Time-of-flight neutron powder diffraction with milligram samples: the crystal structures of NaCoF3and NaNiF3post-perovskites

Using the recently upgraded Polaris diffractometer at the ISIS Spallation Neutron Source (Rutherford Appleton Laboratory), the crystal structures of the post-perovskite polymorphs of NaCoF3and NaNiF3have 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 NaNiF3has 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 NaNiF3obtained from the refinements of the X-ray and neutron data. Relative to the commonly used oxide post-perovskite analogue phases having calcium as theAcation, the axial ratios and derived structural parameters of these fluoride post-perovskites are more consistent with those of Mg0.91Fe0.09SiO3at high pressure and temperature. The structures of NaCoF3and NaNiF3are very similar, but the unit-cell and CoF6octahedral volumes of NaCoF3are 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 NaNiF3post-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 thebaxis of the unit cell, corresponding to rotational motion of the NiF6octahedra about theaaxis of the crystal. Although they must be synthesized at pressures above about 18 GPa, theseABF3compounds are strongly metastable at atmospheric pressure and room temperature and so are highly suitable for use as analogues for (Mg,Fe)SiO3post-perovskite in the deep Earth, with significant advantages over oxides such as CaIrO3or CaPtO3.