Effects of Cation Disordering in a Natural MgAl2O4 Spinel Observed by Rectangular Parallelepiped Ultrasonic Resonance and Raman Measurements

At moderate temperatures, the elastic properties of natural MgAl₂O₄ spinel differ in several significant ways from properties of synthetic spinels. Below 1000 K, the ultrasonic resonant frequencies of an ordered natural spinel change significantly after heat treatment; at higher temperatures, both types of spinels have similar resonant responses. The temperature derivatives of the elastic constants of an ordered spinel also differ from those of disordered spinels at moderate temperatures; again, at higher temperatures, both types of spinels have similar behaviors. The Raman spectra also differ below 1000 K for ordered natural and disordered spinels and are similar at higher temperatures and after cooling to ambient temperature. We associate these changes in ultrasonic resonance and Raman spectra of spinel with cation disordering at high temperature which may be quenched by cooling. We deduce estimates of the inversion parameter from the relative intensities of the two A_lg Raman modes in very good agreement with estimates made from other measurements. We find that C₁₁ and C₁₂ decrease by 4 and 8%, respectively, with 20% inversion in spinel; C₄₄ is less sensitive to cation order. These results imply that previous measurements of the adiabatic elastic constants of spinels at ambient conditions have been affected by the state of cation disorder of the specimen.