Lattice dynamics of tetragonal PbTiO3

The phonon dispersion relations for tetragonal PbTiO3 have been determined along the [100], [110], and [001] directions at room temperature by inelastic neutron scattering. The zone-boundary TA phonon energies for PbTiO3 are considerably lower than those for BaTiO3 and SrTiO3, indicating the dominant role of Pb atoms in TA phonons. The zone-center TO-A(1) phonon energy is higher than the zone-center TO-E phonon energy. A preliminary high-temperature experiment has revealed the gradual softening of the TO-A(1) phonons and the stability of TO-E phonons up to 643 K (0.84 T-c). The TO-A(1) phonons have an extremely broad linewidth, in marked contrast to the well-defined TO-E phonons. The TO phonons at the zone-boundary M point indicate that the energy for the rotational mode of the oxygen octahedra is approximately half of that for the distortion of the octahedra along the c axis. The X-point TO phonon energies for PbTiO3 are much higher than those for Pb(Zn1/3Nb2/3)O-3, suggesting that the B-site atoms in ABO(3) contribute largely to the TO phonons in Pb-based perovskites. Present phonon dispersion relations are generally in agreement with the first-principles calculations for the zone-center and zone-boundary phonon energies.