Title: Spin crossover in the Earth: A perspective from computational materials physics

Speaker: Dr. Han Hsu (Department of Physics, National Central University, Taiwan)

Abstract:
Iron, the most abundant transition metal in the Earth, plays an important role in a wide range of chemical and physical systems, including coordination complexes and lower-mantle minerals. Due to its incomplete 3d shell, iron in these systems can adopt multiple oxidation (Fe2+ and Fe3+) and spin states, exhibiting different characteristics. In particular, the total electron spin (S) of iron can vary with many factors, including temperature, pressure, and chemical environments, and thus change materials properties accordingly. This phenomenon, known as spin crossover, was not well recognized in geoscience until the early 2000s, when spin crossover in ferropericlase (Mg,Fe)O was observed in experiments. Ever since, spin crossover of iron-bearing minerals has attracted tremendous attention, as it directly affects the structural, electronic, optical, elastic, and thermodynamic properties of the host mineral, and iron diffusion and partitioning. So far, spin crossover in many lower-mantle minerals has been reported, including bridgmanite, which is the major mineral phase in the Earth, (Mg,Fe)CO3 ferromagnesite, which is believed to be the major carbon carrier in the mantle, and the new hexagonal aluminous (NAL) phase, which is considered as the major aluminum host in mid-ocean ridge basalt (MORB) in lower-mantle conditions. Geophysical and geochemical effects of spin crossover are thus anticipated. In this talk, I will discuss how first-principles calculations have successfully elucidated spin crossover in these complex materials [1-5], and how first-principles calculations can be applied to other spin-crossover minerals of geophysical/geochemical importance as well.