Study of Critical Fluctuations in BaTiO3by Neutron Scattering

Neutron scattering from BaTi${\mathrm{O}}_3$ crystal has been investigated in the critical region ($T_c=130\mathrm{}°\mathrm{C}<T<\mathrm{}270\mathrm{}°\mathrm{C}$) above the ferroelectric Curie temperature. In this temperature range, strong quasi-elastic scattering was observed. The temperature dependence of the quasi-elastic scattering indicates that it is associated with a critical polarization fluctuation in the crystal. An energy analysis of the cross section proves that the polarization fluctuation in BaTi${\mathrm{O}}_3$ is of the relaxation type in this temperature region, with a relaxation time longer than 5×${10}^{-11\mathrm{}}$ sec. This result is in marked contrast to the case of SrTi${\mathrm{O}}_3$ and KTa${\mathrm{O}}_3$. The neutron cross sections, observed as functions of temperature and wave number, can be interpreted equally well with either an overdamped phonon model or a tunneling-mode model. The strong anisotropy of the quasielastic scattering in reciprocal space indicates that the directions of strongest correlation of the fluctuation are along the (100) axes.