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

Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany

Correspondence: ^* E-mail: tmalch{at}min.uni-heidelberg.de

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 reflections, 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(Ge_xSi_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.