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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

^* E-mail: anton.beran{at}univie.ac.at

	Abstract

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 
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+2x^IVT⁴⁺_2–2x)O_10-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.

Key-words: mullite compounds, ceramics, FTIR spectroscopy, single-crystal refinement, Rietveld refinement.

	Introduction

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 
Mullite is one of the most promising engineering materials for structural and functional ceramics. Its composition is commonly described as a solid solution series, Al₂(Al_2+2xSi_2–2x)O_10-x with x ranging from about 0.18 to 0.82. The Al-poor limit with a corresponding Al₂O₃ content of 57 mol% is separated from the structurally closely related sillimanite, Al₂SiO₅, i.e. x = 0. The Al-rich end-member mullite with a corresponding Al₂O₃ content of 89 mol% ("high-Al mullite") is close to a hypothetical alumina phase, -Al₂O₃, of mullitetype structure with x = 1 (Fischer et al., 1996; Rehak et al., 1998). The frequently observed solid solution compositions with x = 0.25 and x = 0.40 are designated as 3:2-mullite (3Al₂O₃.2SiO₂) and 2:1-mullite (2Al₂O₃.lSiO₂), respectively. Mullitetype compounds also exist in the systems Al₂O₃-GeO₂, Ga₂O₃-GeO₂ (Schneider & Werner, 1982; Michel et al., 1996), Al₂O₃-Me₂O (Me = Na, K) and Al₂O₃-B₂O₃ (Mazza et al., 1992). A detailed review on mullite and mullite ceramics is given by Schneider et al. (1994).

Based on the work of Sadanaga et al. (1962) the average crystal structures of Al-Si mullite phases have been extensively studied by Burnham (1964), Saalfeld & Guse (1981), Angel & Prewitt (1986), Balzar & Ledbetter (1993) and Fischer et al. (1994). Crystallographic data on the Al-Ge analogues are reported by Durovic & Fejdi (1976) and Saalfeld & Gerlach (1991). Mullite crystallises in the orthorhombic space group Pbam. The cell parameters exhibit an anisotropic relationship to the chemical composition. Whereas the a lattice constant increases linearly with increasing alumina content, the corresponding b lattice constant decreases in a non-linear relationship, thus yielding a crossover point at about 78 mol% Al₂O₃ with a pseudo-tetragonal cell metric. Most recent equations based on the linear relationship between a lattice constant, cell volume and the molar alumina content are given by Rehak et al. (1998).

The structure of mullite is best described in relation to the sillimanite structure (Winter & Ghose, 1979). It resembles the Al₂SiO₅ structure in possessing columns of edge-sharing AlO₆ octahedra forming chains parallel to the c-axis. The octahedral columns are cross-linked by tetrahedrally coordinated Si and Al, establishing double chains that also run parallel to [001]. In deriving the mullite structure from that of sillimanite, some oxygens must be removed from their sites. The ability to accommodate oxygen vacancies (V_o), commonly described by the relationship 2 Si⁴⁺ + O^2- = 2 Al³⁺ + V_o, is provided by the crystallo-graphic Oc site, the central oxygen position of the tetrahedral pairs. By removing an oxygen atom from this Oc site, the two adjacent cation sites T are now coordinated by only three oxygen atoms and therefore displaced to the T* position. Consequently the Oc site is now coordinated by three tetrahedral cations and displaced towards the T* site to the Oc* position, forming T₃O groups with 2T + 1T* (Fig. 1). Combined with these displacements is a partial substitution of Al by Si on the T site. This is proven by X-ray structural refinements (Angel & Prewitt, 1986) and by NMR spectroscopic data (Rehak et al., 1998) revealing a preference of the larger Al for the T* site with respect to the average structure. In Al-rich mullites with x > 0.67, the Al-Si substitution and the increase of O vacancies alone are not sufficient to describe the structural relationships. An interstitial incorporation of additional Al connected with the bridging Oc* atom is postulated, thus forming non-symmetrical T₄O groups with 2T + 1T* + 1T** (Fischer et al., 1994). This model would permit the extension of the solid solution series to a pure -Al₂O₃ structure.

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

The aim of this paper is to propose a modified assignment of the observed absorptions in the mid-IR spectral range of Al-Si, Al-Ge and Ga-Ge mullite-type compounds based on polarised FTIR microspectroscopy of oriented ultrathin Al-Si 2:1-mullite single-crystal slabs and on FTIR powder spectroscopy of polycrystalline Al-Si, Al-Ge and Ga-Ge mullite samples. As precise structural data are an essential basis for our approach, the investigated mullite-type compounds were characterised by single-crystal crystallography or by Rietveld refinements (Rietveld, 1969) of X-ray powder data (XRD). Furthermore, the comparison of the X-ray powder data of Al-Si and Al-Ge mullite with the observed and reported single-crystal data provides a good estimate on the reliability of the results for the Ga-Ge mullite, for which no detailed structural information was available.

	Experimental techniques

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 

Sample preparation

The mullite single-crystal (Al-Si 2:1) was synthesised by F. Wallrafen, University of Bonn, Germany using the Czochralski technique. Starting materials were SiO₂ (22.7 wt.%; Alfa, 88777) and Al₂O₃ powders (77.3 wt.%; Alfa, 38368). The starting powders were homogeneously mixed, pressed and subsequently melt in an iridium crucible by means of high-frequency heating in an Ar/10 CO₂ gas atmosphere. The crystal was pulled by an automatic system with crystal diameter control. Pulling velocities varied between 1.5 and 2.5 mm/h at a given crystal rotation of 10/min. After the growth process the crystal was cooled down to room temperature in a two-step process (60 and 20 °C/h, respectively) and had a diameter of 18 mm and a length of 22 mm. Details of the crystal growth process were published by Guse & Mateika (1974). The single-crystal composition of 65.5 mol% Al₂O₃ and 34.5 mol% SiO₂, which closely corresponds to 2:1-mullite, was determined by microprobe analysis.

The mullite powder samples of the present study were prepared by heat-treatment of sol-gel derived precursor powders and by reaction sintering of oxide powders. The Al-Si mullite was prepared from a precursor powder, synthesised by a sol-gel process using aluminium sec-butoxide, Al(OBu^s)₃ (Merck, 820054) and tetraethoxysilane, TEOS (Merck, 800658) as starting materials. Both metal alkoxides were admixed and diluted with isopropanol (Merck, 109634) (Al/Si = 3/1). After homogenisation, the mixture was allowed to hydrolyse only by air contact in a glove box at a relative humidity of about 40 %. After a period of 10 days the solution gelled. The resulting gel was dried (150°C, 15 h), powdered and finally heattreated at 1450°C for 15 h (Voll et al., 1998).

For the synthesis of Al-Ge mullite, powders of -alumina (Merck, 101095) and germanium oxide, GeO₂ (Merck, 12177) (Al/Ge = 3/1) were intimately mixed, using an agate mortar and pestle. This powder mixture was pressed to a pellet and heat-treated at 1325°C for 15 h. The Ga-Ge mullite (ICDD 50–354) was prepared by mixing gallium oxide, -Ga₂O₃ (Alfa, 32102) and GeO₂ powders, pressing the oxide mixture to a pellet and then heat-treating of the sample at 1360°C for 2 h. Since GeO₂ evaporates in the temperature range of mullite formation, the sample was prepared with an excess of GeO₂ (Ga/Ge = 2.65/1). The pellet was separated into pieces and packed closely together in a small alumina crucible covered with a lid.

Qualitative XRD revealed a small amount of an andalusite-type impurity, which prevented a reliable chemical characterisation of the Ga-Ge sintering product. The Al-Si and Al-Ge powders were impurity-free according to XRD examinations; however, the XRD detection limits for chemically similar compounds and a probable amorphous content are 1–2 vol% and 5–10 vol%, respectively. Therefore chemical characterisation of the title compounds by means of structural refinement procedures was chosen.

FTIR spectroscopy

KBr micropellets with a sample/KBr weight ratio of 0.0025 were produced for the powder measurements. Both, powdered samples and KBr were dry-powdered by hand-grinding in an agate mortar, carefully homogenised and dried at 110°C for 2 h prior to pressing. For single-crystal measurements with transmitted non-polarised and polarised light, two slabs oriented parallel (100) and (001) from the mullite single-crystal were cut with a low-speed diamond saw. After grinding and polishing procedures with 15 µm SiC abrasive paper and 5 µm alumina coated lapping films, plane parallel self-supporting plates with a thickness of about 100 µm were produced. Further thinning was provided by ion milling using a Gatan PIPS model 691. The final thickness for a usable measuring field with about 100 x 50 µm² in size in the (100) slab and with about 70 x 70 µm² in size in the (001) slab was 3 µm. The orientation of the faces was proved by optical methods.

FTIR powder spectra were recorded from 1400 to 400 cm^–1 by means of the Perkin-Elmer FTIR spectrometer 1760 X equipped with a TGS detector and a CsI microfocus accessory. Background and sample spectra were obtained from 64 scans each with a nominal resolution of 4 cm^–1. The powder spectra were deconvoluted into single Gaussian-shaped absorption bands using the program PeakFit (Jandel Scientific). The nonpolarised and polarised single-crystal spectra of the two oriented slabs were recorded with a rectangular sample aperture of 50 x 30 µm² from 1400 to 600 cm^–1 on a Perkin-Elmer FTIR microscope equipped with 0.60 numerical aperture mirror lenses (Cassegrains), a liquid-nitrogen cooled MCT detector, and a gold-wire grid polariser. Background and sample spectra were averaged over 128 scans each with a resolution of 4 cm^–1. The data handling was managed by the program IRDM (Perkin-Elmer).

Single-crystal X-ray diffraction

From the mullite 2:1 single-crystal a fragment of 60 x 80 x 100µm³ was separated and mounted on a Kappa diffractometer (Nonius, Delft) using MoK radiation improved by a polycapillary, parallel beam collimator (IFG, Germany). For the data collection with a CCD detector (Princeton Instruments, Trenton), evaluation, and refinement the programs Collect (Nonius, 1999), Denzo (Otwinowski & Minor, 1997) and Shelxl-97 (Sheldrick, 1997) were used. To fill the whole sphere a triclinic strategy of one and four -scans with a total of 328 frames was applied for the measurement. A counting time and -increment of 90 s and 2° per frame, and a crystal to detector distance of 30 mm was chosen. After the data reduction, including intensity integration, background and Lorentz-polarisation corrections, the data processing and scaling in P1 revealed a mosaicity of 0.45° and the final merging of the data in Pbam gave an internal R value of 0.0167. No absorption correction was applied to the data (µ Mok = 1.069 mm^–1).

The refinement was started with the coordinates and crystallographic site occupancies (s.o.f.) given by Angel & Prewitt (1986) for the average structure of the Al-Si 2:1-mullite. The complex scattering factors for neutral atoms were taken from Wilson (1992). Using isotropic displacement parameters and fixed s.o.f. the refinement converged at a R1-value of 0.045, whereas the use of anisotropic displacement parameters reduced R1 to 0.0239. To set the s.o.f. in accordance with the average mullite model M³⁺₂ (T³⁺_2+2xT⁴⁺_2–2x)O_10-x, a mixed occupation of the T³⁺ and T⁴⁺ cations on the T-site, and a preference of the larger T³⁺ cations on the T*-site was assumed. Therefore the constraints on the s.o.f. values of the affected crystallographic sites were defined as T⁴⁺ = +2x, Oc = +3x, T* and Oc* = -2x. The addition of this constraint resulted only in a slight improvement of the final R1-value with 0.0236 and a refined formula very close to the known chemical composition (x = 0.01). Further data on this Al-Si 2:1-mullite single-crystal refinement are compiled in Table 2.

The three samples were prepared as 15 x 15 x 1 mm³ thick layers of the fine grained powders, fixed with grease and pressed on a zero-back-ground silicon holder. The data were collected on a Philips X'Pert diffractometer (r = 171.9 mm) using Cukä_1,2-radiation (40 kV, 40 mA) and equipped with an automatic divergence slit, a sample spinner, a diffracted beam curved graphite monochromator, and a scintillation counter. A receiving slit of 0.1 mm (= 0.033° 26) and an anti-scatter slit of 4° was chosen. On both sides of the sample stage sollerslits with 0.04 rad were inserted. The irradiated sample area was 12 x 12 mm² and measurements were performed in stepscan mode over the range 2–155° 2 in steps of 0.02° with a counting time of 8 s/step.

The Rietveld refinements (Rietveld, 1969) were started with the coordinates of the Al-Si 2:1-mullite single-crystal using the respective initial cell parameters from Schneider & Werner (1982). The structure refinements were performed with PC-Rietveld Plus (Fischer et al., 1993), and standard deviations were corrected according to Berar & Lelann (1991). For intensity correction, the divergence slit conversion according to Fischer (1996) was applied to the Lorentz-polarisation factor. For the peak-shape the pseudo-Voigt function was used and data points within five times the full width at half maximum were considered to contribute to the respective peak. The complex X-ray scattering factors of Al³+, Si⁴⁺, Ga³+ and Ge⁴⁺ were taken from Wilson (1992), and for O^2- the values given by Hovestreydt (1983) were used. According to the single-crystal data refinement one constraint was used for the s.o.f. of the T, T*, Oc, and Oc* position. The isotropic displacement factors were allowed to vary freely.

As known from the preliminary XRD measurements of the Ga-Ge mullite powder, a small Ga₂GeO₅ andalusite-type impurity was present. Therefore a multiphase refinement procedure was applied for this sample. The structural parameters were obtained from a Rietveld refinement of a pure Ga₂GeO₅ andalusite sample (ICDD 50–352), starting from the structural and cell parameters given by Winter & Ghose (1979) and by Schneider & Werner (1982), respectively. The refined cell values for Ga₂GeO₅, a = 8.114(2), b = 8.289(2) and c = 5.828(1) Å, slightly deviate from the values reported by Schneider & Werner (1982). However, the refined values are intermediate between the cell parameters given in the ICDD entries 35–386 and 36–291. The quantitative evaluation according to Hill (1991) revealed an amount of 2.8(5) wt.% Ga₂GeO₅. Further data on X-ray powder data collection are also compiled in Table 2.

	Results

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 

FTIR powder spectroscopy

The FTIR powder spectra in the 1400–400 cm^–1 range of Al-Si, Al-Ge, and Ga-Ge mullite are shown in Fig. 2. The spectra are characterised by three groups of bands, labelled (a), (b) and (c) following Schneider (1981). For Al-Si mullites, group (a) bands are centred in the 1200–1100 cm^–1 region, group (b) bands in the 1000–700 cm^–1 region, and group (c) bands in the 650–400 cm^–1 region. For AlGe mullite, the group (a) band system shifts to the 1100–1000 cm^–1 region, group (b) absorptions shift to 950–650 cm^–1 and group (c) bands to 600–400 cm^–1. In Ga-Ge mullite the corresponding band systems are in the 1050–950 cm^–1, 850–600 cm^–1 and 550–400 cm^–1 ranges.

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).

Polarised single-crystal spectra in the 1400–600 cm^–1 range of the oriented Al-Si 2:1-mullite slabs are shown in Fig. 4. The strongly orientation-dependent bands can be largely correlated with the bands (A-H) of the powder spectrum. Group (a) bands are strongly polarised parallel [100] and [010] and show no essential component parallel [001]. The presence of a third group (a) band on the low-wavenumber side (B’) is confirmed by the polarised single-crystal spectra. The three bands are centred at 1161, 1135 and 1116 cm^–1.

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.

The absorption behaviour is in good agreement with the data published by Rüscher (1996) on the basis of polarised reflection spectra. The reported band positions for the high-energy triplet are at 1162, 1133, and 1108 cm^–1. The band at 1040 cm-1 is only slightly indicated in the reflection spectra. The C, D, E, and F bands are centred at 978, 900, 795 and 723 cm^–1. Slight differences between the spectra measured in transmittance and in reflectance also exist in the relative band intensities.

Single-crystal data refinement

As can be seen from the most chemistry-sensitive cell parameters (Table 2) a = 7.5817(8) ų and V = 168.10(4) ų, these values of the investigated single-crystal are close to the respective reported data for Al-Si 2:1-mullites, e.g. Angel & Prewitt (1986) from single-crystal and Ban & Okada (1992) from powder data. According to the most recent relationships of Rehak et al. (1998) one can calculate a chemical composition with 65.3 and 65.2 mol% Al₂O₃, respectively. This finding agrees well with the result of the constrained s.o.f. refinement with a composition of 67.1 mol% Al₂O₃ (x = 0.41), which is in a very good agreement to the known 2:1 composition of 66.7 mol% Al₂O₃ (x = 0.40). It is worth noting that in the case of area detectors the intensity-dependent refinement of s.o.f. values is more reliable than the evaluation of cell parameters, which are strongly influenced by aberrations from the ideal experimental geometry.

According to the refined structural parameters (Table 3, first line), almost all selected interatomic distances and bond angles of Al-Si 2:1 (Table 4) agree within their standard deviations with the results of the previous investigation by Angel & Prewitt (1986). A significant positive deviation can be only observed for the T*-Oc* values found to be 1.869(8) and 1.852(3) Å] respectively. As the T-T* distance [1.343(2), A&P: 1.347(1) Å] and the O-T*-O angles remain nearly the same values, this small enlargement is mainly caused by a reduced shift of the Oc* site from the special Ocposition [Oc-Oc*: 0.529(8), A&P: 0.543(3) Å]. The pronounced differences with the Al-Si 3:2-mullite of Saalfeld & Guse (1981) can be summarised in a decrease of the M-Od [1.9349(8), S&G: 1.9429(8) Å] and an increase of the T-Oc [1.6697(4), S&G: 1.6582(8) Å] and T-Od [1.7277(5), S&G: 1.7249(6) Å] distances.

The final Rietveld plot of the e.g. Ga-Ge mullite is shown in Fig. 5, and significant refinement values, crystallographic data, and R-values are listed in Table 2. Using again the relationships of Rehak et al. (1998) for Al-Si mullites, the refined cell dimensions a = 7.5655(4) Å and V = 167.81(3) ų revealed a chemical composition of 63.0 and 63.9 mol% Al₂O₃, respectively. The obtained cell parameters of Al-Ge and Ga-Ge slightly deviate from the corresponding values of Schneider & Werner (1982) for a 3:2 Al-Ge and 3:2 Ga-Ge mullite and are almost identical to the results obtained for a 3:2 Al-Ge single-crystal by Durovic & Fejdi (1976). In comparison to the AlSi mullite, the cell volumes for Al-Ge and Ga-Ge mullites exhibit a relative increase of 3.8 and 13.5 %, respectively.

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.

The structural parameters of the investigated mullites are given in Table 3 (second to fourth line) and the most relevant interatomic distances and bond angles are listed in Table 4. Additionally, corresponding values are tabulated for the single-crystal data refinement of the Al-Ge 2:1-mullite by Saalfeld & Gerlach (1991). Generally it can be stated that all obtained data are in best agreement with the values of the single-crystal results. Minor differences can be only observed for the bond distances and angles of the partially occupied T* tetrahedra. The data revealed from the Rietveld refinements show a less distorted coordination of this partially occupied cation site. Especially the usually enlarged T*-Oc* distances exhibit a reduction of about 0.035 Å towards the mean value (Table 4). Although the atomic coordinates of the Oc* site were allowed to vary freely, the shifts of Oc* from Oc closely follow the crystallographic relationship x = 1/2-y, within their standard deviations. The reasonable s.o.f. results for this crystallographic site support the assumption of the non-random cation distribution over the T and T* sites.

In both Al-containing mullite compounds the cation site within the octahedral chain is fully occupied by Al. Consequently the mean M-O₆ bond length in the Al-Si mullite with 1.908 Å is almost identical to the respective value of the Al-Ge mullite with 1.910 k Å and is in good agreement with the theoretical value of 1.895 Å (Table 4). For the Ga-Ge mullite the complete replacement of Al by Ga on the M position causes a significant increase of the M-O₆ bond lengths with a mean distance of 1.969 Å, which is close to the calculated value of 1.98 Å.

	Discussion

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 
The strong shift of the group (a) bands A and B from Al-Si to Al-Ge mullite and the weaker shift from Al-Ge to Ga-Ge mullite yield strong evidence for an assignment of these bands to Si-O and Ge-O stretching vibrations of the respective tetrahedral units. The similar behaviour of band C (Fig. 4) and its relatively weak intensity indicates that this band is also determined by Si-O and Ge-O stretching vibrations (Table 1). This behaviour perfectly agrees with the significant increase of the mean T-O distances of Al-Ge (+ 2.4 %) and Ga-Ge mullite (+ 5.3 %) as compared to that of Al-Si mullite and is also in good accordance with the calculated distances (Table 4). The lack of an absorption component parallel [001] of group (a) bands in the polarised Al-Si 2:1 single-crystal spectra (Fig. 4) is interpreted as a result of bands A and B (and B’ in Al-Si mullite) being essentially determined by high-energy stretching vibrations as can be expected from the extremely short Si(Ge)-Oc tetrahedral bond within (001). The reductions relative to the mean T-O bond distances (Table 4) are -2.3 % (2:1 single-crystal) and -2.5 % for Al-Si, -2.8 % for Al-Ge, and -3.2 % for Ga-Ge mullite. According to the structural refinement of sillimanite by Winter & Ghose (1979) the Si-Oc distance amounts to 1.574 Å or -3.3 % with respect to the mean tetrahedral size. From the close relationship with the crystal structure of sillimanite and with its absorption spectrum (Fig. 6) this distance can be expected also for the disordered structure of mullite. The complete lack of group (a) bands in Si-free mullite-type alkali aluminates (evident from preliminary measurements of mullite-type compounds in the Al₂O₃-Na₂O and Al₂O₃-K₂O system) strongly support the proposed assignment. As stated by Rüscher et al. (1996), the intensity ratio (1130 cm^–1/1170 cm^–1), which is equivalent to the B/A band ratio, strongly increases with an increasing Al₂O₃ content of mullite. A possible conclusion is that the B band is related to the Si-O stretching vibration in the tetrahedral T₃O triclusters of the mullite structure. This model is also in agreement with the results of the NMR measurements of Rehak et al. (1998) where two different Al coordinations at the T site of the triclusters were distinguished. It suggests that a significant amount of Si enters the tetrahedral triclusters. The similar shift behaviour of band C with that of the group (a) bands and its relatively weak intensity indicates that this band is also determined by tetrahedral stretching vibrations. Its strong component parallel [001] is probably due to Si-O stretching modes vibrating roughly along the direction of the tetrahedral double chains, e.g. the elongated Od-T-Od bonds connecting the edge sharing M-O₆ polyhedra along the c-axis.

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).

The predominant band H of group (c) bands is assigned to stretching vibrations of the M cations in the octahedral coordination. This is in accordance to the continuous change of the polyhedral bond angles due to an elongation of the octahedral chain along the c-axis with a reduction of the Oab-Oab common edges and an increase of the Oab-Oab bonds along [001]. The corresponding Oab-M-Oab bond angles range from 80.8(1) to 99.2(1)° in Al-Si, from 79.5(1) to 100.6(1)° in Al-Ge, and from 78.5(2) to 101.5(2)° in the Ga-Ge mullite. The G band which is strongly overlapped by band H is assigned to O-Al(Ga)-O bending vibrations of the respective tetrahedra and band I to O-Si(Ge)-O bending vibrations of the tetrahedral units. Probably this band is influenced by M-O-M bending vibrations of the respective octahedra. The band assignment proposed on an empirical base is in partial agreement with the band positions calculated by MacKenzie (1972) (Table 1).

On the basis of the present band assignment it is interesting to compare the IR spectrum of mullite with that of the structurally related sillimanite. Fig. 6 demonstrates the close similarities between the sillimanite and Al-Si mullite spectrum. The irreducible representations for sillimanite (Salje & Werneke, 1982) explain the enhanced number of bands. On the basis of theoretical considerations we have to expect 40 IR-active modes (10B_1u + 15B_2u + 15B_3u) compared to a reduced number of 22 for an "ideal" mullite with x = 0 (4B_1u + 9B_2u + 9B_3u) (Michel et al., 1996) or 32 (6B_1u + 13B_2u + 13B_3u) for a more adequate mullite structure (Rüscher, 1996).

From the comparison of the spectra it can be deduced that the "isolated" absorption centred at around 1200 cm^–1 (i.e. group (a) bands) is solely caused by stretching vibrations of the structural SiO₄ units which are determined by the very short Si-Oc distance of 1.574 Å in sillimanite (Winter & Ghose, 1979). The bands grouped in the 1000–650 cm^–1 region must also be due to vibrations of tetrahedral units, essentially to stretching vibrations of AlO₄ tetrahedra and to T-O-T bending vibrations. From the similar band positions of group (b) bands it can be concluded that the mean size of the SiO₄ and AlO₄ tetrahedra in mullite is very close to that of SiO₄ and AlO₄ in sillimanite with 1.627 and 1.763 Å, respectively (Winter & Ghose, 1979). Accordingly the mean T-O distances in the average Al-Si mullite structures determined by X-rays as 1.702 Å for 3:2 (Saalfeld & Guse, 1981) and 1.709 Å for Al-Si 2:1 fit exactly between these values. The bands at around 600 cm^–1 are due to Al-O stretching vibrations of the AlO₆ octahedral units. The practically identical size of the AlO₆ octahedra in mullite and sillimanite with mean Al-O distances of 1.908 and 1.912 Å, respectively, is expressed by the almost identical position of the H band. The sharp and splitted bands in the 550–450 cm^–1 region are assigned to O-Si-O bending modes and Al-O-Al bending modes of the AlO₆ octahedra. The two sharp sillimanite bands at 700 and 450 cm^–1, not evident in the spectrum of mullite, must be exclusively controlled by Si-Al ordering effects over the T sites.

	Acknowledgements

Top Abstract Introduction Experimental techniques Results Discussion Acknowledgements References

 
G. Paul, Köln, and A. Wagner, Wien, prepared the ultrathin oriented slabs. D.V. was supported by the Austrian ‘Fonds zur Förderung der wissenschaftlichen Forschung’ (project no. P12919-PHY). Financial support by the International Centre for Diffraction Data (Newtown Square, PA) through Grant-in-Aid no. 90–03 is acknowledged. The authors are grateful to two anonymous reviewers.

Received 4 July 2000
Modified version received 3 January 2001
Accepted 12 January 2001

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