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New examples of non‐cooperative octahedral tilting in a double perovskite: phase transitions in K 3 GaF 6
Author(s) -
King Graham
Publication year - 2020
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520620009695
Subject(s) - octahedron , crystallography , crystal structure , lattice (music) , phase transition , lattice constant , perovskite (structure) , anisotropy , diffraction , synchrotron , materials science , chemistry , condensed matter physics , physics , optics , acoustics
The crystal structures of three polymorphs of K 3 GaF 6 and the transition temperatures between these phases are reported for the first time. Synchrotron powder diffraction data clearly show that at 300 K α‐K 3 GaF 6 crystallizes in space group I 4 1 / a with lattice parameters of a = 19.1124 (3) Å, c = 34.4165 (6) Å, and Z = 80. The structure is based on the double perovskite but with two fifths of the GaF 6 octahedra rotated by ∼45°. This phase remains stable until ∼460 K, above which it undergoes a transition to I 4/ m with lattice parameters of a = 13.6088 (4) Å, c = 8.6764 (3) Å, and Z = 10 at 485 K. β‐K 3 GaF 6 has a similar structure but with only one fifth of the GaF 6 rotated by ∼45°. Above ∼510 K, the cubic Fm 3 m δ‐K 3 GaF 6 structure is stabilized, with a lattice parameter of a = 8.6649 (1) Å at 550 K. The F atoms have highly anisotropic displacement parameters which suggest dynamic octahedral tilting is occurring. This work expands the fairly small group of double perovskite compounds which display non‐cooperative patterns of octahedral tilting.