Intermediate spin state and the B1-B2 transition in ferropericlase

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 similar to 3TPa with iron concentrations (x(Fe)) varying from 0.03 to 0.12. Calculations were performed using LDA + U-sc and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (U-sc) 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 x(Fe) > 0.12, this approach is no longer valid.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 x(Fe).