Ab initio Raman spectrum of the normal and disordered Mg Al$_2$ O$_4$ spinel

Michele Lazzeri (1, 2) , Pascal Thibaudeau (3)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
3 CEA Le Ripault - CEA Le Ripault

Ab initio calculations, based on density functional theory, of the vibrational Raman spectra of both normal and cationic disordered MgAl$_2$ 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 cm$^{-1}$ which is due to the presence of cationic disorder and which is commonly attributed to a breathing mode (BM) of Al O$_4$ tetrahedra. Contrary to previous interpretations, the 727 cm$^{-1}$ 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 Mg O$_4$ tetrahedra.

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Michele Lazzeri, Pascal Thibaudeau. Ab initio Raman spectrum of the normal and disordered Mg Al$_2$ O$_4$ spinel. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2006, 74, pp.140301(R). ⟨10.1103/PhysRevB.74.140301⟩. ⟨hal-00130125⟩