Mullite: Crystal Structure and Related Properties

Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al 2 (Al 2+2 x Si 2‐2 x )O 10‐ x . Main members are sillimanite ( x = 0), stoichiometric 3/2‐mullite ( x = 0.25), 2/1‐mullite ( x = 0.40), and the SiO 2 ‐free phase ι‐alumina ( x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite‐type structural backbone of parallel chains consisting of edge‐sharing MO 6 octahedra and their specific cross‐linkage by TO 4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.