CRYSTAL-STRUCTURE OF AL-RICH MULLITE

Mullite has been synthesized in a sol-gel process yielding an alumina- rich phase with 89 mol% Al2O3 corresponding to Al5.65Si0.35O9.175, wit h 0.825 O vacancies, space group Pbam, a = 7.7391(6), b = 7.6108(5), c = 2.9180(1) angstrom, and Z = 1. This is the highest alumina content observed so far in the mullite solid solution series. The correlation of lattice parameters with alumina content shows a strictly linear inc rease of a beyond the crossover point of a and b at 78 mol% Al2O3. Thi s clearly proves that the average crystal structure remains orthorhomb ic and does not transform to a tetragonal form. The crystal structure has been analyzed by the Rietveld method on the basis of X-ray powder diffraction data. The refinement shows that about half an Al atom per unit cell is statistically distributed on a new T* site that is linke d to T3O groups (T2TO) forming the crosslinks between the chains of A lO6 octahedra. T* has five bonds to O atoms between 1.81 and 2.42 ang strom. These long distances in the average crystal structure are assum ed to be shorter in the local environment of T*, which could be achie ved by split O positions.