High-pressure structure and bonding in CaIrO3: The structure model of MgSiO3 post-perovskite investigated with time-of-flight neutron powder diffraction

Abstract

The structure of CaIrO 3 (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc2 1 is inconclusive. As CaIrO 3 (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca 2+ -centered polyhedra are an order of magnitude more compressible than Ir 4+ -centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO 3 post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra.

The structural diversity of ABS3 compounds with d electronic configuration for the B-cation

“…Supporting this idea is the existence of another ABX 3 compound with A = Ca, CaIrO 3 , which is created in the mixed edge-and corner-sharing phase through the use of elevated pressures. [17] Next, we compare our theoretical space group and structure predictions of stoichiometric ternary sulfides in which the A cations have a lone pair electron configuration, (and with B not equal to Zr), with experimental literature for those systems where non-stoichiometric phases are reported.…”

“…A second type of mixed motif phase, one with mixed edge-and corner-sharing connectivity, is also considered. The Cmcm phase of the compound CaIrO 3 , (which is proposed to exist under high pressures for MgSiO 3 ), [16] and its subgroup Cmc2 1 , (in which atoms are no longer confined to high symmetry positions along the z-axis), [17] have this motif and are respectively designated M M Cmcm and M M Cmc2 1 , with M M signifying "mixed motif". This phase is characterized by planes of BS 6 octahedra, in which the octahedra are connected by edges in one direction in the plane, and by corners in the other planar direction.…”

The isothermal equation of state of CaPtO3 post-perovskite to 40GPa

“…Analogues studied to date include CaIrO 3 (Boffa Ballaran et al, 2007;Lindsay-Scott et al, 2007;Martin et al, 2007Martin et al, , 2007aMartin, 2008;Sugahara et al, 2008;Hirai et al, 2009) and CaPtO 3 (Inaguma et al, 2008;Ohgushi et al, 2008). CaIrO 3 may be synthesised at atmospheric pressure (see e.g.…”