Raman spectroscopy of CaSnO3 at high temperature: a highly quasi-harmonic perovskite
Résumé
Calcium stannate perovskite (CaSnO3) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8 nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Gruneisen parameters and thermal line broadening following Gamma = A coth theta/T, where the quantum temperature theta is determined by the phonon branch without further coupling with other degrees of freedom. The geometrical nature of phonon lines has been identified. High-temperature powder x-ray diffraction measurements provide thermal expansion coefficients of alpha(x) = 13.9 x 10(-6) K-1, alpha(y) = 2.7 x 10(-6) K-1, alpha(z) = 14.3 x 10(-6) K-1. The strongly quasi-harmonic behaviour observed and the lack of any indication of instability with respect to the post-perovskite structure points to the strongly first-order character of the reported perovskite to post-perovskite phase transition in this material, which appears to behave as a very good analogue to MgSiO3 in the Earth's interior.