Infrared band assignment and structural refinement of Al-Si, Al-Ge, and Ga-Ge mullites

Abstract

A new band assignment of the IR spectrum of mullite is proposed on the basis of FTIR powder spec- troscopy of Al-Si, Al-Ge, and Ga-Ge compounds and polarised FTIR single-crystal spectroscopy of oriented ultrathin Czochralski-grown Al-Si 2:1-mullite slabs. The structural parameters of the mullite compounds were obtained from a single-crystal data refinement (Al-Si 2:1) and from Rietveld powder data refinements in space group Pbam. The refined chemical compositions varied from x = 0.31 (Ga-Ge), x = 0.34 (Al-Si) to x = 0.36 (Al-Ge) and x = 0.41 (Al-Si 2:1) with respect to the general mul- lite formula VI M3+ 2( IV T3+ 2+2x IV T4+ 2-2x)O10-x (M = Al, Ga; T = Al, Si, Ga, Ge). The FTIR powder spectra in the 1400-400 cm-1 range of Al-Si, Al-Ge, and Ga-Ge mullite compounds are char- acterised by three groups of bands designated as (a), (b) and (c). The deconvolution of the absorption features in the whole spectral range requires a minimum number of nine fitted bands. For Al-Si mullite, group (a) bands centre in the 1200-1100 cm-1 range, group (b) in the 1000-700 cm-1, and group (c) in the 650-400 cm-1 region. A strong shift of group (a), (b), and (c) bands towards lower wavenumbers exist in Al-Ge and Ga-Ge mullite with respect to Al-Si mullite. This is explained with the increasing size of the polyhedra in replacing Si by Ge and Al by Ga. The orientation-dependent bands in the spectra of the Al-Si 2:1-mullite single-crystal slabs can be clearly corre- lated with the fitted bands of the powder spectra. Due to the band shift and the polarisation behaviour, group (a) bands are assigned to high-energy Si-O and Ge-O stretching vibrations occurring along the extremely short bonds of the respective tetrahedral units within the (001) plane. Group (b) bands are essentially determined by stretching vibra- tions of Al and Ga on T-sites and T-O-T bending vibrations, while group (c) bands are due to stretching vibrations of Al and Ga in octahedral coordination and to O-T-O bending vibrations. On the basis of the present band assign- ment the lattice vibrational region of sillimanite is shortly discussed.