Dilithium zirconium hexafluoride Li2ZrF6 at high pressures: A new monoclinic phase

Dilithium zirconium hexafluoride, Li2ZrF6 (P3¯1m, Z=1), is studied at high pressures using synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell at room temperature. At atmospheric conditions, it has a structure with all the cations octahedrally coordinated to fluorine atoms. Above 10 GPa it transforms reversibly to a new polymorph (C2/c, Z=4), in which the coordination polyhedron of the Zr atoms is a distorted square antiprism, while the Li atoms are in the octahedral coordination. The LiF6 octahedra form layers parallel to (100) that are connected by zig-zag chains of the edge-sharing Zr polyhedra running in the [001] direction. The relative change in volumes per one formula unit for both polymorphs is 6% at 11.8 GPa. The relations to other A2BX6-type structures are discussed.