A high-temperature and high-pressure Raman spectroscopic study of CaGeO 3 garnet

Abstract

High-pressure and high-temperature Raman spectra of CaGeO3 tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO3 majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO3 garnet. The observed mode shifts of CaGeO3 garnet were used to determine the isothermal and isobaric mode Gruneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a i , for CaGeO3 garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10−5 K−1 to −1.3 × 10−5 K−1, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO3 majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO3 tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour.