Phase Transition in Hydrogen-Bonded Crystal at Low Temperature under High Pressure

1.KH_2PO_4和KD_2PO_4(1)随着压力的增加，相变机制由有序-无序型向位移型转变。(2)在低温高压下观察到了量子顺电效应的发生。L_1TiC_4H_40_6H_2O(1)发现铁电相变温度随氚浓度x的增加而降低，并在x，0.95时消失。(2)用软模理论解释了相变温度下的同位素效应。C_3H_2PO_4(1)研究了压力-温度相图。发现了两个三相点：一个是顺电相(I相)、铁电相(II相)和压致反铁电相(III相)相交的位置，另一个是I相、II相、III相和第二压致反铁电相(PASEIX)相交的位置。(2)本文得到的压力-温度相图可用准一维Ising模型4很好地描述。PbHPO4(1)研究发现，铁电相变温度随压力的升高而降低，在3 GPa时消失。(2)PbHPO_4的压力-温度相图与KH_2PO_4非常相似。与以往的报道相反，没有迹象表明PbHPO_4是夸塞一维铁电材料

1. KH_2PO_4 and KD_2PO_4 (1) The crossover of the phase transition mechanism from order-disorder type to displacive type has been found with increasing pressure. (2) The occurrence of the quantum paraelectricty has been observed at low temperature under high pressure2. L_1TIC_4H_40_6H_2O (1) It was found that the ferroelectric phase transition temperature decreases with increasing the deuteron concentration x, and disappears at x, o.95. (2) This isotope effect in the phase transition temperature is explained on the soft-mode theory3. C_3H_2PO_4 (1) The pressure-temperature phase diagram was investigated. Two triple points were found; one is the position where the paraelectric phase (phase I), ferroelectric phase (phase II) and pressure-induced antiferroelectric phase (phase III) meet, and the other is the position where the phase I, phaseII, phaseIII and the second pressure-induced antiferroelectric phase (phaseIX) meet. (2) The pressure-temperature phase diagram obtained in the present work is well described by the quasi-one dimensional Ising model4. PbHPO_4 (1) It was found that the ferroelectric phase transition temperature decreases with increasing pressure, and disappears at 3 Gpa. (2) The pressure-temperature phase diagram of PbHPO_4 is very similar with that KH_2PO_4. Contrary to previous reports, there is no indication for that PbHPO_4 is the qusai-one-dimensional ferroelectric