Origin of Incommensurate Phase Transition and the New Viewpoint on the Phase Fluctuation

这个项目的目的是要证明，位相涨落的影响对晶体中无公度结构的出现起着至关重要的作用。我们提出了一个新的模型来解释石英中无公度相变的起源。在该模型中，由于位相涨落引起的德拜-沃勒因子的减小，在衍射实验中观测到的卫星光强强度被强烈衰减。在本研究中，我们通过以下方法证明了这一模型：(1)通过对高对称性β相低频拉曼光谱的详细研究，发现积分强度的温度依赖关系并不完全遵循居里-魏斯定律，而是偏离了无公度相变温度以上的30K。这与我们的模型是一致的。(2)分子动力学计算采用Tsuneyuki势。结果表明，与目前公认的模型相比，短周期调制比长波调制更有可能出现。(3)用特定原子团的转动自由度解释了介电晶体中无公度结构的特点。提出了弹性铰链分子(EHM)模型。利用EHM模型进行了计算机模拟，研究了相变的动力学过程。用该模型和类似的模型研究了晶格的负毒比和几种非线性类孤子行为。

The aim of this project is to show that the effect of phase fluctuation plays the crucial role in appearance of incommensurate structure in crystals. We propose a new model to explain the origin of the incommensurate phase transition in quartz. In this model, the intensity of the satellite intensity observed in diffraction experiments is strongly attenuated due to the decrease of Debye-Waller factor, which is attributed to the phase fluctuation. In this research we have proved this model from the following method.(1) From the detailed investigation of low frequency Raman spectrum in the high symmetry beta phase, the temperature dependence of the integrated intensity was found not to exactly follow the Curie-Weiss law ,but deviates from 30K above the incommensurate phase transition temperature. This is consistent with our model.(2) The MD calculation was performed using Tsuneyuki potential. The result shows that the short period modulation is more likely to appear than long wave modulation in contrast to the currently accepted model.(3) The characteristics of incommensurate structure in' dielectric crystals are explained by the rotational degree of freedom of certain group of atoms. The elastically hinged molecule (EHM) model was proposed. The dynamics of the transition is investigated by the computer simulation using the EHM model. The negative poison ratio and several non-linear soliton-like behavior of lattice were also investigated by this and similar models.