Raman spectroscopy of CaSnO₃ at high temperature: A highly quasi-harmonic perovskite

Calcium stannate perovskite (CaSnO₃) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Grüneisen parameters and thermal line broadening following =Acoth/T, where the quantum temperature 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 _x=13.9×10⁶K¹, _y=2. 7×10⁶K¹, _z=14.3×10 ⁶K¹. 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 Earths interior.