SUCCESSIVE PHASE TRANSITION OF HEXAGONAL TYPE BARIUM TITANATE

钛酸钡的六方改性在To=222K和Tc=74K时表现出连续的相变，在Tc以下发生铁电活性。为了阐明相变的机理，我们进行了介电、比热和拉曼散射的测量。得到如下结果：1.静水压力下的介电性能：研究了单晶c轴上介电常数随静水压力至8kbar和温度从5K至300K的变化规律。过渡点To （222K at 1bar）以-3.0K/kbar的速率线性减小，To处介电异常形状无显著变化。而Tc附近的介电常数随压力的增加而明显减小，在约10kbar时，Tc处的异常消失。Tc的压力导数约为-0.7K/kbar。2. 比热：使用高精度绝热量热计在5K至300K的温度范围内测量多晶样品的比热。比热随温度变化曲线为典型的二阶异常。然而，与铁电相变相关的比热异常尚未被检测到。3. 偏振拉曼散射：为了深入研究Tc附近拉曼光谱的行为，我们使用薄a板和b板样品（约0.3mm厚）进行了偏振拉曼散射测量。在Tc以下的铁电相中，在57cm^<-1>处存在一个额外的声子模式；这条线只在zz偏振中出现在Tc以下，模的强度表现得像一个阶参数。在Tc处未观察到额外的软光模式；而在Tc附近，只有存在于To以下的两种软模态中的低频模态表现出异常行为。

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.