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)量子顺电性的发生在低温高压下被观察到2。L_1TIC_4H_4O_6H_2O（1）铁电相变温度随氘浓度x的增加而降低，在x = 0.95时消失。(2)用软模理论解释了相变温度中的同位素效应。（1）研究了C_3H_2PO_4的压力-温度相图。发现了两个三相点;一个是顺电相（相I）、铁电相（相II）和压力诱导反铁电相（相III）相交的位置，另一个是相I、相II、相III和第二压力诱导反铁电相（相IX）相交的位置。(2)在本工作中得到的压力-温度相图是很好地描述准一维伊辛模型4。PbHPO_4（1）铁电相变温度随压力的增加而降低，在3Gpa时相变温度消失。(2)PbHPO_4的压温相图与KH_2PO_4的压温相图非常相似。与以前的报道相反，没有发现PbHPO_4是准一维铁电体的迹象

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