Dielectric dispersion in tetragonal BaTiO 3 measured by double-axis brillouin spectroscopy

The dispersion of the dielectric constant ϵa of the tetragonal BaTiO3 was determined at room temperature from Brillouin scattering data of the TA phonons propagating along the [101] direction. The dielectric constant ϵa was obtained by measuring the shift of the velocity of the TA phonon due to its piezoelectric coupling to the overdamped soft TO phonon of E-symmetry. The resultant values of ϵa are consistent with earlier data, and the dispersion of ϵa above 107 Hz was found to be well described by a one oscillator a dielectric function due mainly to the soft TO phonons. The appropriate value of the truly static dielectric constant ϵa was determined to be 2100.