Performing a state of the art study in geo- materials poses several challenges to both the human and the computer system:
Hydrous phases are among the most important Earth components. They have technological and societal utility and are important for a broad suite of Earth processes, including the origin of life. From both thermodynamic and structural perspectives, however, they represent some of the most complex naturally occurring materials: their bonding often includes a combination of strong covalent, weak ionic, van der Waals, and hydrogen bonding, all within large unit cells. Most are solid solutions, and many are prone to variations in layer packing. Many prior studies of these materials have emphasized experimental measurements and analytic modeling of their thermodynamics. Such thermodynamic studies represent a fundamental tool for understanding present and past natural processes, including those that shaped—and continue to shape—the structure and evolution of our planet. Nevertheless, many properties of these materials and solid solutions are difficult to measure experimentally or model analytically. To make significant new progress and attain a deep and predictive understanding of these materials requires a more atomistic and theoretical approach.
Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of minerals that altogether constitute ∼ 95 vol% of the Earth’s lower mantle: ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate (MgSiO3) perovskite.
Using density functional theory+Hubbard U (DFT+U) calculations, we investigate how aluminum affects the spin crossover of iron in MgSiO3 perovskite (Pv) and post-perovskite (Ppv), the major mineral phases in the Earth’s lower mantle. We find that the presence of aluminum does not change the response of iron spin state to pressure: only ferric iron (Fe3+) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe3+ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe3+ in these mineral phases. The most stable atomic configuration has all Al3+ in the B-site and all Fe3+ in the A-site (thus in the HS state). Metastable configurations with LS Fe3+ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe33+ at high pressures in Al-bearing Pv require diffusion of iron from the A-site to the B-site and should be sensitive to the annealing temperature and schedule. In the Earth’s lower mantle, the elastic anomalies accompanying the B-site HS-LS crossover exhibited in Al-free Pv are likely to be considerably reduced, according to the B-site Fe3+ population.
 N. Tosi, D. A. Yuen, N. de Koker, and R. M. Wentzcovitch, Mantle dynamics with pressure- and temperature-dependent thermal expansivity and conductivity, Phys. Earth & Planet. Int. 217, 48–58 (2013). DOI:10.1016/j.pepi.2013.02.004
 H. Hsu, Y. Yu, and R. M. Wentzcovitch, Spin crossover in iron in aluminous MgSiO3 persovskite and post-perovskite, Earth & Planet. Sc. Lett. 359-360, 34-39 (2012). DOI: 10.1016/j.epsl.2012.09.029