ANHARMONICITY OF THE LOWEST-FREQUENCY A(1)(TO) PHONON IN PBTIO3

Detailed Raman studies of the optical phonons in high-quality single-crystal PbTiO3 indicate that the mode energies and symmetry assignments in the literature [G. Bums and B. Scott, Phys. Rev. B 7, 3088 (1973)] must be revised. In this study, all k congruent-to 0 optical-phonon modes are directly observed and are shown to obey rigorously the Raman selection rules. The angular dependence of the phonon frequencies indicates that the energy of the lowest-frequency A1(TO) mode is higher than the energy of the lowest-frequency E(LO) mode at 300 K. This indicates that these two k congruent-to 0 modes will cross at elevated temperature. The A1(TO) mode exhibits an anomalous line shape, which we attribute to the anharmonic nature of the lattice. The phonon frequencies determine the clamped dielectric constants, epsilon(a)(0) and epsilon(c)(0), using the Lydanne-Sachs-Teller relation. These values, when compared to those obtained from impedance measurements, indicate no dielectric dispersion in PbTiO3 between the MHz region and the Raman region.