Lattice instability and kinetic study of mantle substances in the nonequilibrium process under high-pressure and high stress field

本文讨论了由塑性变形和弹性变形引起的晶格不稳定性引起的EarthAfs内部的相变、分解、非晶化或固相反应等结构变化。它们的非热结构变化成为阐明地球物理动力学过程的关键问题。用同步辐射研究了原位压力引起的结构变化。各向异性结构，如链状结构或层状结构，如辉石和橄榄石，受地幔剪切应力的影响较大。非静水条件下的点阵变形与静水条件下的点阵变形不同。以下实验结果已在科学期刊上发表，并在国际会议或国内科学年会上报告：(1)在动力学低温下原子迁移率不足以产生热力学稳定相。在应力场作用下，通过非扩散马氏体相变或较小的原子位移，可以实现更大的相变位置。铁三斜辉石在静水条件下向高压相（拟钛铁矿）可逆转变，是新发现的。然后转变为钛铁矿和进一步的钙钛矿结构。这个过渡路径是首次被发现的。(2)由于外压力与原子间力之间的维里定理，结构变化主要是由弹性变换引起的。利用新设计的金刚石顶砧和同步辐射，明确了在高达50GPa高压下SiO2硅辉石的电子密度测量。通过单晶结构分析，证实了键合电子密度随压力的变化。(3)在SPring-8多砧高压仪器上，采用时间分辨衍射实验对石英-粘土过渡动力学进行了研究。分别对成核和生长进行了分析，得出了转变速率和活化能。结果表明，晶核开始于晶面或晶界。(4)对水合物的压力诱导非晶态化进行了研究，发现两种不同类型的晶格转变的弹性变形和塑性变形会导致非晶态化。(5) MD计算表明，这些相变及其动力学过程反映了弹性不稳定的机理，这是过渡过程中的前驱原子运动。一些子格如密集区被保留，而其他子格则以随机方式变形。由MD计算得到的弹性常数cij作为压力的函数，揭示了结构转变过程中的弹性失稳。晶格不稳定性可以根据BomÅfs准则用弹性张量来解释。在某些弹性张量或它们的组合为负的情况下，结构变得不稳定，从而导致变形。(6)结构各向异性通过OstwaldÅfs阶跃规律导致了压力作用下相变动力学中亚稳相的变化。那么热力学稳定相就不仅仅是在自然界中发现的候选相了。在非流体静力或准流体静力条件下，相变证明为亚稳相或中间相。(7)高压EXAFS光谱带来了固体离子的局部结构。通过高温高压研究，观察了金金属对电势的非谐波热效应。少

Structure changes of EarthAfs interiors under compression, such as phase transformation, decomposition, amorphization or solid reactions have been discussed by the lattice instability due to the plastic and elastic deformation. Their athermal structure changes become more crucial problems in order to elucidate geophysical dynamical process. In situ pressure-induced structure changes have been investigated by synchrotron radiation. Anisotropic structures such as chain structure or layered structures like pyroxene and olivine are subjected to a large effect of the shear stress in the mantle. Nonhydrostatic condition gave an anisotropic lattice deformation, which is different from those hydrostatic compression. The following experimental results have been published in science jourals and reported at international conference or domestic science anual meetings :(1) At kinetically low temperatures atomic mobility is not large enough to generate thermodynamically stable phases. The phase tran … More sition is possible by the non-diffusion martensic transformation or small atomic displacement under stress field. FeGeO3 clinopyroxene showed a reversible transformation to high-pressure phase (pseudo ilmenite) under hydrostatic condition, which was newly found. Then it transforms to ilmenite and further perovskite structure. This transition path was firstly found.(2) The structure changes are resulted from the elastic transformation mainly due to the virial theorem between external pressure and interatomic forces. Electron density measurement of SiO2 stishovite under high pressures up to 50GPa was clarified by using the newly devised diamond anvil and synchrotron radiation. Through single-crystal structure analysis the bonding electron density changes with pressure was confirmed.(3) Kinetic study of quartz-coesite transition was carried out by time-resolved diffraction experiment using multianvil high-pressure apparatus at SPring-8. Nucleation and growth were separately analyzed resulting in giving the transition rate and activation energy. It is clarified that the nucleation starts at the crystal surface or grain boundary.(4) Pressure-induced amorfization of hydrates were examined and two different types of the elastic and plastic deformation of lattice transition bring the amorfization.(5) MD calculation suggested that those phase transformations and their dynamical processes reflect the mechanism of elastic instability, which is a precursor atomic movement on the transition process. Some sublattices such as dense zone are preserved and the others are deformed in the random fashion. Elastic constants cij as a function of pressure calculated from MD reveal elastic instability in the course of structure transformation. The lattice instability can be interpreted by elastic tensors based on BomÅfs criterion. Under the condition that some elastic tensors or their combination become negative, structure becomes unstable resulting in the transformation.(6) Structure anisotropy brings the variation of metastable phases in the kinetics of phase transformation under pressure by OstwaldÅfs step rule. Then thermodynamically stable phases are not only candidates found in nature. Nonhydrostatic or quasihydrostatic condition brings metastable or intermediate phases in the proen of phase transition.(7) High-pressure EXAFS spectroscopy brought the local structure of solid ionics. Anharonic thermal effect on the pair potential of gold metal was observed by high-tcmperature and pressure study. Less