Temperature Dependence of the Transverse and Longitudinal Optic Mode Frequencies and Charges in SrTiO3and BaTiO3

Infrared reflectivity measurements have been made on SrTi${\mathrm{O}}_3$ at 300 and 85°K and on BaTi${\mathrm{O}}_3$ at 300 and 475°K. Analysis of these and other data shows that the lowest frequency transverse mode shifts in a manner consistent with Cochran's theory. All higher frequency modes have frequencies which are stable to at least 1 or 2%. A detailed macroscopic theory for the polar modes is presented and used to evaluate the transverse and longitudinal phonon frequencies including damping. The theory is also used to derive the longitudinal effective charges and the electron-phonon coupling parameters which are then calculated for the phonons in several perovskite crystals.