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

The positional disorder of Mg²⁺ and Al³⁺ cations between the tetrahedral and octahedral sites in natural MgAl₂O₄ spinel has been investigated by ²⁷Al MAS NMR at room temperature and in-situ high-temperature ²⁷Al NMR spectroscopy up to 1600 °C. The inversion parameter describing the disorder, x, where x stands for the positional disorder between Mg and Al cations in Mg_1–xAl_x(Mg_xAl_2–x)O₄, increased with temperature. Below 1100 °C the inversion parameter, x, can be determined from MAS NMR measurements of quenched samples at room temperature. Above 1100 °C, x was estimated from the peak position in the high-temperature ²⁷Al NMR spectra up to 1600 °C. The observed dependence of x with temperature was fitted using the model of O’Neill and Navrotsky (1983). The coefficients of the model obtained are α = 35 ±5 kJ and β = –32 ±5 kJ, which are approximately equal in magnitude and opposite in sign. The x values observed in the present investigation are in agreement with the model. However the introduction of an additional entropy term, ΔS_D, improved the fitting. ΔS_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 °C a narrow peak appeared at about 60 ppm. This peak became narrower with increasing temperature up to 1600 °C. This behavior might suggest that a rapid exchange process among the fourfold-coordinated Al sites occurs in this temperature range.