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

Abstract

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 Fm{\overline 3}m 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.

Polymorphism in A3MF6 (A = Rb, Cs; M = Al, Ga) grown using mixed halide fluxes

“…Rb 3 GaF 6 also only crystallizes in a single polymorph at room temperature. t -Rb 3 GaF 6 crystallizes in the α-K 3 GaF 6 structure type 14 with space group I 4 1 / a and lattice parameters a = 19.7947(4) Å and c = 35.7471(8) Å at 303(2) K. The structure, shown in Fig. 5, contains columns of isolated GaF 6 octahedra down the c -direction separated by Rb cations, akin to the other analogues.…”

“…Many of the A 3 GaF 6 compounds have also been studied and exhibit similar behavior. [10][11][12][13][14] Synthetic cryolite is produced on an industrial scale using several aqueous routes, typically involving HF as a fluorinating agent. Other polycrystalline A 3 AlF 6 samples have been synthesized from the melt starting with stoichiometric AlF 3 and AF, 2 via high-energy ball milling, 15 and by hydrothermal synthesis.…”

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study

“…It should be pointed out that, unlike K 3 AlF 6 but like K 3 GaF 6 , β-Rb 3 ScF 6 transforms directly into a cubic γ phase without going through an intermediate γ-K 3 AlF 6 with the Fddd (No. 70) space group.…”

“…It is worth noting that Rb 3 ScF 6 is another new member of a small family but, expanding in recent years, of a double perovskite with NCOT. To the best of our knowledge, this group currently includes (K or Rb) 3 AlF 6 , ,, Sr 3 WO 6 , (Ba or Sr) 3 TeO 6 , Rb 2 K­(Cr or Ga)­F 6 , K 3 GaF 6 , (K or Rb) 3 MoO 3 F 3 , Rb 3 InCl 6 , and Cs 1.17 In 0.81 Cl 3 …”

“…Similar to the K 3 AlF It should be pointed out that, unlike K 3 AlF 6 but like K 3 GaF 6 , 30 β-Rb 3 ScF 6 transforms directly into a cubic γ phase It is worth noting that Rb 3 ScF 6 is another new member of a small family but, expanding in recent years, of a double perovskite with NCOT. To the best of our knowledge, this group currently includes (K or Rb) 3 AlF 6 , 1,2,18 Sr 3 WO 6 , 31 (Ba or Sr) 3 TeO 6 , 32 Rb 2 K(Cr or Ga)F 6 , 33 K 3 GaF 6 , 30 (K or Rb) 3 MoO 3 F 3 , 34 Rb 3 InCl 6 , 35 and Cs 1.17 In 0.81 Cl 3 . 36 Solid-State NMR.…”