Pressure-induced Fe⇆Cu cationic valence exchange and its structural consequences: High-pressure studies of delafossite CuFeO2

The present high-pressure studies of CuFeO2 to 30 GPa using x-ray diffraction, along with Fe-57 Moumlssbauer and Fe and Cu K-edge x-ray absorption spectroscopy methods, reveal a sequence of intricate structural/electronic-magnetic pressure-induced transitions. The low-pressure R3m structure (0-18 GPa) is composed of sheets of Fe-S=5/2(3+) ions alternating with layers of O-CuS=01+-O dumbbells, the latter oriented along the c axis. This structure is characterized by an unusual positive d(c/a)/dP. At 18 GPa a structural transition takes place to a more isotropic C2/c structure with the O-CuS=01+-O axis tilted 28 degrees from the c axis and with negative d(c/a)/dP. This transition corroborates with the onset of long-range antiferromagnetic order. Starting at similar to 23 GPa, with an initial volume reduction in similar to vertical bar Delta V/V-0 vertical bar=0.16, the Cu-Fe bands overlap and this leads to a (CuS=01+FeS=5/23+)->(CuS=1/22+FeS=22+) interionic valence exchange in about 1/3 of the C2/c-CuFeO2 at 27 GPa. As a result: (i) the Cu2+-O becomes fourfold coordinated and is in a new crystallographic structure with space group P3m, and (ii) the Neacuteel temperature increases above twofold [T-N(CuS=1/22+FeS=22+)approximate to 2.2T(N)(CuS=01+FeS=5/23+)]. This sequence of transitions is reversible with minimal hysteresis.