Development of the crystal structure model-assembly program which does not need any initial model

根据粉末x射线衍射数据，利用蒙特卡罗法和r因子法建立了晶体结构模型。为了用Rietveld方法研究未知的晶体结构，必须事先准备好正确的结构模型。当没有关于结构的信息时，这通常是非常困难的。自动构建目标材料晶体结构模型的程序将简化用Rietveld法求解粉末材料结构的任务。该程序通过蒙特卡罗方法设置原子位置，需要XRD数据，细胞常数，空间群，化学式和z数（公式在单位细胞中的数量）。用随机数选择单晶胞中独立原子的Wyckoff位置和坐标，并计算理论XRD数据。根据理论和实测的XRD数据计算了模型的r因子。数百种低R值的模型被选择和存储。由于存储模型的原子集中在一个特定的区域，搜索区域被限制在这些原子的附近，这一过程使总计算时间最小化。利用该程序和个人计算机求解了未知高压相（CaMg_2Al_6O_<12>）的结构，表明该方法是求解粉末材料结构的有力工具。

A computer program which constructs crystal structure models using the Monte Carlo and the R-factor method from powder X-ray diffraction data was developed. In order to study an unknown crystal structure by the Rietveld method, a correct structure model must be prepared in advance. This is usually very difficult when no information is available about the structure. A program which automatically constructs a crystal structure model of the object material would simplify the task of solving the structure of the powdered material by the Rietveld method. This program, which sets atomic positions by the Monte Carlo method, requires XRD data, cell constants, a space group, a chemical formula, and a Z-number (the number of the formula in a unit cell). Wyckoff positions and coordinates of independent atoms in a unit cell are selected by random numbers, and theoretical XRD data are calculated. The R-factor of the model is calculated from theoretical and observed XRD data. Hundreds of models of a low R value are selected and stored. As atoms of the stored model concentrate in a particular area, search areas are restricted to the vicinity of these atoms, a process which minimizes the total calculation time. Using this program and a personal computer, the structure of unknown high pressure phase (CaMg_2Al_6O_<12>) was solved, which demonstrates that this method is a powerful tool to solve the structure of powder materials.