The resemblance between powder patterns because of similarity of crystal structures is well known and widely used. This phenomenon facilitates the determination of unit cells and is frequently used to predict crystal structures of new substances. At present the matching of diffraction analogues is done mainly by hand. Some approaches have been considered in this paper for applying a computer to the problem. Four numerical criteria for resemblance of powder patterns are suggested. Powder patterns are matched with patterns in a database by making use of a computer program based on these criteria.

The procedure results in a short list of powder patterns to be examined by the expert. The efficiency of the program is illustrated by examples of calculations for substances of both high and low symmetry. The search system may find an important application in X-ray powder diffraction analysis for the identification of solid solutions, of substances documented under unusual conditions, of structure analogues and for classification of patterns in a database.

Yttrium aluminium garnet, Y3A15O12– YAG, has a powder diffraction pattern that is easy to analyse in principle, but shows very interesting problems. These problems are revealed using an interactive graphics system, UNIRAS, intregated with some of the usual procedures for crystallographic data processing. For example, indexing with the APPLEMAN procedure is improved when the high-angle peaks are properly identified, and the cell parameters then refine to a high precision. Iterations between refinements and graphical analysis allows the operator to identify even some of the very weak peaks in the pattern. Using the peak positions from the refinement, the experimental data can be analyzed for instrumental aberrations and corrections can be applied to improve the accuracy of the cell parameter.

The paper describes a stepping motor unit which can replace the mechanical connection between a goniometric circle (ϑ or 2ϑ) and the rotatory movement of the Automatic Divergence Slit attachment.

Recent developments in the Rietveld method for the analysis of powder diffraction data have seen the method evolve from its original purpose of crystal structure refinement to include the determination of phase abundance in polycrystalline mixtures and the estimation of crystal size and strain parameters. However, the Rietveld method is not easy to use and may deter many powder diffractionists, who are not interested in structure refinement per se, from using the method in its non-structural applications.

In order to overcome the difficulties in using the Rietveld method, a program, QPDA (for Quantitative Powder Diffraction Analysis), has been written that sets the conditions necessary for a single or multi-phase refinement, runs the Rietveld program and extracts phase abundance and size/strain information from the refined parameters. The program comprises a user-friendly, default-driven system of subroutines, written initially in VAX Fortran, and operates from a database of inorganic materials frequently encountered in a wide range of minerals and materials science industries.

The texture index introduced by Harris (1952) is reconsidered from the point of view of the orientation distribution of crystallites in a flat powder specimen. It is shown that the texture index can be used as an approximate measure of texture degree or as a texture correction factor in cases of a weak texture. Its failure in both these aspects for the strong textures is caused by an incorrect averaging of pole densities contained in the original definition of this quantity.

This note discusses the use of the powder-diffractometer intensity-response function reference standard to be released shortly by the National Institute of Standards and Technology (NIST) as a Standard Reference Material (SRM). Its purpose is to encourage use of the standard and to stimulate discussion and comments from readers with regard to the procedure for its use. Comments may be addressed to either of the authors.

CdTexSe1-xsolid solutions with (x) ranging between zero and one were prepared by solid state diffusion under vacuum and their precise lattice constants and X-ray powder diffraction data were determined. It was found that alloys with 0≤x≤0.4 possess the hexagonal wurtzite structure while those with 0.5≤x≤1.0 have the cubic zincblende structure. The lattice parameters obeyed Vegard's law according to the following formulae

Extrapolated lattice constants were a = 6.066(6) Å for the cubic form of CdSe and a = 4.563(6) Å and c = 7.502 (10) Å for the hexagonal form of CdTe.

Observed and calculated X-ray powder diffraction data are given for cis-[bis (7,9 dimethylhypoxanthine) (ethylene diamine) platinum (II)] dihexafluorophosphate. The paper details how the powder data were obtained with CrKα1radiation and how the quality rating of the pattern was assessed with respect to unique indexing and with regard to the agreement between calculated and observed relative intensities.

X-ray powder diffraction and optical data are presented for alkali-deficient schorls from two tourmaline-dumortierite deposits. The compositions of both schorls reflect the presence of an alkali-defect substitution, whereby the alkali cation deficiencies in the crystal structure are charge balanced by trivalent for divalent cation substitutions in the octahedral sites. Refined unit cell and optical data are as follows: a = 15.9523(15), c = 7.1466(2) Å, F30 = 104(0.009, 32), M20= 68, ε = 1.638(2), ω = 1.660(2), Dx= 3.12 for the schorl from Jack Creek, near Basin, Montana, U.S.A. and a = 15.9800 (15), c = 7.1504(2) Å, F30= 156(0.006, 32), M20= 152, ε = 1.642(2), ω = 1.668(2), Dx= 3.17 for the schorl from Ben Lomond, Hervey Range, North Queensland, Australia. Indexed X-ray powder diffraction patterns are also presented.

Three orthovanadates ATh2(VO4)3with A = Li, Na, Ag have been synthesized by solid state reaction. Single crystals of AgTh2(VO4)3were obtained. This compound is isotypic with sheelite whose space group is I41/a(88). The two other compounds (A = Li, Na) have a zircon type structure: I41/ amd(141). Unit-cell parameters and powder diffraction data for the three compounds are reported.

The crystal structure of the spinel polymorph of Fe2SiO4, synthesized at high temperature (900°C) and high pressure (70 kbar), was studied by the Rietveld analysis of X-ray powder diffraction data collected with a Guinier-Hägg camera. The compound is cubic, space group , with cell dimension: a= 8.2413(6) Å, V= 559.8(1) Å3, Z= 8, Cell Wt. = 1630.2, Dx= 4.835 g·cm−3, Do= 4.75 g·cm−3. The figure of merit is F10= 92(0.011, 10). The final R value is RF= 0.058. The crystal has a mixed normal-inverse spinel structure. The site occupancy refinement showed that 37.9% of the silicon was found in the octahedral site (M site), while 18.9% of the iron occupied the tetrahedral site (T site). Due to the larger displacement of Si4+ion by Fe2+ion, the positional parameter of oxygen atom (0.3689) is smaller than that of X-ray single crystal structure (0.3658), and the average Si-O bond (1.697(1)Å) is longer and Fe-O bond (2.112(1)Å) is shorter than those of X-ray single crystal structure.

Observed and calculated X-ray powder diffraction data are reported for ZnGa2S4. After corrections to the observed data, refined tetragonal unit cell parameters ao= 5.2779(2), co= 10.4179(8) Å (λ = 1.54051 Å) were obtained. The powder pattern for ZnGa2S4was calculated in space group Iwith a stoichiometric amount of Zn and Ga randomly distributed amongst the two cation sites.

A new X-ray powder diffraction pattern for α-Si3N4is presented and compared with the current PDF pattern. The necessity for an updated pattern is discussed.

X-ray powder diffraction data for Bi2Mo3O12·nH2O (n = 4.75) were obtained. The crystal system was determined to be monoclinic with space group P21or P21/m. The unit cell parameters were refined to a = 6.334(2) Å, b = 11.593(2) Å, c = 5.777(2) Å, and β= 113.166(8)°.

Notwithstanding expectations of a pure pyrochlore structure (cubic space group Fd3m (227) with Z=8) terbium titanate turns out, like many pyrochlore compounds, to be slightly disordered in the cation and the oxygen lattice so that it partially (for 10%) resembles a structure with fluorite properties (space group Fm3m (225) with Z = 2).