Spectroscopic study of ordering in non-stoichiometric magnesium aluminate spinel

FTIR and RAMAN spectroscopic methods were used to study the ordering of non-stoichiometric nano-magnesium aluminate spinels (MgOnAl₂O₃, 0.4 < n < 12) synthesized using a combustion synthesis method. It was established that the degree of structural disorder (i.e., the inversion parameter, i) can be quantified using the intensities of the γ₁ and γ₅ IR modes or 670 and 723 cm^-1 Raman shifts. The results indicated that the as-synthesized materials were heavily disordered and obey earlier conclusions that the defect chemistry of non-stoichiometric spinels is dominated by clusters formed from anti-site defects. Analysis of the temperature dependency of cation distribution in the Mg- and Alrich samples showed that the spinel phase moved toward equilibrium upon increases in temperature. Where decomposition occurred, the disordered level decreased at temperatures up to 1000 °C. Above this temperature, the order level dropped far below the expected equilibrium value and the γ₃ mode (a mode that is characterized for ordered structures, such as a natural spinel) that appears. These findings, together with Raman results of partly decomposed Al-rich samples, support the hypothesis that a MgAl₂O₄-γ-Al₈/₃□_1/3O₄ solid solution comprises of series of complex micro-phases with considerable short-range order.