The low-temperature form of Rb{sub 2}KCrF{sub 6} and Rb{sub 2}KGaF{sub 6}: The first example of an elpasolite-derived structure with pentagonal bipyramid in the B-sublattice
- Departamento de Fisica de la Materia Condensada, Facultad de Ciencia y Tecnologia, Universidad del Pais Vasco, Apdo 644, 48080 Bilbao (Spain)
- Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB, UPR 9048 CNRS), Universite de Bordeaux 1, 87 avenue du Docteur Albert Schweitzer, 33608 Pessac Cedex (France)
The crystal structure of the low-temperature forms of Rb{sub 2}KCrF{sub 6} and Rb{sub 2}KGaF{sub 6} has been solved on single crystal. The symmetry is tetragonal with F4/m space group; the unit cell parameters are: a=19.407(3)A, c=8.833(2)A for Rb{sub 2}KCrF{sub 6} at T=90K and a=19.375(4)A, c=8.782(2)A for Rb{sub 2}KGaF{sub 6} at T=90K. The relationships between the parameters of the prototype cubic elpasolite, which is stable at high temperature, and the tetragonal superlattice of the low temperature form have been established. Considering the general formulation A{sub 2}BB'F{sub 6}, the cationic positions in the A and (B,B') sublattices remain identical in the two allotropic varieties. The main originality of the structure concerns the environment of 4/5 of the potassium atoms (B sublattice) which is transformed from octahedra into pentagonal bipyramids sharing edges with adjacent B'F{sub 6} octahedra containing Cr or Ga. The displacive phase transition is simply explained by the rotation of 45 deg. in the (a,b) plane of 1/5 of the B'F{sub 6} (B'=Cr, Ga) octahedra. The similarity of this phase transition and the transformation of perovskite into tetragonal tungsten bronze (TTB) will be discussed.
- OSTI ID:
- 20902484
- Journal Information:
- Journal of Solid State Chemistry, Vol. 179, Issue 12; Other Information: DOI: 10.1016/j.jssc.2006.07.032; PII: S0022-4596(06)00424-5; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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