A new band assignment of the IR spectrum of mullite is proposed on the basis of FTIR powder spectroscopy 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 mullite formula VIM3+2(IVT3+2+2xIVT4+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 characterised 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 correlated 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 vibrations 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 assignment the lattice vibrational region of sillimanite is shortly discussed.

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