Brillouin and Raman spectroscopy of the ferroelastic rutile-to-CaCl2 transition in SnO2 at high pressure

The high-pressure behavior of SnO2 has been examined by Brillouin and Raman scattering spectroscopy to pressures of 30 GPa at ambient temperature. The elastic constants c(11), c(12), c(44), and c(66) of rutile-type SnO2 were determined as a function of pressure to 13 GPa by Brillouin spectroscopy and the Raman-active modes have been measured up to 30 GPa. The transition from the rutile- to the CaCl2-type structure was observed. The Raman-active soft mode and the elastic soft mode behavior were fit to an order parameter model with bilinear coupling of the Raman-active order parameter and the elastic strain. It is shown that the pressure dependence of the Raman-active soft mode is controlled in part by the characteristic frequencies of the two coupled soft modes. No further indications for soft-mode-driven phase transitions at higher pressures were found.