Raman Spectrum of Single-Domain BaTiO3

Room-temperature Raman spectra of high-quality single-domain BaTi${\mathrm{O}}_3$ crystals are reported. The frequencies of the Raman-active long-wavelength optical phonons are determined and shown to be in reasonable agreement with the Lyddane-Sachs-Teller (LST) relation as well as infrared reflectivity data. The effects of mode damping on the Raman line shape are discussed using a classical lattice-polarization-fluctuation model for the Raman scattering process. Based on this model, it is shown that the soft $E$ mode is overdamped, and at 25°C has an undamped mode frequency ${\omega }_{\mathrm{TO}}$ of 36±3 ${\mathrm{cm}}^{-1\mathrm{}}$ and a damping constant $\Gamma$ of 1.1±0.1. The temperature dependence of ${\omega }_{\mathrm{TO}}$ is given and found to be consistent with the LST relation. The soft $A_1$ mode is not observed, but is estimated from the LST relation and infrared reflectivity data to be heavily damped ($\Gamma \approx 0.5$) and to have an undamped mode frequency of 180±10 ${\mathrm{cm}}^{-1\mathrm{}}$.