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 Fe²⁺ 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 Fe²⁺ oxidation takes place, starting at ~8 GPa reaching 70 % Fe³⁺-abundance at 40 GPa. Based on XRD and R(P,T) data it is unequivocally concluded that this non-reversible process, Fe²⁺ → Fe³⁺ + e^-, results in Fe²⁺ converting into Fe³⁺ with no structural transition. HP studies of CuFeO ₂ using XRD, MS and K-edge XAFS spectroscopy, reveal a sequence of structural/electronic-magnetic pressure-induced transitions. The low pressure R3̄m structure (0-18 GPa) is composed of sheets of Fe_S=5/2³⁺ ions alternating with layers of O-Cu_S=0¹⁺-O dumbbells oriented along the c axis with d(c/a)/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 (Cu_S=0¹⁺ Fe_S=5/2³⁺) → (Cu_S=1/2²⁺ Fe_S=2²⁺) inter-ionic valence exchange in about 1/3 of the C2/c-CuFeO₂. As a result: (i) the Cu²⁺-O becomes 4-fold coordinated and is in a new crystallographic structure with space group P3̄m, and (ii) the Néel temperature increases twofold (Cu_S=1/2²⁺ Fe_S=2²⁺) ≈ 2.1 T_N (Cu_S=0¹⁺ Fe_S=5/2³⁺). This transition is reversible.