钙钛矿CaTiO3-Na1/2Ce1/2TiO3体系在高温高压下的晶体化学特征研究

The CaTiO3-Na1/2Ce1/2TiO3 system is a binary system of two typical end members from natural minerals "Perovskite Space", which has not been systematically studied. The corresponding synthesis Ca1-2xNaxCexTiO3 (0.0≤x≤0.50,?x=0.05) , belongs to perovskite compounds consisting of A-site alkali metal and light rare earth element. At present, the structures and phase transitions of CaTiO3 under normal and extreme conditions are relatively clear, while Na1/2Ce1/2TiO3 is controversial.The purpose of this study is to accurately measure the structures and phase transitions of the end members at ordinary temperature and pressure, then to identify the structure of intermediate components and the phase transitions with the increasing of intermediate components in this system, and then select representative components to measure the structural stability and phase transitions under extreme conditions (high temperature and high pressure),then to find the crystal chemical characteristics, so to study the how components, temperature and pressure impact on the structure and properties.The results are used to find the structure of solid solution components that have not yet found in nature and existing conditions of stable structure, and to predict species and characteristics of new perovskite minerals, and to expand the study scope of perovskites under extreme conditions, so to understand the structural stability, status, physical properties and explore the significance of the crystal chemistry and mineralogy.

CaTiO3-Na1/2Ce1/2TiO3体系是"钙钛矿空间"的2个典型端元组分形成的二元系，还未系统研究过，对应合成的Ca1-2xNaxCexTiO3（0.0≤x≤0.50,?x=0.05）属于A位替代的含碱金属和轻稀土元素的钙钛矿化合物。目前，左端元CaTiO3在常规和极端条件下的结构和相变比较明确，而右端元组分Na1/2Ce1/2TiO3的情况不明确。本研究拟在常温常压下精确测定右端元组分的结构的基础上，确定中间组分的结构，以及该体系随组分的增加的系列相变，进而选取代表性组分，测定其在极端条件下（高温、高压）的结构稳定性，相变，揭示其晶体化学特征，探讨组分、温度、压力对结构、性质的影响。利用得到的温压结果，研究自然界中尚未发现的固溶体组分的结构，稳定结构的赋存条件，预测新的钙钛矿矿物种属和特征，扩展极端条件下钙钛矿相的研究范围，了解其物质组成和状态，探讨其晶体化学特性和矿物学的意义。

本项目研究的CaTiO3 - Na1/2Ce1/2TiO3体系，其系列化合物(Ca1-2xNaxCex)TiO3（x = 0-0.5, Δx = 0.05），是典型的A位离子替代型的含稀土元素的钙钛矿型氧化物。通过高温固体反应法合成了(Ca1-2xNaxCex)TiO3（x = 0-0.5, Δx = 0.05）共11个钙钛矿型化合物，利用电子探针和X射线衍射分析，确认了这是一个完全固溶体系列。从组分变化的角度，对CaTiO3 - Na1/2Ce1/2TiO3体系利用粉晶X射线衍射，同步辐射X射线衍射、透射电镜、拉曼光谱、红外光谱和热分析等研究，得出钙钛矿(Ca1-2xNaxCex)TiO3的晶体结构特征和一些相变规律。对于端元组分Na1/2Ce1/2TiO3的Rietveld拟合，确定其为I4/mcm结构。常温常压下，对于CaTiO3 – Na1/2Ce1/2TiO3体系，(Ca1-2xNaxCex)TiO3随着Ce含量（x）的增加，结构从Pbnm结构转变为I4/mcm结构，这种相变属于二级相变，其相变边界为x = 0.4 ~ 0.44。拉曼光谱和红外光谱显示了CaTiO3 – Na1/2Ce1/2TiO3体系的短程有序特征的结果与XRD研究的结论完全一致。利用金刚石压腔技术，将CaTiO3压力加至40GPa依然未观察到相变，只有三个晶轴不同程度的压缩，此后有利用同步辐射和拉曼技术，升压到60GPa依然保持着斜方结构。其理论模拟，预测CaTiO3在30GPa会发生相变，实测中并未发现。对端元组分Na1/2Ce1/2TiO3的高压研究表明，右端元Na1/2Ce1/2TiO3在室温常压下和不同压力下进行的XRD衍射实验，并未观察到明显相变。晶胞参数a和c随压力的增大逐渐减小，c比a减小得明显。理论计算的状态方程与高压实验基本一致，实验结果与理论计算结果基本一致。Ti-O-Ti键角随压力增大而减小。