In situ observation and kinetics of the phase transformation under high pressure and stress

本研究旨在探讨构成地壳和地幔的硅酸盐和水合物的相变动力学。已经知道，当热能不足以为相变提供合适的激活能时，根据Ostwald阶跃规则，相变往往停留在亚稳态甚至不稳定状态。为了了解上述力学过程，通过模拟地球内部的物理化学过程作为温度、压力和应力的函数，进行了定量的动力学研究。(1)利用单晶，在300K的静水压条件下，用DAC研究了与蛇纹石等镁铁质水合物相似的氢氧化钙的相变过程。压缩导致了高密度的非晶态，但没有b…更多的是形成CaCl2型高压稳定相。这种压力诱导的非晶化是由晶格的弹性变形引起的，是一种可逆的相变。(2)安装在DAC中的内部铂加热系统能够更精确地模拟考虑地温梯度的地幔相变。用时间分辨衍射法研究了石英到柯石英在不同压力和温度下结构变化的动力学，观察了它们的热膨胀系数、体积弹性系数和弹性常数等物理性质。阐明了不同的压缩路径和加热过程带来了不同的阈值转变临界压力和温度。(3)目前的拉曼光谱研究和EXAFS实验解释了高压下晶格的非晶化是由弹性或塑性晶格变形引起的。通过对玻璃和GeO2高密度非晶相的局域结构的研究，可以解释压力诱导非晶化的不同机制。(4)通过分子动力学计算和基于DX-Xα方法的分子轨道计算，模拟了具有金红石型结构的SiO_2、GeO_2、SnO_2和PbO_2在变应力场下的结构变化。较少

Present investigation aims to discuss the kinetics of the phase transformation of silicates and hydrates constituting earth crust and mantle. It has been known that the phase change often stays in the metastable or even unstable state by the Ostwald step rule, when the thermal energy is not large enough to supply the proper activation energy for the phase transformation. For the purpose of understanding the above mentioned mechanical process, quantitative kinetic investigations have been undertaken by the simulation of physico-chemical process found in the earth interiors as a function of temperature, pressure and stress. The following research projects have been performed :(1) X-ray diffraction study of the phase transition of Ca(OH)2, which is an analog material to mafic hydrates such as serpentine, has been carried out using the single crystal under hydrostatic pressure condition at 300K with diamond-avil cell (DAC). The compression induced a high density amorphous state but never b … More rought a CaCl2 type high pressure stable phase. This pressure-induced amorphization was caused by elastic deformation of the lattice and it is a reversible transformation.(2) An internal Pt heating system installed in the DAC enable to simulate the more precise phase transformation in the earth mantle in consideration of the geothermal gradient. Kinetics study of the structure change from quartz to coesite under various pressures and temperatures was undertakek by the time resolved diffraction study, Their physical properties such as thermal expansibity, bulk moduli and elastic constants were observed. It has clarified that the various compression paths and heating processes bring a different threshold transition critical pressure and temperature.(3) The present Raman spectroscopic study and EXAFS experiment accounts for the elastic or plastic lattice deformation causes the amorphization under high pressure. The investigation of the local structure of glass and high density amorphous phase of GeO2 could explain the different mechanism of the pressure-induced amorphization.(4) Simulation of structure change of SiO2, GeO2, SnO2 and PbO2 having a rutile structure has been executed under variable stress field through the molecular dynamical calculation using optimized pair potential and molecular orbital calculation based on DX-Xalpha method. Less