Inelastic neutron scattering from single-domain BaTiO3

The lattice dynamics of tetragonal BaTiO3 have been investigated by inelastic neutron scattering from a single-domain crystal. Measurements were carried out on two projections, (hk0) and (h0l); the study revealed extreme anisotropy in energy and damping of the soft optic mode, depending upon the polarization vector (e) over right arrow. The lowest transverse optic branch with the wave vector (q) over right arrow parallel to [110] and (e) over right arrow parallel to[1(1) over bar 0] shows well-defined phonons except for small q values (<0.05 angstrom(-1)), where they become overdamped. The soft-mode energy increases rapidly with q and extrapolates back to 4.5 meV at the zone center, in agreement with the previous Raman measurements. On the other hand, the soft modes with <(e)over right arrow>parallel to [100] (with (q) over right arrow parallel to [010] or [001]) have unusually low energies and high damping constants for the entire zone. They cross over the acoustic branches, resulting in marked anomalies for the latter. The present study establishes the validity of the soft-mode model for ferroelectricity in BaTiO3, as was the case for SrTiO. The important feature for BaTiO3 is the greatly enhanced polarization correlation along the [100] direction compared with other perovskites.