Dynamics of the Cation Mixing of MgAl ₂ O ₄ and ZnAl ₂ O ₄ Spinel

The kinetics of the site exchange reaction of Al 3+ cations between tetrahedral and octahedral sites in MgAl 2 O 4 and ZnAl 2 O 4 spinel in natural and synthetic powdered samples was determined by 27 Al MAS‐NMR spectroscopy with a high‐speed magic‐angle‐spinning probe using samples heat‐treated for different time periods. Three site fittings to the 27 Al 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 ZnAl 2 O 4 spinel, the observed degree of cation disorder and its temperature variation are quite small, which is consistent with previous thermodynamic measurements. For the MgAl 2 O 4 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 MgAl 2 O 4 spinel.