Dynamics of the Cation Mixing of MgAl2O4 and ZnAl2O4 Spinel

Abstract

The kinetics of the site exchange reaction of Al3+ cations between tetrahedral and octahedral sites in MgAl2O4 and ZnAl2O4 spinel in natural and synthetic powdered samples was determined by 27Al MAS‐NMR spectroscopy with a high‐speed magic‐angle‐spinning probe using samples heat‐treated for different time periods. Three site fittings to the 27Al NMR spectra, one tetrahedral site [4]Al and two octahedral sites ([6]Al(1) and [6]Al(2)), reasonably reproduced the observed spectra. Among these three sites, [6]Al(2) and [4]Al showed a broad distribution of the nuclear‐quadrupole coupling constant, which suggests a distorted coordination environment of the Al ions. In the ZnAl2O4 spinel, the observed degree of cation disorder and its temperature variation are quite small, which is consistent with previous thermodynamic measurements. For the MgAl2O4 spinel, the change in the concentration of [4]Al versus time was fitted well by a single exponential function. This suggests that the reaction [6]Al +[4]Mg →[4]Al +[6]Mg is a first‐order reaction. An almost linear relationship between the inverse temperature and the logarithm of the reaction rate yielded an activation energy of 260 ± 30 kJ/mol. The reverse rate for the reaction [4]Al +[6]Mg →[6]Al +[4]Mg was also determined. The reverse rate was also shown to be first order and the reaction rates were 2 orders greater than those of the forward reaction. The ratio of the rate constants for the forward and reverse reactions gives a reasonable nonconfigurational free energy change for the cation exchange of MgAl2O4 spinel.