Lattice Dynamics and Phase Transitions of Strontium Titanate

The crystal dynamics of strontium titanate has been studied both experimentally and theoretically. The frequency versus wave-vector dispersion curves for some of the normal modes propagating along the 0, 0, 1 direction have been measured by neutron spectrometry at 90 and 296^. The experiments were performed at the Chalk River Laboratories of Atomic Energy of Canada Ltd. , using a triple-axis crystal spectrometer. The temperature dependence of the transverse optic mode of the lowest frequency has been found to be in agreement with the temperature dependence of the dielectric constant, as predicted by Cochran. The experimental results have been used to obtain the parameters of several models, more than one of which gives reasonable agreement with the experimental results. It is suggested that the anomalous behavior of the elastic properties and the phase transition at 110^ are associated with an accidental degeneracy of two branches of the dispersion curves; the longitudinal acoustic branch and the transverse optic branch of lowest frequency. The origin of the temperature dependence of this transverse optic mode and the relevance of lattice dynamics to the phase transitions in other perovskites are discussed.