New examples of non-cooperative octahedral tilting in a double perovskite: phase transitions in K₃GaF₆

The crystal structures of three polymorphs of K₃GaF₆ and the transition temperatures between these phases are reported for the first time. Synchrotron powder diffraction data clearly show that at 300 K α-K₃GaF₆ crystallizes in space group I4₁/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 GaF₆ octahedra rotated by ∼45°. This phase remains stable until ∼460 K, above which it undergoes a transition to I4/m with lattice parameters of a = 13.6088 (4) Å, c = 8.6764 (3) Å, and Z = 10 at 485 K. β-K₃GaF₆ has a similar structure but with only one fifth of the GaF₆ rotated by ∼45°. Above ∼510 K, the cubic Fm3m δ-K₃GaF₆ 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.