TEMPERATURE AND COMPOSITION DEPENDENCE OF THE CATION DISTRIBUTION IN SYNTHETIC ZNFEYAL2-YO4 (O-LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-1) SPINELS

Members of the spinel solid solution series ZnFeyAl2-yO4 (y = 0, 0.2, 0.4, 0.6, 1.0, 1.5, and 2) were synthesized by direct solid state reaction of the simple metal oxides in air at 1400 K. Two aliquots of the single-phase spinels obtained for each composition were annealed at 1350 and 1000 K for 48 hr and then quenched in water. The structural study of the quenched samples was performed by powder XRD, using the Rietveld method of structure refinement. The inversion degrees estimated for ZnFe2O4 up to 1350 K are in good agreement with those published by O'Neill (Eur. J. Mineral. 4, 571 (1992)) in the range 773-1223 K. The nonlinear enthalpy of disordering model of O'Neill and Navrotsky (Am. Mineral. 68, 181 (1983); Am. Mineral. 69, 733 (1984)), recently applied to several studies of simple spinel oxides, could successfully describe the cation arrangement in spinels of the binary solid solution series prepared in this work, with parameters alpha(Zn-Al) = +82 kJ mole-1, alpha(ZN-Fe3) = +46 kJ mole-1, beta = -20 kJ mole-1, and sigma = 0. Fe-57 Mossbauer spectra collected at 300 and 80 K consisted of only one doublet. Although the contribution of tetrahedral Fe3+ could not be resolved, the variation of the estimated average hyperfine parameters with composition and quenching temperature could be correlated with the cation arrangement deduced from XRD data. (C) 1994 Academic Press, Inc.