Softening and mode crossing of the lowest-frequency A_(1)(transverse-optical) phonon in single-crystal PbTiO_(3)

摘要

The polarized Raman spectra of high-quality single-crystal PbTiO_(3) were measured in the temperature range between 300 and 700 K. The soft-mode behavior of the A_(1)(1TO) (transverse-optical) phonon was observed directly. Above 400 K, however, an intensity transfer between the subpeaks of the A_(1)(1TO) mode was observed, and the line shape of the A_(1)(1TO) mode strongly deviated from the theoretical prediction based on the harmonic approximation in the interatomic potential. In addition, the mode crossing between the A_(1)(1TO) phonon and the E(1LO) (longitudinal optical) phonon was observed. The observed mode crossing behavior clearly supports the assignment of the E(1LO) and A_(1)(1TO) modes, which was originally suggested by Foster and co-workers Phys. Rev. Lett. 71, 1258 (1993).