High-pressure behaviour of a CaIrO₃ perovskite-type structure

Abstract. Structural refinements of single-crystal X-ray diffraction data collected at room temperature both under ambient and under high pressures up to 10 GPa have been used to characterise the octahedral tilting and site distortions of the orthorhombic Pbnm CaIrO3 perovskite-type phase. The octahedral tilting has been evaluated using both the unit-cell lattice parameters and the displacements of the oxygen and A site from their hypothetical positions in an ideal cubic perovskite structure. The difference between the two types of calculations shows that the assumption of rigid and undistorted octahedra made for determining the tilt angles from the unit-cell parameters is valid only as a first approximation, especially in the case of CaRuO3, CaRhO3 and CaIrO3 perovskites, for which polyhedral distortions are significant. CaIrO3 perovskite appears to have much larger tilting and distortion than other CaO perovskites, likely causing the structure to be stiffer than expected from the general trend of the bulk moduli defined by other CaO perovskites. Moreover, in contrast to other CaO perovskites for which the Ca polyhedron is more compressible than the octahedral site, the compressibility of the IrO6 octahedra is very similar to that of the CaO12 polyhedra, giving rise to an octahedral tilting that is independent of pressure at least up to 10 GPa. This behaviour may help reduce the electronic interactions among oxygens of neighbouring octahedra during compression, given the already large tilts present in the CaIrO3 perovskite structure, and it is likely responsible for the bulk modulus's first pressure derivative being smaller than 4. Larger tilting and distortions are also observed for CaO perovskites that have a Pt-group element occupying the octahedral site. Such perovskites have been found to undergo a phase transformation to an orthorhombic Cmcm post-perovskite phase (CaIrO3-type structure) at pressures below 25 GPa, suggesting that a large octahedral tilting and polyhedral distortion may be a requisite for stabilising the orthorhombic post-perovskite phase of CaO perovskite-type materials.