The crystal structures of pressure-induced LiSrAlF6-II and LiCaAlF6-II

The crystal structures of LiCaAlF6-II and LiSrAlF6-II (both P 21/c, Z = 4) occurring at high pressures and room temperature were studied with synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells. The structure of LiSrAlF6-II stable between 1.6 and 3.0 GPa was solved with a global optimisation algorithm and group theory considerations, and refined with the Rietveld method in the rigid-body approximation. It is a distorted variant of the ambient pressure polymorph (LiSrAlF6-I, ), in which each cation occupies a deformed octahedral site. LiCaAlF6 transforms to this monoclinic polymorph II above about 7 GPa. The differences in the high-pressure behaviours of LiCaAlF6 and LiSrAlF6 are discussed by considering the ionic radii.