Establishment of the New Viewpoint on the Dynamical Mechanism of Structural Phase Transitions in Hydrogen-bonded Crystals

(1)对于以KH_2PO_4为代表的氢键晶体的结构相变动力学机制，传统的质子隧穿模型和我们提出的H_2PO_4有序-无序模型之间存在着长期的争论。非相干中子散射的实验结果已证明不存在质子隧穿运动。(2)Kaminow-Damen和Peercy观察和解释的KH_2PO_4的拉曼光谱长期以来一直被认为是前一模型的证据。在宽温区和高压下获得了KH_2PO_4、KD_2PO_4及其类质同象晶体的拉曼散射、超拉曼散射和远红外反射的新的精确实验数据。从临界弛豫模式和在相变温度以上由于无序结构而引起的PO_4自由基局域对称性降低两个方面成功地分析了这些数据，得出了对KH_2PO_4型晶体适用的新观点。这一观点将推广到所有氢键晶体。(3)用晶体学、量热法和光学方法研究了零维氢键系统M_3H（XO_4）_2（M=K，Rb，Cs ; X= S，Se）的相变。D取代H引起的大同位素效应的起因是H（XO_4）_2自由基中原子几何构型的不同，包括氢键上电子密度的不同。(4)在某些氢键晶体中，用D取代H，在高压下发现了一些新的相和转变温度的变化。

(1) A long controversy had been held between the proton tunneling model (traditional) and the H_2PO_4 order-disorder model (the new viewpoint presented by us) with respect to the dynamical mechanism of structural phase transition in KH_2PO_4, which is a representative of hydrogen-bonded crystals. The existence of tunneling motions of protons has been proved to be excluded by the experimental results of incoherent neutron scattering.(2) The Raman spectra of KH_2PO_4 observed and interpreted by Kaminow-Damen and Peercy had been long regarded as the evidence of the former model. New precise experimental data in Raman scattering, hyper-Raman scattering, and far infrared reflection have been obtained on KH_2PO_4, KD_2PO_4 and their isomorphous crystals in wide temperature region and under high pressure. These data have been successfully analysed in terms of the critical relaxation mode and the lowering of the local symmetry of PO_4 radicals due to the disordered structure above the transition temperature.From both results, the new viewpoint has been concluded to hold as far as KH_2PO_4 type crystals are concerned. This viewpoint will be extended to all hydrogen-bonded crystals.(3) Phase transitions in zero-dimensional hydrogen-bonded systems M_3H(XO_4)_2 (M=K, Rb, Cs ; X= S, Se) have been found and studied by crystallographical, calorimentrical and optical methods. The origin of the large isotope effect due to the substitution of H by D has been attributed to the difference in geometrical configurations of atoms in the H(XO_4)_2 radical including that of electron densityon the hydrogen bond.(4) Several new phases and variations of transition temperatures have been found by subsituting H by D and under high pressure in some hydrogen-bonded crystals.