Pressure stimulated charge crossover in iron oxides and hydroxide

High pressure (HP) studies carried out with ⁵⁷Fe Mössbauer Spectroscopy led to the serendipitous discovery of self oxidation of Fe^2 + in Fe(OH)₂, and self reduction in the CuFeO₃ delafossite. Mössbauer spectroscopy, x-ray diffraction (XRD), and electrical resistance (R(P,T)) studies in Fe(OH)₂ to 40 GPa revealed an unforeseen process by which a gradual Fe^2 + oxidation takes place, starting at ~8 GPa reaching 70 % Fe^3 +-abundance at 40 GPa. Based on XRD and R(P,T) data it is unequivocally concluded that this non-reversible process, Fe^2 + → Fe^3 + + e⁻, results in Fe^2 + converting into Fe^3 + with no structural transition. HP studies of CuFeO₂ using XRD, MS and K-edge XAFS spectroscopy, reveal a sequence of structural/electronic-magnetic pressure-induced transitions. The low pressure $R\overline{3}m$ structure (0–18 GPa) is composed of sheets of ${Fe}_{S=5/2}^{3+}$ ions alternating with layers of $O-{Cu}_{S=0}^{1+}-O$ dumbbells oriented along the c axis with $d\left(c/a\right)/dP>0$. At 18 GPa a transition takes a place to an isotropic C2/c structure. This transition corroborates with the onset of the long-range antiferromagnetic order. Starting at ~23 GPa and 84 % of the initial volume, Cu-Fe bands overlap and at 27 GPa leads to $\left({Cu}_{S=0}^{1+}{Fe}_{S=5/2}^{3+}\right)\to \left({Cu}_{S=1/2}^{2+}{Fe}_{S=2}^{2+}\right)$ inter-ionic valence exchange in about 1/3 of the C2/c-CuFeO₂. As a result: (i) the Cu^2 +-O becomes 4-fold coordinated and is in a new crystallographic structure with space group $P\overline{3}m$, and (ii) the Néel temperature increases twofold ($T_N\left({Cu}_{S=1/2}^{2+}{Fe}_{S=2}^{2+}\right)\approx 2.1T_N\left({Cu}_{S=0}^{1+}{Fe}_{S=5/2}^{3+}\right))$. This transition is reversible.