The High-Temperature Polymorphs of K₃AlF₆

Abstract

The crystal structures of the three high-temperature polymorphs of K(3)AlF(6) have been solved from neutron powder diffraction, synchrotron X-ray powder diffraction, and electron diffraction data. The β-phase (stable between 132 and 153 °C) and γ-phase (stable between 153 to 306 °C) can be described as unusually complex superstructures of the double-perovskite structure (K(2)KAlF(6)) which result from noncooperative tilting of the AlF(6) octahedra. The β-phase is tetragonal, space group I4/m, with lattice parameters of a = 13.3862(5) Å and c = 8.5617(3) Å (at 143 °C) and Z = 10. In this phase, one-fifth of the AlF(6) octahedra are rotated about the c-axis by ∼45° while the other four-fifths remain untilted. The large ∼45° rotations result in edge sharing between these AlF(6) octahedra and the neighboring K-centered polyhedra, resulting in pentagonal bipyramidal coordination for four-fifths of the K(+) ions that reside on the B-sites of the perovskite structure. The remaining one-fifth of the K(+) ions on the B-sites retain octahedral coordination. The γ-phase is orthorhombic, space group Fddd, with lattice parameters of a = 36.1276(4) Å, b = 17.1133(2) Å, and c = 12.0562(1) Å (at 225 °C) and Z = 48. In the γ-phase, one-sixth of the AlF(6) octahedra are randomly rotated about one of two directions by ∼45° while the other five-sixths remain essentially untilted. These rotations result in two-thirds of the K(+) ions on the B-site obtaining 7-fold coordination while the other one-third remain in octahedral coordination. The δ-phase adopts the ideal cubic double-perovskite structure, space group Fm ̅3m, with a = 8.5943(1) Å at 400 °C. However, pair distribution function analysis shows that locally the δ-phase is quite different from its long-range average crystal structure. The AlF(6) octahedra undergo large-amplitude rotations which are accompanied by off-center displacements of the K(+) ions that occupy the 12-coordinate A-sites.

Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23

“…In materials such as Sr 3 WO 6 large rotations of the small BX 6 octahedra occur to give the larger A cation, which resides on a B-site, a coordination number larger than 6. [11][12][13][14][15] There are also oxygen deficient cryolites and related double perovskite compositions, such as Sr 3 SbO 5.5 , which have simple cubic crystal structures but locally have a large rearrangement of the atoms around the oxygen vacancy such that the actual coordination environments of the cations are similar to those seem in stoichiometric cryolites. 5 There exists a small class of materials which have cryolite-like formulas but with small concentrations of vacancies on the A-sites and anion sites.…”

Room-temperature ferrimagnetism of anti-site-disordered Ca2MnOsO6

“…The double perovskite oxide A 2 BB'O 6 [A: alkali earth, B: 3d, B': 4d(5d) elements] normally crystallizes into a monoclinic (P2 1 /n), or tetragonal (I4/m), or cubic (Fm-3m) structure, in which two distinct octahedra BO 6 and B'O 6 align in a rock-salt fashion [37]. The degrees of the octahedral distortion and rotation seem to have a major impact on the lattice symmetry [38]. Based on the general trend, we attempted to characterize the crystal structure of Ca 2 MnOsO 6 by applying these models.…”

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

“…K 3 AlF 6 has an especially complex roomtemperature structure based on $45 tilting of two fifths of the AlF 6 (Abakumov et al, 2009). It undergoes three phase transitions at higher temperature to structures where the number of AlF 6 rotated by $45 is one fifth, then one sixth, ISSN 2052-5206 # 2020 International Union of Crystallography and finally the cubic double perovskite structure with no octahedral tilting (King et al, 2011). Very recently Rb 3 AlF 6 was reported to have the same structure as -K 3 AlF 6 at room temperature (Rakhmatullin et al, 2020).…”