MULLITE WITH LATTICE-CONSTANTS A GREATER-THAN B

The mullite phase was synthesized in a sol-gel process using aluminum sec-butylate and silicon chloride in proportions corresponding to an Al/Si ratio of 4:1. Heat treatment at 1000-degrees-C yielded a mullite-type phase, gamma-alumina, and amorphous fractions. Conventional indexing based on the orthorhombic (Pbam) mullite unit cell with b > a showed that X-ray diffraction peaks with high k and l but low h indices are strongly shifted toward higher d-spacings as compared with a ''normal'' mullite. Furthermore, the relative positions of reflection pairs hkl and khl are interchanged. The Rietveld analysis yielded the following constants: a = 7.760(2) angstrom, b = 7.595(2) angstrom, c = 2.9192(4) angstrom confirming a > b.