LiSrAlF6 with the LiBaCrF6-type structure

The crystal structure of LiSrAlF6-III (P21/c, Z = 4) occurring above 3.0 GPa at room temperature was studied with synchrotron angle-dispersive x-ray powder diffraction in a diamond anvil cell. It was solved by combining a global optimization and a topological analysis with the Rietveld method using rigidbody AlF6 geometrical constraints. LiSrAlF6-III, related to LiBaCrF6 (P21/c, Z = 4), is built of deformed SrF12 icosahedra within a three-dimensional framework of corner-sharing distorted AlF6 octahedra and LiF4 tetrahedra, whereas the low-pressure phases I (P31c, Z = 2) and II (P21/c, Z = 4) have cations exclusively in distorted octahedral coordinations. The pressure-induced changes of the coordination polyhedra in the series LiSrAlF6-I, LiSrAlF6-II to LiSrAlF6-III are similar to the differences in coordination polyhedra due to the increase of the ionic radii of the Sr 2+ and Ba 2+ cations in LiSrAlF6-I and LiBaM �� F6 (M �� = Al, Ga, Cr ,V , Fe, or Ti) at ambient conditions. These observations are discussed on the basis of the high-pressure high-temperature systematics in AB2X6 compounds. (Some figures in this article are in colour only in the electronic version)