Evidence for competing orderings in strontium titanate from hyper-Raman scattering spectroscopy

The frequencies of the softest polar optic phonons of tetragonal SrTiO3 are accurately determined using high-resolution hyper-Raman spectroscopy on or iented-domain crystals. The splitting of the polar soft mode in the tetrago nal phase is quantitatively related to the 105 K structural transition, i.c ., to tie rotation of the oxygen octahedra and to the associated macroscopi c strains. In addition to quantum fluctuations, these tetragonal distortion s are found to be essential in preventing the ferroelectric transition.