Mullite; a substituted alumina Available to Purchase

Mullite solid-solution can be represented by the formula Al_4+2xSi_2-2xO_10-x, Z = 1 where x is the number of oxygen atoms missing per unit cell. The presence of a miscibility gap between sillimanite (x < 0.04) and siliceous mullite (x = 0.17) suggests that their different ordering schemes, deduced from SAED studies, may be the underlying cause, and that the true end-member is aluminum-rich. Supporting evidence lies in the lengthening of the modulation wavelength, interpreted as a systematic coarsening of mullite antiphase domains, with increasing Al content. The concomitant trend toward greater sharpness and complexity of the superstructure reflections defining domain size may be a consequence simply of chemical fluctuations near domain boundaries, but is also very likely to result from the long-range ordering of oxygen vacancies. Mullite cell dimensions vary almost linearly with composition, and when extrapolated beyond those for the most aluminous one found in this study (76 mole percent Al₂O₃; x = 0.59), fall very close to those of a metastable polymorph of alumina, t-Al₂O₃. A structure for this mullite end-member is postulated, and a hypothetical phase diagram given for the system Al₂SiO₅-Al₂O₃ in the absence of corundum.