A new assignment of IR vibrational modes in mullite

A new assignment of IR absorption bands of mullite is presented on the basis of empirical studies in the 1400-400 cm(-1) vibrational range of mullite-type compounds in the systems Al2O3-SiO2, Al2O3-GeO2, Ga2O3-GeO2, and Al2O3-Me2O, Ga2O3-Me2O (Me = Na, K, Rb). The powder samples were prepared by heat treatment of sol-gel derived precursor powders and by reaction sintering of oxide powders. The FTIR powder spectra of Al-Si, Al-Ge, and Ga-Ge mullite compounds are characterized by three band groups, designated as (a), (b) and (c). Due to the lack of group (a) bands in the alkaline aluminate and gallate spectra, this high-energetic band group is assigned to Si-O and Ge-O stretching vibrations. Group (b) bands are essentially determined by Al-O and Ga-O stretching vibrations with Al and Ga on T sites in tetrahedral coordination and by T-O-T bending modes, while the low-energetic group (c) bands are due to Al-O and Ga-O stretching vibrations in octahedral structural units. Details of the vibrational modes are discussed on the basis of the deconvoluted spectra. (C) 2002 Elsevier Science B.V. All rights reserved.