Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO$_3$ perovskite

Raman and Mössbauer spectra from $^{119}$Sn nuclei in CaSnO$_3$ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn-O-Sn bonds in chains of oxygen octahedra SnO$_6$ along the $c$ axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO$_3$ samples with the ilmenite structure have been obtained for the first time.