Observation of relaxation mode in ferroelectric barium dititanate by micro-Brillouin scattering

New ferroelectric single crystals of BaTi2O5 have been studied in detail by high-resolution micro-Brillouin scattering with focus on the central component of the relaxation mode over a wide temperature range (20-600degreesC). A relaxation mode has been clearly observed in these single crystals for the first time. In addition, the usual phonon features are also observed in the Brillouin shifts determined by selection rules for a monoclinic phase. The ferroelectric phase transition at T-c similar to 470degreesC, sharp anomaly has been observed from both the frequency shift and hypersonic damping. The relaxation time slows down and intensity of central peak increases markedly when temperature approaches T, from above and below. Furthermore, the relaxation mode has an A(y) symmetry which implies that the relaxation mode involves interwell motion of Ti ions parallel to the polar axis. The temperature dependence of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model in which the off-center ion in the octahedra tunnels between two sites of a double-well potential.