Intermediate spin state and the B1-B2 transition in ferropericlase

Abstract

Ferropericlase (fp), (Mg1-xFex)O, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of the mantles of super-Earths. Here, we present an ab initio investigation of the structure and magnetic ground state of fp up to ∼3TPa with iron concentrations (xFe) varying from 0.03 to 0.12. Calculations were performed using LDA + Usc and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (Usc) is required. Similar to the end members FeO and MgO, fp also undergoes a B1-to-B2 phase transition that should be essential for modeling the structure and dynamics of the mantles of super-Earths. This structural transition involves a simultaneous change in magnetic state from a low-spin (LS) B1 phase with iron total spin S=0 to an intermediate-spin (IS) B2 phase with S=1. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron B2-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low-concentration fp using a quasi-ideal solid solution model. For xFe>0.12, this approach is no longer valid. At ultrahigh pressures, there is an IS-to-LS spin state change in Fe in the B2 phase, but the transition pressure depends sensitively on thermal electronic excitations and on xFe.