The solid solubility of CaTiOGeO4 with CaZrOGeO4 and the structural phase transitions occurring in this solid solution as a function of temperature and composition were examined using in situ high temperature X-ray powder diffraction. Structural phase transitions were identified based on the determination of spontaneous strain. The transition P21/a-A2/a, which is typical for titanite and CaTiOGeO4, was observed in samples with Zr concentrations up to 10%. The addition of Zr destabilizes the ordered P21/a phase and Tc decreases accordingly. The aristotype structure of titanite in space group symmetry A2/a was observed for intermediate compounds. Compounds with high Zr contents exhibit a triclinically distorted titanite structure. Their structure was modeled in space group A1̅. The triclinic structure appears at Zr contents of about 70% with Tc close to ambient temperature. Further increase of the Zr content stabilizes the triclinic structure and leads to an increase of Tc up to Tc = 488 K in CaZrOGeO4.

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