Spin-State Crossover of Iron in Lower-Mantle Minerals: Results of DFT+U Investigations

Abstract

The lower mantle is the largest layer of the Earth. Pressures and temperatures there vary from 23 GPa to 135 GPa and ~1,900 K to 4,000 K. Aluminous perovskite, Al-Mg1− x Fe x SiO3, and ferropericlase, Mg1− x Fe x O, are the most important phases of the Earth’s lower mantle, comprising ~62 vol% and ~35 vol% of this region, respectively. The remaining consists of CaSiO3-perovskite, according to the pyrolitic compositional model (Ringwood 1982). Silicate perovskite transforms into another polymorph, post-perovskite, at conditions expected to occur near the D″ discontinuity in the deep slower mantle, i.e., 2,500 K and 125 GPa (Murakami et al. 2004; Oganov and Ono 2004; Tsuchiya et al. 2004; Wentzcovitch et al. 2006). The spin-state crossover (also referred to as “spin pairing transition” or “spin(-state) transition”) of iron in ferropericlase under pressure was observed in 2003 by X-ray emission spectroscopy (XES) (Badro et al. 2003). In the following year a similar phenomenon was also identified in iron-bearing perovskite (Mg1− x Fe x SiO3) by the same technique (Badro et al. 2004). This phenomenon had been predicted for decades (Fyfe 1960; Gaffney and Anderson 1973; Ohnishi 1978; Sherman 1988, 1991; Sherman and Jansen 1995; Cohen et al. 1997) but the pressure conditions were challenging for this type of experiment and it took several decades to observe it. The implications of this phenomenon for the properties of this region, or of the entire planet, are yet to be understood. Today, theoretical studies are making decisive contributions to this problem. This article reviews the main theoretical results on the spin-state crossover in the lower mantle phases. Spin changes in strongly correlated oxides and silicates under pressure is the type of problem that …