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 F 57 e Mössbauer and Fe and CuK -edge x-ray absorption spectroscopy methods, reveal a sequence of intricate structural/electronic-magnetic pressure-induced transitions. The low-pressure R 3̄ 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, 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-Cu S=0 1+ -O axis tilted 28° from the c axis and with negative d (c/a) /dP. This transition corroborates with the onset of long-range antiferromagnetic order. Starting at ∼23GPa, with an initial volume reduction in ∼ | ΔV/ V0 | =0.16, the Cu-Fe bands overlap and this leads to a (Cu S=0 1+ Fe S=5/2 3+) → (Cu S=1/2 2+ Fe S=2 2+) 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 P 3̄ m, and (ii) the Néel temperature increases above twofold [TN (Cu S=1/2 2+ Fe S=2 2+) 2.2 TN (Cu S=0 1+ Fe S=5/2 3+)]. This sequence of transitions is reversible with minimal hysteresis.