Electronic and optical properties of theMg2−xFexSiO4spinel: From band insulator to Mott …

The crystal properties and electronic structure of ${\text{Mg}}_{2\ensuremath{-}x}{\text{Fe}}_{x}{\text{SiO}}_{4}$ spinel have been studied using the density-functional theory including the local Hubbard interaction $U$ within the generalized gradient approximation. The lattice constant depends linearly on increasing iron concentration $x$ in agreement with the Vegard's law. The contributions of the $\text{Fe}({t}_{2g})$ and $\text{Fe}({e}_{g})$ orbitals to the electronic density of states have been determined and their crucial role in the band-gap formation has been elucidated. We explain the decrease in the insulating gap with increasing iron concentration on the basis of the Mott-Hubbard picture of correlated insulators. Thus, it was revealed that the electronic structure of ${\text{Mg}}_{2\ensuremath{-}x}{\text{Fe}}_{x}{\text{SiO}}_{4}$ evolves from the band insulator found in ${\text{Mg}}_{2}{\text{SiO}}_{4}$ to the Mott insulator with increasing iron concentration. We present also the absorption spectra derived from the complex dielectric tensor, which are in good qualitative agreement with the previously reported experimental data for a wide range of energies.