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

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 Mössbauer 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 Fe²⁺ in the lower-mantle ferropericlase [(Mg_0.75,Fe_0.25)O] at pressures up to 90 GPa. The pre-edge features from X-ray absorptionspectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe²⁺ (a lower-energy ⁴T_1g state, a ⁴T_2g state, and a higher-energy ⁴T_1g state) and a single ²E _g state for low-spin Fe²⁺, attributed to the (t _2g)⁰(e_g)³ excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe²⁺ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resultingfrom the centrosymmetric character of the Fe²⁺ site. The Kpre-edge RXES spectra at the incident energy of 7112 eV, which are similar to the Fe L-edge spectra, are also used successfully to quantitatively obtain consistent results on the spin transition of Fe²⁺ in ferropericlase under high pressures. Owing to the superior sensitivity of the RXES technique, the observed electronic states andtheir energy separations provide direct information on the local electronic structures and crystal-field splitting energies of the 3d electronic shells of Fe²⁺ in ferropericlase at relevant pressures of the Earth's lower mantle.