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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+2x Si 2-2x )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.

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The thermal expansion coefficients of transition-metalfree sinter mullite and fused mullite, and of chromiumdoped (11.5 wt% CrZO3) and iron-doped (10.3 wt% Fez03) sinter mullites are measured between 25" and 900°C by hightemperature Guinier X-ray diffraction techniques. Most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, =300"C. Although the temperature-induced c-axis expansion coefficients seem to be less dependent on the compositional state x and on transition-metal incorporation of the A14+2rSi2-2r010-r mullite-type phases ( (~( c ) = 5.6 X 10-6/OC to 6.1 X 10-6/"C), thermal a-and b-axis expansion coefficients change more significantly (a(u) = 3.1 X 10-6/"C to 4.1 X 10-6/oC and a(b) = 5.6 X 10-6/"C to 7.0 X 10-6/oC, where the values were calculated between 300" and 900°C). The larger temperatureinduced b than c and a expansions probably are caused by intense lengthening of the relatively long and elastic octahedral AI(1)-O(D) bonds in mullite, which form at an angle of about 30" with b, but of about 60" with a. With increasing x vaiue of the transition-metal-free mullites, the volume thermal expansion decreases, while the anisotropy of thermal expansion is reduced simultaneously. We believe that the variation of the thermal expansion coefficients is controlled by the AP occupancy and by the number of O(C) vacancies in the mullite structure, and also by the disordering distribution of both structural elements. Transition-metal incorporation into mullite has no distinct influence on thermal expansion anisotropy, but does reduce thermal volume expansion. A prestressing of the crystal structure by substitution of A13+ by the larger Fe3+ and Cr3+ ions may be the main reason for the latter effect. [

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