Structural, electronic, and magnetic properties of Fe2SiO4 fayalite: Comparison of LDA and GGA results

We present a first principle investigation of the structural, electronic and magnetic properties of Fe2SiO4 Fayalite, the iron-rich end member of the (Mg,Fe)(2)SiO4 olivine solid solution, naturally occurring in the Earth's upper mantle. Local spin-density approximation and spin-polarized generalized gradient approximation (sigma-GGA) results are compared; sigma-GGA appears to provide an overall better description of the structural properties. The ground-state spin configuration is investigated and the antiferromagnetic spin arrangement consistent with a superexchange mechanism through oxygen orbitals is found to be preferred. Electronic structure calculations using both exchange and correlation functionals predict a metallic ground state, contrary to experimental evidence that indicates a insulating, possibly Mott-Hubbard, behavior. In fact, by comparison of our DFT results with the RPA solution of a simple ad hoc Hubbard model, we were able to estimate the average short-range electron-electron repulsion parameter U. This quantity turns out to be larger than the relevant band width, and therefore, we support the Mott-Hubbard nature of the experimentally observed insulating behavior.