The solid solubility of CaTiOGeOb>4b> with CaZrOGeOb>4b> 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 P2b>1b>/a-A2/a, which is typical for titanite and CaTiOGeOb>4b>, was observed in samples with Zr concentrations up to 10%. The addition of Zr destabilizes the ordered P2b>1b>/a phase and Tb>cb> 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 Tb>cb> close to ambient temperature. Further increase of the Zr content stabilizes the triclinic structure and leads to an increase of Tb>cb> up to Tb>cb> = 488 K in CaZrOGeOb>4b>.
