Structure and phase transitions of LiTaOGeO(4)

The structure of LiTaOGeO(4) has been refined using X-ray diffraction data collected at 293 K and at 173 K. The low-temperature structure is isostructural with LiTaOSiO(4) and closely related to the low-temperature structure of titanite, CaTiOSiO(4). Li occurs in a distorted tetrahedral coordination. The transition to the disordered structure, with space group symmetry C2/c, occurs at T(c) = 231 (1) K. Li is disordered across two symmetry-equivalent positions and Ta is located at the centre of its coordination octahedron in this paraphase. The transition is continuous, and the thermal evolution of the order parameter is well approximated using a tricritical mean-field model. Anharmonic thermal displacement of the Li cation has been analyzed and its one-particle potential has been determined. The height of the potential barrier separating the two Li positions across a curved trajectory is close to RT(c), where R is the universal gas constant.