Iron spin transitions in high-pressure magnetite

The spin state of iron (Fe) in materials deep within Earth’s mantle (primarily ferropericlase (Mg,Fe)O and (Mg,Fe)(Si,Fe,Al)O3 perovskite) has been of recent interest. The Fe high- to low-spin transition in ferropericlase is now well supported by both experiment and theory. However, the spin changes in perovskite are not as well understood, in part because perovskite contains more variables than ferropericlase: different valences (Fe3+ and Fe2+), substitutions (such as Al), two possible Fe sites, and varying Fe content. Experimental evidence using X-ray emission spectroscopy suggests the presence of intermediate-spin Fe in perovskite, but ab initio studies have not found intermediate-spin Fe to be stable with respect to high- or low-spin. In order to better understand these inconsistencies between experimental and computational results, we use ab initio methods to calculate the stable spin state in Fe3O4 magnetite, which also has X-ray emission spectroscopy evidence for a high- to intermediate-spin transition between 12-16 GPa [1]. Magnetite has two Fe sites, a tetrahedral site occupied by Fe3+ and an octahedral site, which has an average valence of 2.5+. At ambient pressure there is a charge ordering transition at 120K, where the low-temperature charge-ordered phase has valences [Fe3+]TET[Fe2+Fe3+]OCTO4 and the high-temperature charge-averaged phase has valences [Fe3+]TET[Fe2.5+Fe2.5+]OCTO4. We use ab initio methods to calculate the spin transition pressure for high- to intermediate-spin, high- to low-spin, and intermediate- to low-spin transitions in magnetite as a function of site (tetrahedral and octahedral), valence (2+, 2.5+, 3+), and charge state (charge averaged and charge ordered). Our results find that the charge-averaged symmetry (Fd-3m) has a spin collapse from high to low-spin on the octahedral site at 50 GPa and a gradual loss of moment on the tetrahedral site between 70 to 150 GPa. Fe in magnetite with charge ordering on the octahedral site (Imma and Pmca symmetries) is stable as high-spin up to 70 GPa (highest pressure of our study). Our calculations do not find intermediate-spin Fe to be stable with respect to high- or low-spin in magnetite. There now exist two systems, magnetite and perovskite, where experimental X-ray emission spectroscopy suggests Fe is in the intermediate-spin state but computational results do not find intermediate-spin Fe to be the most stable spin state. The results on spin transitions in magnetite are important for understanding Fe’s spin behavior in the lower mantle and better characterizing high-pressure regimes like subduction zones or other Fe-rich regions. The results could impact magnetotelluric measurements and geodynamic models. [1] Ding Y., et al Novel Pressure-Induced Magnetic Transition in Magnetite (Fe3O4). PRL. 100, 045508 (2008)