Crystal structures and stability of LiCeF5 and LiThF5 at high pressures: A comparative study of the coordination around the Ce4+ and Th4+ ions

Single crystals of LiCeF5 and LiThF5 have been synthesized using a low-temperature hydrothermal method. Both materials are isostructural with LiUF5 (I41/a, Z = 16). The chains of edge-sharing M4+F9 tricapped trigonal prisms (M4+: Ce4+, Th4+, and U4+) along the c axis are connected with others by corners in a three-dimensional framework, at which the cavities are filled out by lithium atoms. The lithium atoms are octahedrally coordinated by fluorines and the deformed LiF6 ocahedra share their edges. The lithium atoms form a distorted tetrahedral cluster unit. aterials have been studied to about 10 GPa using single-crystal X-ray diffraction in diamond anvil cells. Upon compression, the structure of LiCeF5 is stable in the entire pressure range investigated here and its compressibility is isotropic. The compressibility of LiThF5 is anisotropic, with the a lattice parameter, perpendicular to the ThF9 chains, being more sensitive to pressures. LiThF5 undergoes a reversible phase transition to an as yet undetermined polymorph above 5 GPa.