No abstract

The thermal expansion of CaIrO 3 , the archetype phase of the "post-perovskite" polymorph of MgSiO 3 , has been determined by X-ray powder diffraction from 113 K to 1173 K. For temperatures above 298 K, the volumetric coefficient of thermal expansion,

stabilised by chemical substitution so as to occur within the standard operating range of a multi-anvil press, is briefly discussed. For NaMgF 3 , the transitions to the high-pressure phases occur at pressures outside the normal range of a multi-anvil press. Two different sequences of transitions had previously been suggested from computer simulations. With increasing pressure, we find that the relative stability of the NaMgF 3 phases follows the sequence: perovskite → CaIrO 3 structure → Sb 2 S 3 structure → P6 3 /mmc structure. However, only the perovskite and CaIrO 3 structures are stable with respect to dissociation into NaF and MgF 2 .

Using the recently upgraded Polaris diffractometer at the ISIS Spallation Neutron Source (Rutherford Appleton Laboratory), the crystal structures of the post‐perovskite polymorphs of NaCoF3 and NaNiF3 have been determined by time‐of‐flight neutron powder diffraction from samples, of mass 56 and 16 mg, respectively, recovered after synthesis at ∼20 GPa in a multi‐anvil press. The structure of post‐perovskite NaNiF3 has also been determined by single‐crystal synchrotron X‐ray diffraction for comparison. All measurements were made at atmospheric pressure and room temperature. Despite the extremely small sample size in the neutron diffraction study, there is very good agreement between the positional parameters for NaNiF3 obtained from the refinements of the X‐ray and neutron data. Relative to the commonly used oxide post‐perovskite analogue phases having calcium as the A cation, the axial ratios and derived structural parameters of these fluoride post‐perovskites are more consistent with those of Mg0.91Fe0.09SiO3 at high pressure and temperature. The structures of NaCoF3 and NaNiF3 are very similar, but the unit‐cell and CoF6 octahedral volumes of NaCoF3 are larger than the corresponding quantities in NaNiF3, which supports the hypothesis that the Co2+ ion has a high‐spin state in this compound. The anisotropic atomic displacement parameters of the Na ions in NaNiF3 post‐perovskite are of similar magnitude to those of the F ions. The probability ellipsoid of the F1 ion is a prolate spheroid with its largest component parallel to the b axis of the unit cell, corresponding to rotational motion of the NiF6 octahedra about the a axis of the crystal. Although they must be synthesized at pressures above about 18 GPa, these ABF3 compounds are strongly metastable at atmospheric pressure and room temperature and so are highly suitable for use as analogues for (Mg,Fe)SiO3 post‐perovskite in the deep Earth, with significant advantages over oxides such as CaIrO3 or CaPtO3.

11ABX 3 post-perovskite phases that are stable (or strongly metastable) at room-pressure are of 12 importance as analogues of post-perovskite MgSiO 3 , a deep-Earth phase stable only at very 13 high pressure. Commonly, CaIrO 3 has been used for this purpose, but it has been suggested 14 that CaPtO 3 might provide a better analogue. We have measured the isothermal 15 incompressibility, at ambient temperature, of orthorhombic post-perovskite structured 16 CaPtO 3 to 40 GPa by X-ray powder diffraction using synchrotron radiation.