Raman Spectra of High-Pressure Polymorphs of MgSiO3 at Various Temperatures

Abstract

Raman spectra of two of the three known high-pressure polymorphs (the ilmenite and perovskite modifications) of MgSiO₃ were investigated in the temperature range 105-875 K at atmospheric pressure. Even with extreme care, MgSiO₃-perovskite was found to become amorphous at temperatures greater than about 500 K, but MgSiO₃-ilmenite was still crystalline even after the highest temperature run. Negative variations of all Raman modes with temperature were observed for both polymorphs. Except for the most intense Raman mode of MgSiO₃-ilmenite, only a linear temperature variation can be established within the experimental uncertainty and temperature range spanned. This is the first report of slopes of the variation (dv_i/dT) for both MgSiO₃-ilmenite and MgSiO₃-perovskite. By utilizing these data and others from the literature, the volume thermal expansivity for MgSiO₃-ilmenite was calculated to be (2.8±0.5)×10^-5deg^-1 and that for MgSiO₃-perovskite to be (3.1±0.5)×10^-5deg^-1 at ambient conditions.