Development and Applications of the Pioneering Technology of Structure Refinement from Powder Diffraction Data

Abstract

A couple of Rietveld-analysis programs, RIETAN, for angle-dispersive diffraction and time-of-flight neutron diffraction have been extensively used for structural studies of many compounds. The history of their developments is briefly reviewed. The latest versions, RIETAN-2000 for angle-dispersive diffraction and RIETAN-2001T for the Vega and Sirius diffractometers at KENS have a practical feature of partial profile relaxation. In this technique, primary profile parameters of (nearly) isolated reflections can be individually refined independent of secondary profile parameters. It was combined with split profile functions to improve fits between observed and calculated patterns, particularly in samples exhibiting anisotropic profile broadening and reflections with very large lattice-plane spacings. RIETAN-2000 and our own program for the maximum-entropy method (MEM), PRIMA, were virtually integrated into a structure-refinement system, REMEDY, whereby the pattern calculated from structure factors obtained by MEM is fit to the whole observed pattern. `Observed' structure factors estimated at the end of the whole-pattern fitting are analyzed again by MEM. Whole-pattern fitting and MEM analysis are alternately repeated until R factors in the former no longer decrease. Applications of this technology to three inorganic compounds having highly disordered structures are demonstrated, showing its excellent performance and effectiveness.