B3LYP Periodic Study of the Physicochemical Properties of the Nonpolar (010) Mg-Pure and Fe-Containing Olivine Surfaces

B3LYP periodic simulations have been carried out to study some physicochemical properties of the bulk structures and the corresponding nonpolar (010) surfaces of Mg-pure and Fe-containing olivine systems; i.e., Mg2SiO4 (Fo) and Mg1.5Fe0.5SiO4 (Fo(75)). A detailed structural analysis of the (010) Fo and Fo(75) surface models shows the presence of coordinatively unsaturated metal cations (Mg2+ and Fe2+, respectively) with shorter metal-O distances compared to the bulk ones. Energetic analysis devoted to the Fe2+ electronic spin configuration and to the ion position in the surfaces reveals that Fe2+ in its quintet state and placed at the outermost positions of the slab constitutes the most stable Fe-containing surface, which is related to the higher stability of high spin states when Fe2+ is coordinatively unsaturated. Comparison of the simulated IR and the corresponding reflectance spectra indicates that Fe2+ substitution induces an overall bathochromic shift of the spectra due to the larger mass of Fe compared to Mg cation. In contrast, the IR spectra of the surfaces are shifted to upper values and exhibit more bands compared to the corresponding bulk systems due to the shorter metal-O distances given in the coordinatively unsaturated metals and to symmetry reduction which brings nonequivalent motions between the outermost and the internal modes, respectively.