Origin of anomalous line shape of the lowest-frequency A(1)(TO) phonon in PbTiO3

Abstract

The origin of the anomalous line shape of the A(1)(1TO) soft mode in PbTiO3-based single crystals was investigated by polarized Raman-scattering method over a wide range of temperature between 100 and 700 K. Detailed scattering data indicated that the previously proposed anharmonicity model could not properly explain the anomalous behavior of the lowest-frequency subpeak of the A(1)(1TO) mode that was primarily responsible for the phonon softening of the undoped PbTiO3. Further Raman studies of Ba-doped PbTiO3 crystals showed that the Ba-impurity peak that dominated the soft-mode transition also had the A(1) symmetry of the PbTiO3 host lattice. A careful comparison of these two distinct peaks having the same A(1) symmetry indicates that the observed anomalous line shape of the A(1)(1TO) mode is caused not only by the lattice anharmonicity but also by some lattice defects. Particularly, the lowest-frequency subpeak of the A(1)(1TO) mode is not directly related to the anharmonicity but originates from the existence of thermodynamically unavoidable lattice defects in PbTiO3.