Spin state and structural stability of ferropericlase up to 3 Mbar

Ferropericlase (Fp), (Mg_1-xFe_x)O, the second most abundant mineral in the Earth mantle, is expected to be an essential component of the deep mantle of terrestrial exoplanets. Thus, an understanding of Fp across a wide range of pressures, temperatures, and iron concentrations is crucial for modeling these planets' internal structure and dynamics. Understanding pressure-induced electronic spin transitions in iron in Fp is challenging, given the strongly correlated nature of iron. Here, we present an LDA+U_sc study of Fp's structure and spin state from 200 GPa to 3 TPa and iron concentrations, x_Fe, varying from ~3% to ~12%. The vibrational stability of various spin states is addressed, and the quasiharmonic approximation (QHA) is used to compute the thermodynamic properties and stability field of B1 and B2 phases. Such properties will be useful for modeling the mantle of super-Earth-type planets with up to ~15 M_earth.