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

A new band assignment of the IR spectrum of mullite is proposed on the basi s of FTIR powder spectroscopy of Al-Si, Al-Ge, and Ga-Ge compounds and pola rised FTIR single-crystal spectroscopy of oriented ultrathin Czochralski-gr own Al-Si 2:1-mullite slabs. The structural parameters of the mullite compounds were obtained from a sin gle-crystal data refinement (Al-Si 2:1) and from Rietveld powder data refin ements 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 M-VI(2)3+((T2+2x3+T2-2x4+)-T -IV-T-IV)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 G a-Ge mullite compounds are characterised by three groups of bands designate d 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 str ong 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 sin gle-crystal slabs can be clearly correlated with the fitted bands of the po wder 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 oc curring along the extremely short bonds of the respective tetrahedral units within the (001) plane. Group (b) bands are essentially determined by stre tching vibrations of Al and Ga on T-sites and T-O-T bending vibrations, whi le group (c) bands are due to stretching vibrations of Al anti Ga in octahe dral coordination and to O-T-O bending vibrations. On the basis of the pres ent band assignment the lattice vibrational region of sillimanite is shortl y discussed.