Temperature dependence of cation disorder in MgAl₂O₄ spinel using ²⁷Al and ¹⁷O magic-angle spinning NMR

The disorder of Mg²⁺ and Al³⁺ cations between the tetrahedral and octahedral sites in MgAl₂O₄ spinel has been determined by ²⁷Al magic-angle spinning (MAS) NMR spectroscopy for synthetic powders quenched from temperatures between 700 and 1400°C. Disorder, measured by the fraction of Al in the tetrahedral site and described by the inversion parameter, x, increased with temperature from x = 0.22 (± 0.03) at 700°C to x = 0.29 (± 0.03) at 1000°C. This temperature-dependent increase in disorder is significantly lower than that reported in a previous ²⁷Al MAS NMR study by Wood et al. (1986) as a result of a more appropriate choice of instrument parameters for acquisition of the ²⁷Al MAS NMR spectra.

Cation disorder in MgAl₂O₄ was also investigated by ¹⁷O MAS NMR on ¹⁷O-enriched samples quenched from 700 and 1400°C. The disorder increased from x = 0.16 (± 0.02) at 700°C to x = 0.24 (± 0.03) at 1400°C. This temperature-dependent increase in disorder was similar in magnitude but systematically lower than that obtained by ²⁷Al MAS NMR.