Structure transitions in silica minerals-a joint atudy of X-ray diffraction and computer simulation

本研究旨在通过结合衍射和光谱技术，建立石英及其同构化合物结构转变和相关物理性质波动机理的原子观点。虽然两年的时间不足以完成所有计划的课题，特别是光谱实验，但人们开发了几个功能强大的计算机程序来分析通常分子动力学计算提供的位置-时间数据。对石英型结构的AlPO_4和GaPO_4在不同温度下获得的单晶强度数据进行了详细的晶体结构分析。CRYSTAL软件包包含SDSTR、MDPDF、ORDERP、DTRANS和NOMODE五个程序单元，每个程序单元通过MD计算的数据执行，分别计算散射强度（中子或x射线衍射）、动态结构因子、原子概率密度函数、原子均方位移、有序参数和最后的正态分析（仍在开发中）。将这些程序应用于褐铁矿AlPO_4的MD结果中，发现了一种有序-无序型机制在该矿物的α - β结构转变中起重要作用。这一结果非常重要，目前仍处于初步阶段，但与最近的研究如x射线结果（本作者），超拉曼结果和硬模红外吸收结果等不一致。必须检查平均阻力计算结果的可靠性，然后对平均阻力进行一些长期计算。

This study has been projected to establish the atomic views about the mechanism of structure transition and fluctuation of related physical properties in quartz and its isostructural compounds, by combinatorial application of diffraction and spectroscopic techniques. Although two years were not long enough to perform all the projected subjects, especially spectroscopic experiments, several powerful computer programs were developed to analyze position-time data provided by usual molecular dynamics calculations.Detailed crystal structure analyzes of AlPO_4 and GaPO_4, both having the quartz type structures, were also completed for single crystal intensity data obtained at different temperatures.The software package CRYSTAL has five program-units, SDSTR, MDPDF, ORDERP, DTRANS and NOMODE, each executed by teading in data from MD calculations, to calculate scattering intensities (for neutron or X-ray diffraction), dynamical structure factors, atomic probability density functions, atomic mean-square displacements, order parameters and finally normal mode analysis (still under development), respectively. In application of these programs to the MD results for berlinite AlPO_4, an order-disorder type mechanism was found to play important role in the alpha-betastructure transition in this mineral. This result is so important and still preliminary at present, but is not consistent with recent studies such as X-ray results (by the present author), hyper Raman results and hard mode infrared absorption results and so on. The results of the MD calculations must be examined for their reliability, and then some long-term calculations for MD are currently undertaken.