A high-temperature diffraction study of the solid solution CaTiOSiO₄-CaTiOGeO₄

Abstract

The structure of CaTiOGeO₄ (CTGO) has been refined using single crystal X-ray diffraction data. CTGO is isostructural with titanite, CaTiOSiO₄. The displacive P2₁/a-A2/a phase transition analogous to titanite has been studied by in situ heating X-ray powder diffraction and transmission electron microscopy. The transition is accompanied by the disappearance of superstructure reß ections, k + l = 2n + 1, which are replaced by diffuse scattering for T > T_c. The diffuse scattering is seen as streaks along b* in high-temperature TEM SAD. Lattice parameters as a function of temperature and composition were determined by X-ray powder diffraction between room temperature and a maximum of 1123 K. Strain analysis of CTGO indicates a transition temperature of T_c = 588 ± 4 K and the additional occurrence of an isosymmetric anomaly at T_i = 800 ± 25 K. There is complete solid-solution along the join CaTiO(GexSi_1-x)O₄. The lattice parameters across the solid solution vary continuously and the structural phase transitions were identified based on the determination of spontaneous strain associated with the transitions. The e11 and e13 components dominate the strain tensor. All compositions across the solid solution exhibit close to tricritical behavior and nearly constant scalar strain