Ab initioRaman spectrum of the normal and disorderedMgAl2O4spinel

Abstract

Ab initio calculations, based on density functional theory, of the vibrational Raman spectra of both normal and cationic disordered $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ spinel are reported. The vibrational properties of the disordered system are obtained in two steps. First, the interatomic force constants of small defective supercells are computed in a fully ab initio way. Later, these force constants are used to build the dynamical matrices of larger disordered systems. The calculated Raman spectrum presents the mode near $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ which is due to the presence of cationic disorder and which is commonly attributed to a breathing mode (BM) of $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedra. Contrary to previous interpretations, the $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode is due to phonons which are silent in the perfect crystal and become Raman active in the disordered structure because of the coupling with the BM of the $\mathrm{Mg}{\mathrm{O}}_{4}$ tetrahedra.