Spin state of iron in Fe3O4magnetite andh-Fe3O4

Abstract

The high-pressure behavior of magnetite has been widely debated in the literature. Experimental measurements have found conflicting high-pressure transitions: a charge reordering in magnetite from inverse-spinel to normal-spinel [Pasternak et al., J. Phys. Chem. Solids 65, 1531 (2004); Rozenberg et al., Phys. Rev. B 75, 020102 (2007)], iron high-spin to intermediate-spin transition in magnetite [Ding et al., Phys. Rev. Lett. 100, 045508 (2008)], electron delocalization in magnetite [Baudelet et al., Phys. Rev. B 82, 140412 (2010); Glazyrin et al., Am. Mineral. 97, 128 (2012)], and a structural phase transition from magnetite to $h$-Fe${}_{3}$O${}_{4}$ [Dubrovinsky et al., J. Phys.: Condens. Matter 15, 7697 (2003); Fei et al., Am. Mineral. 84, 203 (1999); Haavik et al., Am. Mineral. 85, 514 (2000)]. We present ab initio calculations of iron's spin state in magnetite and $h$-Fe${}_{3}$O${}_{4}$, which help resolve the high-pressure debate. The results of the calculations find that iron remains high spin in both magnetite and $h$-Fe${}_{3}$O${}_{4}$; intermediate-spin iron is not stable. In addition, magnetite remains inverse-spinel but undergoes a phase transition to $h$-Fe${}_{3}$O${}_{4}$ near 10 GPa. Magnetite has a complex magnetic ordering, multiple valence states (Fe${}^{2+}$ and Fe${}^{3+}$), charge ordering, and different local Fe site environments, all of which were accounted for in the calculations. The lack of intermediate-spin iron in magnetite helps resolve the spin state of iron in perovskite, the major mineral in the lower mantle. In both magnetite and perovskite, x-ray emission spectroscopy (XES) measurements in the literature show a drop in satellite peak intensity by approximately half, which is interpreted as intermediate-spin iron. In both minerals, calculations give no indication of intermediate-spin iron and predict high-spin iron to be stable for defect-free crystals. The results question the interpretation of a nonzero drop in XES satellite peak intensities as intermediate-spin iron.