Molecular dynamics simulation of majorite solid solution

采用分子动力学（MD）方法研究了Mg3 Al 2Si 3 O 12（镁铝榴石组分）-Mg 4Si 4 O 12（辉石组分）体系的主固溶体，预测了组成和阳离子分布对晶胞参数、体积、体积模量和摩尔焓的影响。制备具有以下两种类型的阳离子分布的固溶体晶体Mg 3（Mg[x]Si[x]Al[2-2x]）Si 3 O 12（0 [小于或等于] x [小于或等于]1）：Mg和Si在两个不等价的八面体位置上有序排列的四元固溶体. Mg和Si在八面体位置随机分布的立方固溶体。分子动力学计算在恒定压力和温度下使用二体原子间作用势模型CMAS 94（Matsui，1994）进行。主要研究结果如下：1.四元固溶体的晶胞参数呈a > c的关系。随着Mg_4Si_4O_（12）含量x的降低，这些参数的差异变小。在镁铝榴石组成（x = 0）下，晶体系统变为0.2。在富铝组分（x ≤ 0）中，两种固溶体模型给出了几乎相同的摩尔体积、体积模量和摩尔焓值。随着Mg 4Si 4 O 12含量x的增加，立方固溶体变得显示出比正交固溶体更大的摩尔体积、更小的体积模量和更高的焓。通过高压X射线衍射研究确定的主晶固溶体的体积模量随着x在0 [小于或等于] x [小于或等于] 0.8的组成范围内减小，并且在0.8 [小于或等于] x [小于或等于] 1的范围内快速增加。这是解释的渐进有序的Mg和Si的八面体网站周围的组成x = 0.8。

The molecular dynamics (MD) method was applied to majorite solid solution in the system Mg3A12Si3O12 (pyrope)-Mg4Si4012 (pyroxene component), in order to predict the effects of composition and cation distribution on the cell parameters, volume, bulk modulus, and molar enthalpy. The solid solution crystals Mg3(Mg[x]Si[x]Al[2-2x])Si3O12 (0 【less than or equal】 x 【less than or equal】1) with the following two types of cation distribution were prepared :1. Tetragonal solid solution with the ordering of Mg and Si on the two nonequivalent octahedral sites.2. Cubic solid solution with the random distribution of Mg and Si in the octahedral site.The MD calculation was performed under constant pressure and temperature, by using two-body interatomic potential model CMAS94 (Matsui, 1994). The characteristic results are as follows :1. The cell parameters of tetragonal solid solution shows the relation a > c. The difference in the parameters becomes smaller with the decrease in the Mg4Si4O12 content x. At the pyrope composition (x = 0), the crystal system becomes cubic.2. In Al-enriched composition (x 〜 0), the two solid solution models give almost the same values of molar volume, bulk modulus, and molar enthalpy. With the increase in the Mg4Si4O12 content x, the cubic solid solution becomes to show larger molar volume, smaller bulk modulus, and higher enthalpy, than the orthorhombic solid solution.The bulk modulus of the majorite solid solution determined by high pressure X-ray diffraction studies decreases with x in the compositional range of 0 【less than or equal】 x 【less than or equal】 0.8, and increase rapidly in the range 0.8 【less than or equal】 x 【less than or equal】 1. This is explained by the progressive ordering of Mg and Si on the octahedral sites around the composition of x = 0.8.