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

¹ Institut für Mineralogie und Kristallographie, Universität Wien-Geozentrum, Althanstraße 14, A-1090 Wien, Austria
² Deutsches Zentrum für Luft- und Raumfahrt, Institut für Werkstoff-Forschung, Porz-Wahnheide, Linder Höhe, D-51140 Köln, Germany

View larger version (51K): [in this window] [in a new window]   Fig. 1. Schematic representation along [001] of the average structure of mullite.

View larger version (19K): [in this window] [in a new window]   Fig. 2. FTIR powder spectra in the 1400–400 cm–1 range of Al-Si, Al-Ge, and Ga-Ge mullite, deconvoluted into single Gaussian-shaped bands (A-I). For position of band maxima see Table 1. The subdivision in the three band groups (a), (b), and (c) are related to the band positions in the Al-Si mullite spectrum.

View larger version (10K): [in this window] [in a new window]   Fig. 3. Relation diagram showing the band positions of absorption bands A-I (in cm–1) of the individual mullite compounds (Al-Si, Al-Ge, Ga-Ge).

View larger version (19K): [in this window] [in a new window]   Fig. 4. Polarised FTIR spectra of oriented Al-Si 2:1 mullite single-crystal slabs, deconvoluted into single Gaussian-shaped bands, and compared with a nonpolarised FTIR single-crystal spectrum (top: dashed line) and an FTIR powder spectrum (top: full line). The spectra with E vibrating parallel to [100] and [010] were measured on a (001) slab, and the spectrum with E parallel [001] was measured on a (100) slab, both 3 µm thick. Vertical dotted lines indicate band positions. Absorbance scale for non-polarised single-crystal spectrum and powder spectrum in arbitrary units.

View larger version (23K): [in this window] [in a new window]   Fig. 5. Final Rietveld plot with observed (crosses), calculated (line), and difference (lower continuous line) diffraction profiles for the multiphase refinement of Ga-Ge mullite in the ranges 5–75° 2 (A) and 75–145° 2 (B). Peak positions calculated from the cell parameters are indicated as tick marks, the lower representing the contribution of the Ga-Ge andalusite impurity.

View larger version (23K): [in this window] [in a new window]   Fig. 6. FTIR powder spectrum of Al-Si mullite (dotted line) with band assignment A-I, compared with the powder spectrum of sillimanite (continuous line). The spectra are divided in three band groups (a, b, c).