Raman scattering study of SrTiO₃ at high pressure

The pressure dependence of the Raman spectra of SrTiO₃ was investigated up to 15.5 GPa. The anti-ferrodistortive cubic-tetragonal phase transition (phonon condensation at the R point of the Brillouin zone) occurs at about 6 GPa. The strong, broad bands, present under ambient conditions, sharpen and change their prodle with increasing pressure. They diminish rapidly in their intensity above 13 GPa and disappear by 15.5 GPa. These pressure-induced changes can be due to increasing harmonicity of the stretching and bending Ti-O vibrations, leading to reduced intensity of the second-order Raman scattering, and/or due to the inhibition of the off-center dynamic distortions at the Ti⁴⁺ cation sites, and thus due to the decrease in intensity of the "disorder-induced' first-order Raman scattering. The spectrum at 15.5 GPa shows weak band splittings, which might indicate a further transformation to an orthorhombic phase of SrTiO₃.