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

由塑性变形和弹性变形引起的晶格失稳讨论了土体内部在压缩作用下的结构变化，如相变、分解、非晶化或固相反应。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诱导的结构变化。各向异性结构，如链状结构或层状结构，如辉石和橄榄石，受到地幔剪应力的很大影响。非静力条件下的晶格变形是各向异性的，这与静水压变形不同。下列实验结果已发表在《科学期刊》上，并在国际会议或国内科学年会上发表：(1)在动力学低温下，原子的迁移率不足以产生热力学稳定相。相变…在应力场作用下，通过非扩散马氏相变或原子小位移可以获得更多的位移。FeGeO_3单斜辉石在静水条件下表现出向高压相(拟钛铁矿)的可逆转变，这是新发现的。然后转变为钛铁矿，进而转变为钙钛矿结构。这种转变路径是首次发现的。(2)结构的变化主要是由于外力和原子间作用力之间的维里定理引起的弹性相变。利用最新研制的金刚石顶锤和同步辐射装置，对高压下SiO_2辉石的电子密度测量进行了研究。通过对单晶结构的分析，确定了成键电子密度随压力的变化。(3)在Spring-8高压装置上用时间分辨衍射仪对石英-柯石英转变过程进行了动力学研究。分别对形核和长大进行了分析，给出了转变速率和活化能。(4)研究了水合物在压力作用下的非晶化行为，发现晶格相变的两种不同类型的弹性变形和塑性变形导致了这种非晶化。(5)MD计算表明，这些相变及其动力学过程反映了弹性失稳的机制，这是相变过程中的一种前驱原子运动。一些亚晶格如致密区被保留，另一些亚晶格以随机方式变形。由MD计算的弹性常数CIj与压力的函数关系揭示了结构相变过程中的弹性不稳定性。这种晶格不稳定性可以用基于BOMÅFS准则的弹性张量来解释。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，从而导致相变。(6)结构各向异性导致了OstwaldÅFS阶跃法则在压力下相变动力学中亚稳态相的变化。那么热力学稳定相并不是自然界中唯一的候选相。在非静水或准静水条件下，在相变过程中会产生亚稳态或中间相。(7)高压EXAFS谱带来了固体离子的局域结构。通过高温高压研究，观察到了强子热效应对金属对电势的影响。较少

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