Crystal structure of post-perovskite-type CaIrO3 reinvestigated: new insights into atomic thermal vibration behaviors

Abstract

Single crystals of the title compound, the post-perovskite-type CaIrO3 [calcium iridium(IV) trioxide], have been grown from a CaCl2 flux at atmospheric pressure. The crystal structure consists of an alternate stacking of IrO6 octahedral layers and CaO8 hendecahedral layers along [010]. Chains formed by edge-sharing of IrO6 octahedra (point-group symmetry 2/m..) run along [100] and are interconnected along [001] by sharing apical O atoms to build up the IrO6 octahedral layers. Chains formed by face-sharing of CaO8 hendecahedra (point-group symmetry m2m) run along [100] and are interconnected along [001] by edge-sharing to build up the CaO8 hendecahedral layers. The IrO6 octahedral layers and CaO8 hendecahedral layers are interconnected by sharing edges. The present structure refinement using a high-power X-ray source confirms the atomic positions determined by Hirai et al. (2009) [Z. Kristallogr. 224, 345–350], who had revised our previous report [Sugahara et al. (2008). Am. Mineral. 93, 1148–1152]. However, the displacement ellipsoids of the Ir and Ca atoms based on the present refinement can be approximated as uniaxial ellipsoids elongating along [100], unlike those reported by Hirai et al. (2009). This suggests that the thermal vibrations of the Ir and Ca atoms are mutually suppressed towards the Ir...Ca direction across the shared edge because of the dominant repulsion between the two atoms.

Keywords

• crystal structure
• redetermination
• calcium iridium(IV) trioxide
• post-perovskite
• thermal vibration