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

Abstract

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.

The phase diagrams of KCaF3 and NaMgF3 by ab initio simulations

“…An estimate of the magnitude of these changes can be obtained by considering the CaIrO 3 structure parameterised in terms of the four shortest cation–anion distances and three tilt angles, as suggested by Lindsay-Scott et al ( 2011 ; Equations 4–6). By putting the interatomic distances equal to the sum of the ionic radii (Shannon 1976 ), and assuming the same tilt angles as in NaNiF 3 (Lindsay-Scott et al 2014 ), we obtain a = 3.48 Å, b = 12.12 Å and c = 8.67 Å for CaIrO 3 -type KCaF 3 at P = 0, in good agreement with the cell parameters obtained by fitting the axial EoS to the VASP simulations ( a = 3.47 (1) Å, b = 12.1 (2) Å and c = 8.36 (1) Å). If the AF distance is now reduced to that appropriate for a composition of K 0.5 Na 0.5 CaF 3 , a value of b = 11.00 Å then results ( a and c are unchanged in this approximation), which is equivalent to applying a pressure of ~4.1 GPa.…”

“…For this reason, other ABF 3 fluorides, such as NaCoF 3 and NaNiF 3 (Dobson et al 2011 ; Shirako et al 2012b ; Yusa et al 2012 ), which may be prepared in a multi-anvil press and then recovered to atmospheric pressure, where they remain quite strongly metastable, have been suggested as more suitable analogues for PPV-MgSiO 3 . These compounds have already proved to be of great value in experimental studies, in revealing the relative strength of PV and PPV polymorphs (Dobson et al 2012 ), in demonstrating the topotaxic nature of the PV to PPV transition (Dobson et al 2013 ), in determining the degree of anisotropy in the diffusion coefficients of the PPV phase (Dobson et al 2014 ) and in allowing the determination of accurate PPV crystal structures (Lindsay-Scott et al 2014 ).…”

Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary

“…First, we reiterate that shape anisotropy is seen in all lowpressure analog postperovskite-structured materials (19)(20)(21)24) (and related structures such as stibnite, Sb 2 S 3 ) and suggest that magnesium silicate postperovskite will also grow with a [100]-rod morphology. This is consistent with postperovskite morphologies which would be predicted from growth morphology theories based on lattice spacings (25)(26)(27) or attachment energies (28).…”

“…Fig. 3 shows a single crystal of NaCoF 3 postperovskite embedded in finely crystalline NaZnF 3 from a diffusion experiment at 14 GPa and 823 K. The fluoride postperovskites form with rod or needle habits (with aspect ratios ranging from 4:1 for CaIrO 3 to >10:1 for NaNiF 3 ) with their a axis parallel to the rod/needle axis (19)(20)(21). The observed diffusion profile in the direction of the long axis of the crystal is well fitted, with a codiffusivity in the single crystal of 5.3 ± 0.42 × 10 −14 cm 2 /s and a matrix Zn diffusivity of 2.0 ± 1.0 × 10 −14 cm 2 /s (Fig.…”

“…The inability to determine the crystal orientation in the diffusion couple, due to the small crystal size and weak metastability of fluoride postperovskite, means we are limited to comparing the axial value with the mean radial value. The long axis of postperovskite crystals is invariably the a axis in a range of systems (10,(19)(20)(21), but we cannot identify the orientation of the other 2 axes in our diffusion couple. The observed radial diffusivity displays the most variability in the experiments; if we assume that this arises from differences in the diffusivities in the b and c axial directions, then, by comparison with the simulations, we can ascribe the smallest and largest radial diffusivities to the b and c directions, respectively.…”