Abstract:
The elastic constants of natural MgAl[inf 2]O[inf 4] spinel have been measured using resonant ultrasound spectroscopy at high temperature. Below 1000 K, the ultrasonic resonant frequencies of an ordered natural spinel change significantly after heat treatment. The Raman spectra for ordered natural and disordered spinels also differ below 1000 K but are similar at higher temperatures and after cooling to ambient temperature. These changes in ultrasonic resonance and Raman spectra of spinel can be associated with cation disordering at high temperature that may be quenched by cooling. The estimated inversion parameters from the relative intensities of the two A1g Raman modes are in very good agreement with estimates from other measurements. It is found that C[inf 11] and C[inf 12] decrease by 4% and 8%, respectively, with 20% inversion in spinel; C[inf 44] is less sensitive to cation disorder. 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.