First-principles Study of Intermediate-spin Ferrous Iron in the Earth’s Lower Mantle

English Abstract:

Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of the Earth’s lower-mantle minerals, including ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate (MgSiO₃) perovskite, altogether constituting ~95 vol% of the lower mantle, and ferromagnesite [(Mg,Fe)CO₃], a potential carbon carrier in the lower mantle. Despite great strides made in the past decade, the existence of an intermediate-spin (IS) state in ferrous iron (Fe²⁺) (with total electron spin S = 1) and its possible role in the pressure-induced spin crossover in these lower-mantle minerals still remain controversial. Using density functional theory + self-consistent Hubbard U (DFT + U_sc) calculations, we investigate all possible types of IS states of Fe²⁺ in these minerals, and this talk will be particularly focused on (Mg,Fe)SiO₃ perovskite (also known as bridgmanite). Among the possible IS states in these minerals, the most probable IS state has an electronic configuration that significantly reduces the electron overlap and the iron nuclear quadrupole splitting (QS). These most probable IS states, however, are still energetically disfavored, and their QSs are inconsistent with Mössbauer spectra. We therefore conclude that IS Fe²⁺ is highly unlikely in the Earth’s lower mantle.