Cation mixing in natural MgAl ₂ O ₄ spinel; a high-temperature ²⁷ Al NMR study

Hokkaido University, Division of Chemistry, Sapporo, Japan

The positional disorder of Mg (super 2+) and AL (super 3+) cations between the tetrahedral and octahedral sites in natural MgAl ₂ O ₄ spinel has been investigated by ²⁷ Al MAS NMR at room temperature and in-situ high-temperature ²⁷ Al NMR spectroscopy up to 1600 degrees C. The inversion parameter describing the disorder, chi , where chi stands for the positional disorder between Mg and Al cations in Mg (sub 1-chi ) Al (sub chi ) (Mg (sub chi ) Al (sub 2-chi ) )O ₄ , increased with temperature. Below 1100 degrees C the inversion parameter, chi , can be determined from MAS NMR measurements of quenched samples at room temperature. Above 1100 degrees C, chi was estimated from the peak position in the high-temperature ²⁷ Al NMR spectra up to 1600 degrees C. The observed dependence of chi with temperature was fitted using the model of O'Neill and Navrotsky (1983). The coefficients of the model obtained are alpha = 35+ or -5 kJ and beta = -32+ or -5 kJ, which are approximately equal in magnitude and opposite in sign. The chi values observed in the present investigation are in agreement with the model. However the introduction of an additional entropy term, Delta S _D , improved the fitting. Delta S _D reduces the entropy of disorder relative to a random mixing model. This would reflect either a nonconfigurational entropy contribution or short-range Mg-Al order because of local charge balance. On the other hand, above 1400 degrees C a narrow peak appeared at about 60 ppm. This peak became narrower with increasing temperature up to 1600 degrees C. This behavior might suggest that a rapid exchange process among the fourfold-coordinated Al sites occurs in this temperature range.

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