Mullite with Lattice Constants a > b

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Abstract
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°C yielded a mullite‐type phase, γ‐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) Å, b= 7.595(2) Å, c= 2.9192(4) Å confirming a > b.

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