The local post-perovskite structure and its temperature dependence:: atom-pair distances in CaIrO3 revealed through analysis of the total X-ray scattering at high temperatures

The temperature-dependent post-perovskite structure model of MgSiO3 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 CaIrO3. 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 CaIrO3, visible from Rietveld structure refinement, is distinct from the short-range structure (<= 18 angstrom) 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 (similar to 50 angstrom). 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.