Title
The crystal structure of <tex>$\alpha-K_{3}AIF_{6}$</tex>: elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework The crystal structure of <tex>$\alpha-K_{3}AIF_{6}$</tex>: elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework
Author
Faculty/Department
Faculty of Sciences. Physics
Publication type
article
Publication
Easton, Pa ,
Subject
Chemistry
Source (journal)
Inorganic chemistry / American Chemical Society. - Easton, Pa
Volume/pages
48(2009) :19 , p. 9336-9344
ISSN
0020-1669
ISI
000270091000039
Carrier
E
Target language
English (eng)
Full text (Publishers DOI)
Affiliation
University of Antwerp
Abstract
The crystal structure of α-K3AlF6 was solved and refined from a combination of powder X-ray and neutron diffraction data (a = 18.8385(3)Å, c = 33.9644(6)Å, S.G. I41/a, Z = 80, RP(X-ray) = 0.037, RP(neutron) = 0.053). The crystal structure is of the A2BB′X6 elpasolite type with the a = b ≈ ae√5, c = 4ae superstructure (ae, parameter of the elpasolite subcell) and rock-salt-type ordering of the K and Al cations over the B and B′ positions, respectively. The remarkable feature of α-K3AlF6 is a rotation of 2/5 of the AlF6 octahedra by π/4 around one of the crystal axes of the elpasolite subcell, coinciding with the 4-fold symmetry axes of the AlF6 octahedra. The rotation of the AlF6 octahedra replaces the corner-sharing between the K and Al polyhedra by edge-sharing, resulting in an increase of coordination numbers of the K cations at the B positions up to 7 and 8. Due to significant deformations of the K polyhedra, the corner-sharing connectivity of the octahedral elpasolite framework is broken and the rotations of the AlF6 octahedra do not have a cooperative character. Elpasolites and double perovskites with similar structural organization are discussed. The difference in ionic radii of the B and B′ cations as well as the tolerance factor are proposed to be the parameters governing the formation of elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework.
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