Atomistic study of vibrational properties of γ-Al2O3

A study of the vibrational density of states (DOS) of c-Al2O3 is presented. Four structural models from the recent literature are considered: vacant spinel model and three nonspinel models. The vacant spinel and one of the nonspinel models have unit cells with 40 atoms, while the other two models have 160 atoms. The inter- atomic interactions are computed using classical force fields that include Coulomb and van der Waals attractive interactions, short range repulsive interactions, as well as three-body terms. The oxygen polarizability is included via a core-shell potential. The DOS is compared with ab initio calculations recently published for the vacant spinel model. The classical and ab initio DOS show some differences for frequencies higher than 200 cm -1 , the ab initio having more peaks and having a frequency cutoff 100 cm -1 lower than the classical DOS. The DOS of all models present some small differences. While the 160-atoms nonspinel models present a rather structureless DOS, 40-atoms models present peaks and dips relative to the 160-atoms models. The elastic constants of polycrystalline c-Al2O3 are also estimated using several force fields. In general, the classical force field predict higher elastic moduli than the ab initio method. The infrared spectra of the four models are calculated.