Papers by Keyword: Maximum-Entropy Method

Abstract: A multi-purpose pattern-fitting system, RIETAN-2000, has been extensively utilized to contribute to many structural studies. It offers a sophisticated structure-refinement technique of whole-pattern fitting based on the maximum-entropy method (MEM) in combination with a MEM analysis program PRIMA. We have recently completed a successor system, RIETAN-FP, to RIETAN-2000, adding new features such as standardization of crystal-structure data, an extended March-Dollase preferred-orientation function, and automation of imposing restraints on bond lengths and angles. Further, we have been developing a new three-dimensional visualization system, VESTA, using wxWidgets as a C++ application framework. VESTA excels in visualization, rendering, and manipulation of crystal structures and electron/nuclear densities determined by X-ray/ neutron diffraction and electronic-structure calculations. VESTA also enables us to display wave functions and electrostatic potentials calculated with part of these programs.

Abstract: Crystal structures of SrAl2O4, BaAl2O4 and their solid solutions have been reviewed in terms of the linkage pattern of [AlO4] tetrahedra. With SrAl2O4 the hexagonal-to-monoclinic phase transformation occurs at 950K during cooling. The space group change from P63 to its subgroup P21 eliminates the triad axis of the former phase, which involves a reduction in the symmetry of the trigonally distorted rings. The hexagonal structures of SrAl2O4 and BaAl2O4 differ distinctly in the linkage pattern of the [AlO4] tetrahedra. In the former structure, all of the tetrahedral rings are equivalent. In the latter, there are two types of tetrahedral rings; trigonal rings and asymmetrical ones. The trigonal rings, comprising 25% of the total number of rings, contain in their centers the Ba atoms with the special position. This implies that the triad axes exist in the centers of the rings, and hence they are distorted trigonally as in the hexagonal SrAl2O4. On the other hand, the Ba atoms in the asymmetrical rings are located at the general position site. The structural disorder in Ba0.6Sr0.4Al2O4 (space group P6322) was investigated by the combined use of Rietveld method and maximum-entropy method. The electron density distribution was satisfactorily expressed by the split-atom model, in which the strontium/barium and oxygen atoms were split to occupy the lower symmetry sites.

Abstract: The structure of a high ionic conductor Rb₄Cu₁₆I_7.2Cl_12.8 at low temperature has been reinvestigated by use of angle-dispersive neutron powder diffraction. All the diffraction data have been analyzed with integrated software REMEDY consisting of RIETAN-2000 for whole-pattern fitting and MEED for calculating densities of electrons or atomic nuclei by the maximum-entropy method. The resulting nuclear-density maps have reconfirmed that Cu1-Cu2 chains are the main conduction pathways in this material, as previously suggested from the Rietveld analysis of neutron powder diffraction data.