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 FeS=5/23+ ions alternating with layers of O-CuS=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 (CuS=01+ FeS=5/23+) → (CuS=1/22+ FeS=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 (CuS=1/22+ FeS=22+) ≈ 2.1 TN (CuS=01+ FeS=5/23+). This transition is reversible.
- Fe Mössbauer spectroscopy
- Pressure-induced REDOX
- Structural/electronic transitions
- X-ray diffraction and spectroscopy