Effects of cation disordering in magnesium aluminate spinel on the rectangular parallelepiped resonance and Raman measurements of vibrational spectra.

摘要

The effects of cation disordering of a natural MgAl{dollar}sb2{dollar}O{dollar}sb4{dollar} spinel on acoustic and optic vibration were measured for the first time using the rectangular parallelepiped resonance method and Raman measurements. In the resonant frequency measurements of a natural spinel at high temperatures over the temperature range 298 to 1068 K, a discontinuous increase in the measured acoustic resonant vibrations of the lower harmonic modes and a discontinuous decrease in the measured acoustic resonant vibrations of the higher harmonic modes were observed at around 1000 K. Similar differences among the resonant frequencies were also observed at ambient conditions between a less disordered spinel and the highly disordered states of a natural spinel.; In the Raman measurements of the same natural spinel over the temperature range 298 to 1424 K, plots of the Raman vibrational frequencies of the external and internal vibrational modes versus temperature change slopes at around 1000 K.; These two measurements clearly indicate that a major change occurred at 1000 K, which I label as a transition temperature. I interpret the change that occurred around 1000 K as the onset of cation disordering in the natural spinel.; The interpretation is consistent with the following observations: (1) an abrupt decrease in oxygen positional parameter in an x-ray single crystal structure analysis of a synthetic spinel between 873 and 973 K; (2) a discontinuous decrease of linear thermal expansion coefficients in a synthetic spinel at 933 K by dilatometry, and (3) a discontinuous decrease of the unit cell parameter of a natural spinel at around 1073 K by x-ray diffraction.; The adiabatic elastic moduli found here for the natural spinel are different from results which have been previously reported by others, however, the moduli of a disordered natural spinel are similar to those previously reported for synthetic spinels. These observations demonstrate that cation disordering of a spinel clearly affects the elastic moduli. Thus, with the accurate resonant frequency measurement using an automatic scan, it is possible to demonstrate the effects due to cation disordering in a spinel.; I made consistent mode assignments of the Raman active vibrational modes of a spinel, based on the assumption of weak coupling between Mg{dollar}sp{lcub}2+{rcub}{dollar} and Al{dollar}sp{lcub}3+{rcub}{dollar} ions in tetrahedral sites. I interpret the broadening, the relative intensity change, and the extra Raman modes as originating mostly from cation disordering and possibly from electronic Raman scattering from impurity ions in spinels. This interpretation is consistent with systematics and observations of Mg-O and Al-O vibrations. I also suggest that Raman intensities can be used to estimate the amount of cation disorder, which is important for a model calculation of thermodynamic properties. Thus I find consistency between the RPR and the Raman measured properties.