Lattice Absorption in Strontium Titanate

The optical absorption spectrum of SrTiO₃ is presented over the energy range 0.05-0.3 eV, well below the fundamental edge. Nine absorption bands are observed. We interpret all but one of them as being due to multiphonon creation processes. The sole exception is a band at 0.068 eV, which we attribute to a single-phonon process. Selection rules for simultaneous creation of either two or three phonons are obtained in dipole approximation, at selected critical points in the Brillouin zone. The observed bands are assigned using these rules together with the experimental phonon dispersion curves of Cowley and other published values for the higher-energy branches obtained at specific points in the zone by other techniques.