of twisted U-shaped enantiomers, UR
and UL
, i.e. similar to the arrangements found in the α and β phases, with the low-lying saddle point between them corresponding to the situation in the α′ phase. These calculations also indicated that the most stable CHCl3-solvated conformation for the related dibromo-5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione was Z-shaped, in agreement with the crystal structure determined earlier for its DMSO solvate [Wang et al. (1998). Synthesis, pp. 1615–1618].
Commensurate structure of Ca2CoSi2O7, a new twinned orthorhombic structure
Authors: Hagiya Kenji , Kusaka Katsuhiro , Ohmasa Masaaki , Iishi Kazuaki
Published: July 2002

The crystal structure of the commensurate phase of Ca2CoSi2O7, dicalcium cobalt disilicate, has been derived from the modulated structure described in (3 + 2)-dimensional space. The structure is orthorhombic P21212; a = 23.510 (4), b = 23.510 (4), c = 5.025 (1) Å (at 170 K), Z = 18. Since the crystal is twinned and the apparent diffraction symmetry is 4/mmm, the parameters were refined by a newly developed least-squares program for the refinement of twinned crystals. The structure is essentially similar to the known structure of the melilite group, but with regular arrangement of the bundles along [001] formed with four arrays of the sixfold coordinated Ca polyhedra and an array of CoO4 tetrahedra. The distribution of the bundles found in the present structure is different from that reported by Riester et al. [(2000), Z. Kristallogr.
215, 102–109].
A new set of molecular descriptors
Authors: Blatova O. A. , Blatov V. A. , Serezhkin V. N.
Published: July 2002

A new set of criteria for quantitative analysis of molecular interactions is proposed, which is based on the conceptions of atomic and molecular Voronoi–Dirichlet polyhedra. It is shown that the calculation of solid angles of ligands and complexes as a whole allows one to estimate screening effects and the probability of forming intra- and intercomplex non-valent contacts. The set proposed was used to study the influence of steric factors on the stability of complex groups as well as on the presence or absence of agostic contacts in crystal structures of 808 rare-earth π-complexes.
Coupling of the lactone-ring conformation with crystal symmetry in 6-hydroxy-4,4,5,7,8-pentamethyl-3,4-dihydrocoumarin
Authors: Budzianowski Armand , Katrusiak Andrzej
Published: July 2002

Conformational disorder and inversions of the lactone ring induce structural transformations in the crystals of 6-hydroxy-4,4,5,7,8-pentamethyl-3,4-dihydrocoumarin, C14H18O3. The onset of ordering of the lactone ring at 300 K proceeds continuously, changes the space group from P21/m to P21/c and doubles the unit cell; the abrupt inversion of the lactone rings at 225 K changes the crystal translational symmetry in the (010) plane. The mechanism combining the molecular conformation and dynamics with the crystal structure, its symmetry, and phase transitions is presented.
Planar packing of tetrachlorodicyanobenzene isomers
Authors: Britton Doyle
Published: July 2002

Ortho-, meta- and para-tetrachlorodicyanobenzene (TBD, tetrachlorobenzenedicarbonitrile) form 1/1 complexes with hexamethylbenzene (HMB). Seven polymorphic forms of the crystals, one of o-TDB, two of m-TDB and four of p-TBD, have been found. All of the complexes involve layers of two-dimensional arrays, and in six of the complexes the layers are homogeneous, alternate layers being entirely TDB or entirely HMB. The layers can be described in terms of an hexagonal tiling and in five of the homogeneous layers the TDB are ordered so that one CN and two Cl come together at each corner of the tiles. In the sixth the m-TDB is disordered, but a plausible arrangement can still be found with one CN and two Cl coming together in the same fashion.
[Ni(cyclam)(OCOR)2], a finite molecular complex: hydrogen-bonded supramolecular aggregation in one, two and three dimensions, and coordination polymers in one and two dimensions. Erratum
Authors: Zakaria Choudhury M. , Ferguson George , Lough Alan J. , Glidewell Christopher
Published: July 2002

In the paper by Zakaria et al. (2002) Acta Cryst. (2002), B58, 78–93 the title was printed incorrectly. The correct title should read: [Ni(cyclam)(OCOR)2]: a finite molecular complex; hydrogen-bonded supramolecular aggregation in one, two and three dimensions; and coordination polymers in one and two dimensions.
New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design
Authors: Belsky Alec , Hellenbrandt Mariette , Karen Vicky Lynn , Luksch Peter
Published: July 2002

The materials community in both science and industry use crystallographic data models on a daily basis to visualize, explain and predict the behavior of chemicals and materials. Access to reliable information on the structure of crystalline materials helps researchers concentrate experimental work in directions that optimize the discovery process. The Inorganic Crystal Structure Database (ICSD) is a comprehensive collection of more than 60 000 crystal structure entries for inorganic materials and is produced cooperatively by Fachinformationszentrum Karlsruhe (FIZ), Germany, and the US National Institute of Standards and Technology (NIST). The ICSD is disseminated in computerized formats with scientific software tools to exploit the content of the database. Features of a new Windows-based graphical user interface for the ICSD are outlined, together with directions for future development in support of materials research and design.
Electron-density study of m-nitrophenol in the orthorhombic structure
Authors: Hamzaoui F. , Baert F. , Wojcik G.
Published: July 2002

m-Nitrophenol (m-NPH) occurs at room temperature in two solid modifications: monoclinic P21/n and orthorhombic P212121 [Pandarese, Ungaretti & Coda (1975). Acta Cryst. B31, 2671–2675]. The thermal vibrations and electronic-density distribution of the molecule in the orthorhombic structure have been analyzed in terms of Stewart's rigid pseudoatom model, using restricted Slater radial functions and angular multipole terms extending to actapoles for C, N and O and dipoles for H pseudoatoms. The net atomic charge and the in-crystal molecular dipole moment have been determined in order to understand the nature of the inter- and intramolecular charge transfer. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of the H atoms, in the absence of complementary neutron diffraction data.
Crystal and magnetic structure of piezoelectric, ferrimagnetic and magnetoelectric aluminium iron oxide FeAlO3 from neutron powder diffraction
Authors: Bouree F. , Baudour J. L. , Elbadraoui E. , Musso J. , Laurent C. , Rousset A.
Published: July 2002

The crystal structure of FeAlO3 has been determined at T = 298 K by neutron diffraction, using polycrystalline samples prepared in a high state of purity. The space group is Pna21, Z = 8; a = 4.9839 (1), b = 8.5544 (2), c = 9.2413 (2) Å. The structure, which is isomorphous to that of FeGaO3, can be described as a double combination of hexagonal and cubic closed packing of oxygen ions. There are four different cation sites labelled Fe1, Fe2 (predominantly occupied by iron), Al1 and Al2 (predominantly occupied by aluminium). The oxygen environment of Al1 forms an almost regular tetrahedron. The other sites have a distorted octahedral environment, especially irregular for Fe1 and Fe2. The fractions fi
of iron ions over the four cation sites are: f
1 = 0.78 (1), f
2 = 0.76 (1), f
3 = 0.10 (1) and f
4 = 0.34 (1). Neutron diffraction at T = 30 K reveals a classical Néel ferrimagnetism, the direction of easy magnetization being a, with strong `180° cation-anion-cation' super-exchange antiferromagnetic interactions Fe1—O—Fe2 and Fe1—O—Al2 (Al2 being a site occupied by 0.34 Fe). The Néel sublattices are A = Fe1 + Al1 and B = Fe2 + Al2. The average magnetic moment per atom is weak (3.4 ± 0.3 μB
) and the spontaneous magnetization at T = 30 K is extremely weak: 0.38 ± 0.17 μB
per atom. Piezoelectricity probably originates in the bond arrangement of the four tetrahedral All sites in the unit cell, each tetrahedron being oriented with an Al1—O bond parallel to the polar c axis.
Isothermal Compressibility and Pressure Dependence of the Crystal Structures of the Superconducting Charge-Transfer Salt κ-(BEDT-TTF)2Cu(NCS)2 [BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene]
Authors: Rahal M. , Chasseau D. , Gaultier J. , Ducasse L. , Kurmoo M. , Day P.
Published: July 2002

From the pressure dependence of the lattice parameters measured by single-crystal X-ray diffraction at room temperature, the amplitudes and directions of the principal components of the isothermal compressibility tensor of κ-(BEDT-TTF)2Cu(NCS)2, bis(ethylenedithio)tetrathiafulvalene copper dithiocyanate, have been determined from 1 × 105 Pa to 12.9 × 105 kPa. The crystal structure has been determined at 1 × 105 Pa and 7.5 × 105 kPa. From the latter, the pressure dependence of the intermolecular electron-transfer integrals and the band structure have been calculated and compared with those at 1 × 105 Pa and 15 K.
Synthèse et caractérisation structurale d'un phosphate d'indium In3P2O8 présentant des paires In–In
Authors: Peltier V. , L'Haridon P. , Marchand R. , Laurent Y.
Published: July 2002

The crystal structure of the new indium phosphate In3P2O8 has been determined from single-crystal data. The unit cell is cubic [space group I{\bar 4}3d (Z = 8)] with a = 11.152 (1) Å. The structure is characterized by a three-dimensional arrangement of isolated PO4
3− tetrahedra forming the anionic network, while the electroneutrality is insured by (In—In)4+ cationic pairs. The In—In distance, equal to 2.630 (1) Å, is the shortest In—In bond ever found. Powdered In3P2O8 can be synthesized from a mixture of 4 InPO4:1 In2O3 after thermal reduction under H2.
Incommensurate modulated disorder in Ba0.85Ca2.15In6O12
Authors: Baldinozzi G. , Goutenoire F. , Hervieu M. , Suard E. , Grebille D.
Published: July 2002

The tunnel structure of the oxide Ba0.85Ca2.15In6O12 has been revisited. A mono-incommensurate modulation has been evidenced by electron diffraction. The simultaneous refinement of X-ray and neutron powder diffraction patterns has allowed a complete determination of the incommensurate structure [P3({{1}\over{3}} {{1}\over{3}}γ)]. The existence of a modulated disorder, affecting Ba and Ca atoms, has been pointed out. The occupancy modulation concerning Ba and Ca atoms located on the hexagonal tunnel axis induces a modulation of the Ba—O bond lengths. The high-resolution electron microscopy study has shown the existence of twinning domains and of crystal regions possessing a defective arrangement of the modulation.
Neutron powder diffraction study of α-Ti(HPO4)2.H2O and α-Hf(HPO4)2.H2O; H-atom positions
Authors: Salvadó M. A. , Pertierra P. , García-Granda S. , García J. R. , Rodríguez J. , Fernández-Díaz M. T.
Published: July 2002

The complete crystal structures, including H-atom positions, of α-Ti(HPO4)2.H2O (α-TiP) and α-Hf(HPO4)2.H2O (αHfP) were determined by Rietveld refinement and Fourier synthesis, using constant-wavelength neutron diffraction data. This work is one of few recent examples of the determination of H atoms using neutron powder diffraction with an intense source. The orientation of water molecules in the cavities of α-TiP is similar to that previously found for α-ZrP. For α-HfP, although the hydrogen-bonding scheme is identical, the orientation of water molecules in similar cavities was found to be different.
Distortion of Crystal Structures of Some CoIII Ammine Complexes. II. Distortion of Crystal Structures of [Co(NH3)5NO2]X
2 (X = Cl, Br) on Cooling
Authors: Boldyreva E. , Kivikoski J. , Howard J. A. K.
Published: July 2002

For two solid complexes, [Co(NH3)5NO2]X
2 (X = Cl−, Br−), pentaamminenitrocobalt(III) dichloride/dibromide, the anisotropy of structural distortion on cooling the samples from 290 to 150 K was studied by variable-temperature single-crystal X-ray diffraction. The changes in lattice parameters were used to calculate the strain tensors and to scan linear strain in various crystallographic directions. The changes in different intra- and intermolecular distances in the structure were followed. Lattice strain on cooling was compared with lattice strain in the same structures under high hydrostatic pressure and with structural distortion in the course of homogeneous nitro–nitrito linkage isomerization. The effect of isomorphous substitution (Br− for Cl−) with respect to structural strain was discussed. Structural strain in the monoclinic chloride and bromide was compared also with the previously obtained data [Boldyreva, Kivikoski & Howard (1997). Acta Cryst. B53, 394–404] on strain in the orthorhombic structure of another compound of the same CoIII nitropentaammine series, [Co(NH3)5NO2]CI(NO3).
Structural properties of synthetic ionophore GL
2
E
4 diastereomers, 18-membered cyclic lactone tetraepoxides, by X-ray crystal analyses: possible relation between crystal structure and Ca2+-transportation ability. Addendum
Authors: Doi M. , In Y. , Nishino T. , Ishida T. , Inoue M. , Shibuya H. , Ohashi K. , Narita N. , Kitagawa I.
Published: July 2002

Structure factors, anisotropic displacement parameters, H-atom coordinates and complete geometry for the paper by Doi, In Nishino, Ishida, Inoue, Shibuya, Ohashi, Narita & Kitagawa [Acta Cryst. (1995), B51, 1045–1050] have been deposited with the IUCr (Reference: AS0697). Copies may be obtained through The Mangaging Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.