engleski

Structural study of chromium doped mullite

Mullite, 3Al_2O_3.2SiO_2, is a material with important role in technology of classical and advanced ceramics for high temperature applications due to its low thermal expansion and high creep resistance. The orthorhombic symmetry of mullite (space group Pbam) is built up with chains of Al-O octahedra (AlO_6), running parallel to the crystallographic c-axis. These are linked by chains of Si-O tetrahedra (SiO_4) and Al-O tetrahedra (AlO_4). The connection between octahedral and tetrahedral chains creates wide channels along the c-axis. Depending on the synthesis procedure, the mullite structure is able to incorporate considerable amounts of transition metal cations [H. Schneider, Ceramics Transactions 6 (1990) 135]. The charge and size of metal ion determine the accessible site and concentration in mullite [N. N. Greenwood, A. Aernshaw, "Chemistry of elements", Pergamon Press, Oxford, 1984]. The structural distribution of chromium in mullite is of special interest because the low thermal expansion of mullite can be further reduced by its incorporation. The samples of pure mullite and of mullite with 2, 4, 6 and 8 mol% Cr_2O_3 were derived from the single-phase precursors, and examined by X-ray powder diffraction at RT and EPR (Electron Paramagnetic Resonance) spectroscopy. The unit-cell parameters were determined using Si as internal standard, and refined by the whole- powder-pattern fitting [WPPF program, H. Toraya, J. Appl. Cryst. 19 (1986) 440]. It was found that c-axis increased in all Cr doped samples, the percentage increase ranged from 0.222 to 0.520 %. The b-axis was found to increase similar as a-axis for the sample with 2 mol% Cr_2O_3 (percentage increase of 0.075%), while for other Cr-doped samples it increased much more than the a-axis, with the percentage increase between 0.177 and 0.278%. Pecentage increase of the a-axis amounted 0.012-0.148%, with the minimum for the sample containing 4 mol% Cr_2O_3. Rietveld method was performed to refine the crystal structure of the samples [PFLS program, H. Toraya and F. Marumo, Rep. Res. Lab. Eng. Mater. Tokyo Inst. Technol. 5 (1980) 55] using the starting structure model after T. Ban and K. Okada for pure mullite [J. Am. Ceram. Soc. 75 (1992) 227], with Cr^3+ ions incorporating into AlO_6 octaedra. Refinement converged with Rwp= 0.082, 0.091, 0.098 and 0.118 for the samples containing 2, 4, 6 and 8 mol% Cr_2O_3, respectively. It was noticed that for samples containing more than 2mo% Cr_2O_3 the calculated intensities of lines 120 and 210 were larger than the experimental intensities, and that the additional lines appeared in the diffraction patterns with d=4.06, 2.85, 2.09 and 1.749 &Aring ; ; . EPR measurements of Cr-doped mullite samples indicated that Cr^3+ ions substitute for Al^3+ in the AlO_6 octahedra of mullite structure, and also incorporate in octahedrally coordinated interstices in the structural channels along the c-axis. The structure refinement using such model was also performed.

Mullite; Chromium doping; X-ray diffraction; EPR; Rietveld refinement

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