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Time‐of‐flight neutron powder diffraction with milligram samples: the crystal structures of NaCoF 3 and NaNiF 3 post‐perovskites
Author(s) -
LindsayScott Alex,
Dobson David,
Nestola Fabrizio,
Alvaro Matteo,
Casati Nicola,
Liebske Christian,
Knight Kevin S.,
Smith Ronald I.,
Wood Ian G.
Publication year - 2014
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576714021803
Subject(s) - neutron diffraction , perovskite (structure) , crystallography , powder diffraction , powder diffractometer , analytical chemistry (journal) , diffractometer , materials science , chemistry , neutron , crystal structure , single crystal , ion , diffraction , optics , physics , nuclear physics , organic chemistry , chromatography
Using the recently upgraded Polaris diffractometer at the ISIS Spallation Neutron Source (Rutherford Appleton Laboratory), the crystal structures of the post‐perovskite polymorphs of NaCoF 3 and NaNiF 3 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 NaNiF 3 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 NaNiF 3 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 Mg 0.91 Fe 0.09 SiO 3 at high pressure and temperature. The structures of NaCoF 3 and NaNiF 3 are very similar, but the unit‐cell and CoF 6 octahedral volumes of NaCoF 3 are larger than the corresponding quantities in NaNiF 3 , which supports the hypothesis that the Co 2+ ion has a high‐spin state in this compound. The anisotropic atomic displacement parameters of the Na ions in NaNiF 3 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 NiF 6 octahedra about the a axis of the crystal. Although they must be synthesized at pressures above about 18 GPa, these AB F 3 compounds are strongly metastable at atmospheric pressure and room temperature and so are highly suitable for use as analogues for (Mg,Fe)SiO 3 post‐perovskite in the deep Earth, with significant advantages over oxides such as CaIrO 3 or CaPtO 3 .