Resonant X-ray emission study of the lower-mantle ferropericlase at high pressures

Abstract

ab ST rac T Electronic states of iron in Earth’s mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mossbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe 2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25) O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe 2+ (a lower-energy 4 T1g state, a 4 T2g state, and a higher-energy 4 T1g state) and a single 2 Eg state for low-spin Fe 2+ , attributed to the (t2g) 0 (eg) 3 excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe 2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe 2+ site. The K pre-edge RXES spectra at the incident energy of