Crystal chemical studies on incommensurately modulated mullite

莫来石（2：1-莫来石，即2Al 2 O3. SiO2，Al4.8Si1.2O9.6，Z=1）的特征是由（Al，Si）O 4四面体的双链连接的共边AlO 6八面体链，受桥氧位置上空位的影响。Angel等人1991（Amer.Mineral.76，332-342）研究了不可调结构，结论是阳离子和氧空位在莫来石中是部分有序的。然而，他们使用低质量的莫来石单晶进行研究，因此不能提供充分了解莫来石的真实的结构所需的信息。本研究的目的是通过基于布拉格中子衍射数据和超结构反射的高维空间晶体结构分析来获得进一步的理解。一个明确的夹杂物无直拉法生长的莫来石单晶可用于此目的。实验工作将伴随着氧空位模型计算。有序排列的结果将与莫来石的晶格参数和性能，如弹性和热膨胀。

The crystal structure of mullite (2:1-mullite, i.e. 2Al2O3 . SiO2, Al4.8Si1.2O9.6, Z=1) is characterized by chains of edge-sharing AlO6 octahedra linked by double chains of (Al,Si)O4 tetrahedra affected by vacancies on the bridging oxygen position. Angel et al. 1991 (Amer. Mineral. 76, 332-342) studied the incommensurately modulated structure with the conclusion that the cations and oxygen vacancies are partially ordered in mullite. However, they used a mullite single crystal of low quality for their studies, and thus were not able to provide the information required to fully understand the real structure of mullite. It is the aim of this study to gain further understanding by crystal structure analysis in higher dimensional space based on neutron diffraction data of Bragg and super-structure reflections. A clear inclusion-free Czochralski-grown mullite single crystal is available for that purpose. The experimental works will be accompanied by oxygen vacancy modeling calculations. The findings of the ordering patterns will be correlated with the lattice parameters and properties of mullite like elasticity and thermal expansion.