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Cobalt incorporation in mullite

Mullite samples doped with cobalt were derived from diphasic gels with constant atomic ratio (Al+Co)/Si=3:1, where 0, 1, 2 and 3 at% of aluminum was replaced by cobalt. X-ray powder diffraction showed that the samples contained mullite phase and some amount of alpha-Al_2O_3 (for pure and doped samples) and CoAl_2O_4 (for doped samples). Co doping caused the increase in unit-cell parameters of the mullite phase. The Rietveld method was performed for quantitative phase analysis of the samples and for structure refinement of the mullite phase in the samples. It was found that a small amount of Co^(2+), 0.7 at% (0.55 wt%), substituted for Al^(3+) in the AlO_6 octahedra of the mullite structure. Simultaneously, the same amount of tetrahedral Al^(3+) ions was likely substituted by Si^(4+) in the (Al, Si)O_4 tetrahedra for the purpose of charge compensation. The rest of cobalt contained in the samples reacted with alumina forming CoAl_2O_4. Chemical formula for Co-doped mullite is proposed in the form Co_yAl_(4+2x-2y)Si_(2-2x+y)O_(10-x).

Co-doped mullite; ceramics; X-ray diffraction; Rietveld method

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