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The local post‐perovskite structure and its temperature dependence: atom‐pair distances in CaIrO 3 revealed through analysis of the total X‐ray scattering at high temperatures
Author(s) -
Martin C. David
Publication year - 2008
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889808019365
Subject(s) - pair distribution function , rietveld refinement , atom (system on chip) , crystallography , chemistry , scattering , bond length , crystal structure , perovskite (structure) , diffraction , x ray crystallography , molecular physics , physics , optics , quantum mechanics , computer science , embedded system
The temperature‐dependent post‐perovskite structure model of MgSiO 3 is reinvestigated through analysis of the atom‐pair distances observed experimentally via Fourier transformation of X‐ray diffraction and diffuse scattering, the total X‐ray scattering, from CaIrO 3 . In contrast to the results of a previous Rietveld structure refinement, which shows a negative or null thermal expansion of Ir—O and Ca—O bond lengths within the average long‐range structure, visual inspection of these atom‐pair distances in the pair‐distribution function, in addition to structure models fitted through least‐squares refinement to this local‐structure data, strongly suggests that these distances between atom pairs increase with temperature. The average long‐range structure of CaIrO 3 , visible from Rietveld structure refinement, is distinct from the short‐range structure (≤ 18 Å) at all of the temperatures examined in this study (325–1114 K) and is reproduced in structure models fitted to the pair‐distribution function extending to sufficiently long atom‐pair distances (≥ 50 Å). While previous data obtained with Rietveld structure refinement show the iridium coordination octahedra to distort with increasing temperature, models of the short‐range structure demonstrate that these polyhedra instead reduce distortion and rotate in a manner similar to that occurring in the perovskite structure.