Raman Spectrum of Strontium Titanate

The Raman spectrum of single-crystal SrTiO3 has been investigated as a function of temperature down to about 25°K. The lattice dynamics of this crystal have been extensively studied in recent years primarily because of its unusual dielectric and acoustic properties. Both nominally pure and impure samples were examined since these properties seem to be sensitive to small amounts of impurities. An argon-ion laser served as the exciting source and an in-tandem grating spectrometer was used to disperse the scattered light. These instrumental techniques provided rather complete data including the polarization characteristics of the individual lines and peaks. The above data, together with the known phonon dispersion curves of SrTiO3, made it possible to obtain a reliable interpretation of the Raman spectrum. At room temperature we find the Raman spectrum to be entirely second order, in agreement with the selection rules for the cubic perovskite structure. Further, the phonons contributing to the second-order scattering have wave vectors at or near the Brillouin zone boundary and are for the most part transverse polarized. The same combinations and overtones tend to give intense peaks in both SrTiO3 and KTaO3, but the different dispersions of the phonon branches in the two crystals make their spectra appear different. On cooling nominally pure SrTiO3 below the phase transition at 110°K, three sharp lines appear in the spectrum which are due to scattering from local modes rather than from the polar transverse-optic modes. The energy shifts measured for these three sharp lines do not agree with those for the polar TO modes. In the impure sample at low temperature (15°K) scattering from local modes and the polar transverse-optic modes are observed simultaneously. Measurements on the impure sample at 78°K yield scattering from the three polar transverse-optic modes but not from the local modes. These results illustrate the great sensitivity of Raman scattering to crystal structure and show that pure and impure SrTiO3 undergo different phase transitions as the temperature is reduced. Since no ferroelectricity or first-order Raman scattering is found in the tetragonal phase of pure SrTiO3, the point group of this phase is most probably C4hor D4h. If the impure sample is tetragonal at 78°K, as is likely the case, then the Raman measurements show the point group to be S4, C4, or C4V. At lower temperatures the crystal structure is undoubtedly of lower symmetry than tetragonal.