[en] Three programs used in the powder diffraction pattern analysis are described. They are helpful in the indexing of X-ray powder patterns and in the refinement of the high symmetry (cubic, hexagonal and tetragonal) unit cell. All the programs are written in Fortran 4. (author)

[en] The correlation between the metallic radius and the ionic radius is used to modify the values of the metallic radii of the rare earth elements. The linear relations between the unit cell dimensions of RM₅ and R₂M₁₇ compounds and the size of rare earth atoms are observed. The exceptions from the linear rules due to the mixed valency and to the off-stoichiometric composition are discussed. (author)

[en] Rietveld refinements for copper indium selenides CuInSe₂ and CuIn₃Se₅ and their mixture were performed using data collected at a Bragg-Brentano diffractometer. The values of lattice parameters, axial ratio and positional parameters are discussed and compared to available literature data for single crystals and polycrystals. Results for CuInSe₂ give a₀=5.78149(1) Angst and c₀=11.61879(4) Angst. The positional parameter x is determined to be equal 0.2271(4). Refinements for CuIn₃Se₅ give an indication concerning the choice of a structural model for this compound. Namely, the best fit is obtained for a model based on I4-bar 2m space group, yielding a composition close to the experimental one and the lattice parameters a₀=5.75812(2) Angst and c₀=11.53593(7) Angst. Parameters of this model are discussed

[en] Powder diffraction data for fine AlN and GaN powders were collected at a Bragg-Brentano diffractometer equipped with a Johansson monochromator and a semiconductor strip detector. Rietveld-refinements were performed in order to determine the structural parameters of these wurtzite-type (space group, P6₃mc) materials. The following crystallographic data were obtained: a 3.11197(2) A, c = 4.98089(4) A, c/a =1.60056(2), u = 0.3869(5) for aluminium nitride and a = 3.28940(1) A, c = 5.18614(2) A, c/a = 1.62606(1), u 0.3789(5) for gallium nitride. These structural data are discussed in the light of earlier experimental and theoretical results

[en] Photoluminescence characteristics as a function of temperature were investigated for Zn_1-xMg_xSe (0< x<0.63) and Cd_1-xMg_xSe (0< x<0.55) crystals grown by the high pressure Bridgman method. In the composition range investigated Cd_1-xMg_xSe crystallizes in a wurtzite structure while Zn_1-xMg_xSe crystallizes in sphelerite and wurtzite for Mg content lower and higher than 0.18, respectively. The temperature dependence of luminescence can be explained by taking into account, that statistical fluctuations of local composition in solid solutions cause spatial fluctuations of potential, leading to localization of excitons at low temperatures. This phenomenon strongly influences radiative recombination processes in such mixed semiconductors. All observed luminescence characteristics can be explained by this exciton localization model

[en] General formulae allowing estimation of the unit-cell volume are derived. They are valid for all Bravais lattices. The precision of the estimation is discussed. The examples illustrate the application of the method. (orig.)

[en] Studies at extreme pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as semiconductors, superconductors or minerals. This is connected with the opportunity of tuning the pressure by many orders of magnitude. Diamond-anvil and large-anvil pressure cells installed at dedicated synchrotron beamlines are efficient tools for examination of crystal structure, equation of state, compressibility and phase transitions. One of basic methods in such studies is powder diffraction. This review is devoted to methods of powder X-ray diffraction at high-pressures generated by devices installed at synchrotron radiation sources, in particular to the principles of operation of high-pressure-high-temperature cells. General information on high-pressure diffraction facilities installed at 11 synchrotron storage rings in the world is provided. Measurement aspects are considered, including (i) pressure generation and calibration, (ii) strain in the sample, the pressure marker and the pressure-transmitting medium and (iii) pressure and temperature distributions within the cells. Sources of interest in high-pressure diffraction studies (design of new materials, observation of new phenomena, confrontation of theory with experiment) are briefly discussed. Recent developments of high-pressure methods make that pressure becomes a variable playing a key role in investigation of condensed matter. The paper ends with some remarks on the possible future developments of the technique

[en] The pseudo-atom model presents the possibility to calculate the different variants of first-coordination-sphere composition. The result obtained provide an indication of the possible compositions of binary intermetallic compounds. The concepts used in the construction of the model are the filling and the symmetry of the coordination sphere. It is recommended for systems where the atoms may be treated as hard spheres. (Auth.)

[en] The optimization method of indexing powder diffraction patterns based on minimizing the suitable criterion function within a required range of unit-cell parameters is presented. The features of the method are discussed. A comupter program is described which performs the optimization in two steps: systematic search and Powell method. The results are refined by the least-squares method. The actual version indexes patterns of orthorhombic or higher symmetry. (orig.)

[en] The expression ''powder diffraction '' denotes the phenomenon of diffraction of any electromagnetic waves or particles on polycrystalline (powdered, bulk or thin film) materials which is used in a wide variety experimental settings. The X-ray powder-diffraction method was devised and developed during the First World War (1916) by a Dutch/Swiss team, Peter Debye and Paul Scherrer, in Goettingen, Germany, and independently, marginally later, by an American, Albert W. Hull in Schenectady, USA. The birth of powder diffraction came four years after the discovery of the phenomenon of single-crystal diffraction made in 1912 by Walther Friedrich, Paul Knipping and Max Laue in Munich and developed from 1912/1913 by William Henry Bragg (father) and William Lawrence Bragg (son), and later by many others. Powder diffraction became a milestone towards an understanding of the nature of materials, especially of those which cannot be prepared in the form of suitable single crystals, and permitted rapid progress in solid state physics and chemistry. The events leading to the discovery of powder-diffraction phenomenon are briefly reviewed. The importance of synchrotron powder diffraction studies, which have developed since 1980s, is emphasised. (author)