第IV副族金属氧化物高温高压相变行为的理论研究

High temperature/pressure crystal phases of the IV subgroup metal oxides (TiO2/ZrO2/HfO2) have potential applications in the field of ultra-hard materials. Thermal or pressure/stress induced phase transformations can occur between two phases at high temperatures/pressures, which lead to their unstable under normal temperature and pressure and then losing their practical values. However, the most important problems such as their phase transition paths and potential barriers between two crystal phases as well as the influence of their nano-size on transformation behaviors have still been unknown. We want to search various new crystal phases of high temperature/pressure with particle swarm optimizations etc. and then study the free energy and enthalpy of supercells and nano-particles with first principles and thermodynamics theory simulations. Their stabilities of high temperature/pressure phases can be determined and predicted through the change curve of free energy with temperature or enthalpy with volume/pressure, respectively. The transition paths from high to low temperature/pressure phases can be defined with both experimental observations and the potential surface of the supercells calculated with G-SSNEB method in the course of supercell distortions, reconstruction of chemical bonds, and atomic movements etc. and then as the circumstantial evidence to support the structures of high temperature or pressure phases as well as understand their origin of thermal and dynamical stabilities. Their free energy or enthalpy of nano-particle system can be used to explain the origin of the significant nano-scale effect after considering several factors such as surface energy, interface energy, and strain energy, etc.

第IV副族金属氧化物（TiO2/ZrO2/HfO2）的高温高压相在超硬材料领域具有潜在的应用前景。在高低温/压相之间，可以发生热诱发和压力/应力诱发相变。相变将导致其失稳而失去使用价值。然而决定高低温/压相之间能否相变的重要指标-相变路径和势垒以及纳米颗粒尺度对相变行为的影响等基本问题目前还都没有得到很好地理解。本项目拟使用粒子群优化等算法寻找各种可能高温/压新相；采用第一性原理和热力学理论计算等方法研究超晶胞和纳米颗粒的自由能/焓。通过自由能/焓随温度/体积变化曲线预测和解释高温/压相稳定结构；通过G-SSNEB等算法来研究超晶胞畸变、化学键重组和原子迁移等过程中势能面和势垒高度等参数并比对实验观测来确定高低温/压相变路径，旁证高温/压相结构和理解其稳定存在的热力学和动力学根源；引入界面能和应变能等因素研究相变过程中纳米颗粒自由能/焓变化，解释第四副族金属氧化物显著纳米尺度效应的起因。

第IV副族金属氧化物（TiO2/ZrO2/HfO2）的高温高压相在超硬材料领域具有潜在的应用前景。在高低温/压相之间，可以发生热诱发和压力/应力诱发相变。相变将导致其失稳而失去使用价值。然而决定高低温/压相之间能否相变的重要指标-相变路径和势垒等基本问题目前还都没有得到很好地理解。本项目拟使用粒子群优化等算法寻找各种可能高温/压新相；采用第一性原理和热力学理论计算等方法研究超晶胞和纳米颗粒的自由能/焓。通过自由能/焓随温度/体积变化曲线预测和解释高温/压相稳定结构；通过G-SSNEB等算法来研究超晶胞畸变、化学键重组和原子迁移等过程中势能面和势垒高度等参数并比对实验观测来确定高低温/压相变路径，旁证高温/压相结构和理解其稳定存在的热力学和动力学根源。我们的研究表明：(1) 二氧化钛金红石晶相到钶铁矿相变过程中，(011) 面钛原子沿着 [0-11] 晶向滑移，氧原子随着钛原子滑移被拖着曲折移动，最终导致钶铁矿晶相形成，在滑移过程中最低势垒为 231 meV/TiO2。相变发生过程中，首先出现金红石孪晶和钶铁矿晶相局部区域。随着应变增加，钶铁矿区域增加。在应变在20%左右，从金红石到钶铁矿晶相瞬时发生。(2) 钶铁矿晶相到单斜晶相的相变过程是所谓二阶段相变即先进行晶胞体积压缩到一定程度，然后再进行原子滑移。相变阻力主要来自于相变过程中晶胞剪切运动。(3) 钶铁矿晶相到高温高压相黄铁矿晶相是二阶段相变即先进行体积压缩然后(001)晶面沿着[010]方向滑移，滑移势垒约为625 meV。