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-无序型向置换型转变。(2)在低温高压下观测到了量子准电性的发生。L_1TIC_4H_40_6H_2O(1)发现铁电相变温度随氘核浓度x的增加而降低，在x， 0.95处消失。(2)用软模理论解释了这种同位素对相变温度的影响。研究了C_3H_2PO_4(1)的压力-温度相图。发现了两个三重点；一个是准电相（phaseI）、铁电相（phase II）和压力诱导反铁电相（phase III）的交汇位置，另一个是I相、phase eii、phase eiii和第二个压力诱导反铁电相（phase eix）的交汇位置。(2)本文得到的压力-温度相图可以用准一维Ising模型很好地描述。phbhpo_4(1)铁电相变温度随压力的增加而降低，在3gpa时消失。(2) phbhpo_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phbhpo_4是准一维铁电体

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