Crystal structure of post-perovskite-type Cairo₃ reinvestigated: New insights into atomic thermal vibration behaviors

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.