Mullite and mullite-type crystal structures

Synthetic mullites (Al4+2 x Si2 –2 x O10 – x ) belong to one of the most important groups of ceramic materials. Their crystal structure consists of chains of edge-sharing AlO6 octahedra interconnected by tetrahedral double chains of TO4 (T = Al, Si) tetrahedra. The occurrence of oxygen vacancies is associated with the formation of triclusters of TO4 groups. The distribution of oxygen vacancies and the distribution of Al and Si on the tetrahedral sites are partially ordered yielding an incommensurately modulated crystal structure with satellite reflections in diffraction patterns. The number of symmetrically independent distribution patterns of oxygen vacancies is given for a 2 × 2 × 2 and a 3 × 3 × 2 superstructure model. Many other compounds have chain structures similar to mullite. These compounds are assigned to subgroups, all derived from a hypothetical tetragonal aristotype structure.