SUCCESSIVE PHASE TRANSITION OF HEXAGONAL TYPE BARIUM TITANATE

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Hexagonal modification of barium titanate exhibits successive phase transition at To=222K and Tc=74K, and below Tc a ferroelectric activity occurs. In order to elucidate the mechanism of the phase transition, we have made dielectric, specific heat and Raman scattering measurements. Following results are obtained. 1.Dielectric properties under hydrostatic pressures : The dielectric constants along the c-axis of the single crystal were investigated as functions of hydrostatic pressure to 8kbar and temperature from 5K to 300K. The transition point To (222K at 1 bar) decreases linearly with increasing pressure at a rate -3.0K/kbar and no significant difference is observed in the shape of the dielectric anomaly at To. While, the dielectric constant at around Tc decreases apparently with increasing pressure and at ca. 10kbar the anomaly at Tc disappears. The pressure derivative of Tc is about -0.7K/kbar. 2. Specific heat : The specific heat of the polycrystalline sample was measured using a high-precision adiabatic calorimeter over the temperature range from 5K to 300K. The specific heat vs T curve shows a typical second-order type anomaly at To. However, the specific heat anomaly associated with the ferroelectric phase transition at Tc has not been detected. 3. Polarized Raman scattering : In order to closely investigate the behavior of the Raman spectra at around Tc, we have made the polarized Raman scattering measurement using thin a- and b-plate samples (ca. 0.3mm thick). In the ferroelectric phase below Tc, existence of an additional phonon mode at 57cm^<-1> has been confirmed ; this line appears below Tc only in the zz polarization and the intensity of the mode behaves like an order-parameter. No additional soft-optic mode was observed at Tc ; however, only the lower frequency mode of two soft modes existing below To shows anomalous behaviors at around Tc.