Title




The hightemperature polymorphs of
 
Author




 
Abstract




The crystal structures of the three hightemperature polymorphs of K3AlF6 have been solved from neutron powder diffraction, synchrotron Xray powder diffraction, and electron diffraction data. The βphase (stable between 132 and 153 °C) and γphase (stable between 153 to 306 °C) can be described as unusually complex superstructures of the doubleperovskite structure (K2KAlF6) which result from noncooperative tilting of the AlF6 octahedra. The βphase 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, onefifth of the AlF6 octahedra are rotated about the caxis by 45° while the other fourfifths remain untilted. The large 45° rotations result in edge sharing between these AlF6 octahedra and the neighboring Kcentered polyhedra, resulting in pentagonal bipyramidal coordination for fourfifths of the K+ ions that reside on the Bsites of the perovskite structure. The remaining onefifth of the K+ ions on the Bsites retain octahedral coordination. The γphase 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 γphase, onesixth of the AlF6 octahedra are randomly rotated about one of two directions by 45° while the other fivesixths remain essentially untilted. These rotations result in twothirds of the K+ ions on the Bsite obtaining 7fold coordination while the other onethird remain in octahedral coordination. The δphase adopts the ideal cubic doubleperovskite structure, space group Fmm, with a = 8.5943(1) Å at 400 °C. However, pair distribution function analysis shows that locally the δphase is quite different from its longrange average crystal structure. The AlF6 octahedra undergo largeamplitude rotations which are accompanied by offcenter displacements of the K+ ions that occupy the 12coordinate Asites. 
 
Language




English
 
Source (journal)




Inorganic chemistry / American Chemical Society.  Easton, Pa
Inorganic chemistry / American Chemical Society.  Easton, Pa
 
Publication




Easton, Pa
:
2011
 
ISSN




00201669
 
Volume/pages




50
:16
(2011)
, p. 77927801
 
ISI




000293493100052
 
Full text (Publisher's DOI)




 
