Determination of the high-pressure properties of fayalite from first-principles calculations

We have calculated the high-pressure properties of fayalite, including band structure, magnetism, equation-of-state and elastic properties using density functional theory within the generalized gradient approximation (GGA) and using the GGA + U method. We show for the first time that the addition of an on-site Hubbard repulsion term to the GGA leads to improved agreement between calculated and experimental values of structural and elastic properties, as well as electronic band gap, particularly at high pressure. High-pressure elastic instability, originating in the vanishing of the elastic constant c44 and previously predicted on the basis of extrapolated experimental data, is found with the GGA + U method. Experimental measurements of the elastic constants agree better with the GGA + U method, than with the GGA, which calls for a reassessment of the structural and elastic properties of iron-bearing minerals calculated using standard density functional theory, which have hitherto been used to interpret the structure and dynamics of the mantle. The improvement can be related to the better description of magnetic structure: without the Hubbard U the magnetic moments on the iron ions decrease with pressure, whereas when it is included they remain almost constant, at least, up to the highest pressure studied. This also leads to better predictions of equation-of-state. Our calculated partial density-of-states indicate that the lowest energy excitation across the electronic band gap lies within the d-orbital manifold, and we argue that this is not observed at ambient pressure because the signal in the optical spectrum is too weak, accounting for apparent discrepancies with reported experimental values. Upon compression the nature of the gap changes due to the broadening of the 3d manifold, and this likely renders the fundamental gap observable in optical spectroscopy at high pressure, accounting for the better agreement between theory and experiment above 20 GPa. We associate the changes in the character of the bands with a change in the crystal structure.