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Upper limit of cobalt incorporation in mullite

Mullite, 3Al_2O_3.2SiO_2, is a material with important role in technology of classical and advanced ceramics. It has orthorhombic symmetry, space group Pbam. 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]. Highest degrees of incorporation were observed for V^3+, Cr^3+ and Fe^3+ followed by Ti^4+, while only a very low amounts of Mn^2+, Fe^2+ and Co^2+ can enter the mullite structure. The charge and size of metal ion determine its accessible site and concentration in mullite [N. N. Greenwood and A. Aernshaw: Chemistry of Elements, Pergamon Press, Oxford, 1984]. Our interest was focused on determination of upper limit of Co^2+ incorporation in mullite. The sample of pure mullite and ones doped with 1, 2, and 3 at% Co were derived from diphasic precursors and sintered at 1600 ^oC for 2 h. They were examined by X-ray powder diffraction at room temperature. Co-doped samples contained mullite phase and small amounts of alpha-Al_2O_3 and CoAl_2O_4. Unit-cell parameters of mullite phase were determined using Si as internal standard, and refined by the whole-powder-pattern fitting method [H. Toraya, J. Appl. Cryst. 19 (1986) 440]. They increased just slightly with increase of cobalt content in the samples. Quantitative phase analysis showed that the samples with 1, 2, and 3 at% Co contained 0.8, 2.5 and 5.1 wt% CoAl_2O_4. It follows that ~0.6 at% Co was incorporated in mullite in prepared Co-doped samples. Upper limit of Co2+ incorporation in mullite was also estimated on the basis of dependence of intensity ratio for selected diffraction lines, I(311, CoAl_2O_4)/I(111, mullite phase), on cobalt content in the samples, and result was the same.

mullite; cobalt doping; X-ray diffraction; whole-powder-pattern fitting

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