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Temperature and composition dependence of structural phase transitions in Ca(Ti_xZr_1–x)OGeO₄

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

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

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 T_c 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 A. The triclinic structure appears at Zr contents of about 70% with T_c close to ambient temperature. Further increase of the Zr content stabilizes the triclinic structure and leads to an increase of T_c up to T_c = 488 K in CaZrOGeO₄.

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