Characterization of order-disorder transition in MgAl2O4:Cr3+ spinel using photoluminescence

Photoluminescence spectroscopy of Cr3+ shows considerable potential for probing local cation arrangement in MgAl2O4 spinel while undergoing order-disorder phase transition. The PL spectrum consists of R peaks (caused by ordered PL centers) and N peaks (caused by disordered PL centers). The peak dominating in the PL spectra dramatically switched from R peaks to N peaks around 850 degrees C and showed rather few changes above 900 degrees C. We defined the percent ratio of the integrated area of N peaks to the total integrated area under all peaks as a parameter to monitor the degree of disordering. This parameter can quantitatively indicate the variation of neighboring atom arrangement around Cr3+ during the order-disorder transition. A thermodynamic model based on this spectroscopic parameter can be built, and we can model the transition with an activation energy of 254 +/- 49 kJ/mol. This study also attempted to determine the ability to restore the ordering of heated spinels by annealing them at 650 degrees C up to 8 days. Our results clearly indicated that the spectra cannot be fully restored back to the ordered state within our experimental time span, thus makes PL spectroscopy a reliable method to distinguish natural spinel from heated ones.