The present high-pressure studies of ${\text{CuFeO}}_2$ to 30 GPa using x-ray diffraction, along with ${}^{57}\text{F}\text{e}$ Mössbauer and Fe and $\text{Cu}K$-edge x-ray absorption spectroscopy methods, reveal a sequence of intricate structural/electronic-magnetic pressure-induced transitions. The low-pressure $R\overline{3}m$ structure (0–18 GPa) is composed of sheets of ${\text{Fe}}_{S=5/2}^{3+}$ ions alternating with layers of ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ dumbbells, the latter oriented along the $c$ axis. This structure is characterized by an unusual positive $d\left(c/a\right)/dP$. At 18 GPa a structural transition takes place to a more isotropic $C2/c$ structure with the ${\text{O-Cu}}_{S=0}^{1+}\text{-O}$ axis tilted $28°$ from the $c$ axis and with negative $d\left(c/a\right)/dP$. This transition corroborates with the onset of long-range antiferromagnetic order. Starting at $\sim 23\text{GPa}$, with an initial volume reduction in $\sim \left|\Delta V/V_0\right|=0.16$, the Cu-Fe bands overlap and this leads to a $\left({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+}\right)\to \left({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+}\right)$ interionic valence exchange in about 1/3 of the $C2/c{\text{-CuFeO}}_2$ at 27 GPa. As a result: (i) the ${\text{Cu}}^{2+}\text{-O}$ becomes fourfold coordinated and is in a new crystallographic structure with space group $P\overline{3}m$, and (ii) the Néel temperature increases above twofold $\left[T_{\text{N}}\left({\text{Cu}}_{S=1/2}^{2+}{\text{Fe}}_{S=2}^{2+}\right)\approx 2.2T_{\text{N}}\left({\text{Cu}}_{S=0}^{1+}{\text{Fe}}_{S=5/2}^{3+}\right)\right]$. This sequence of transitions is reversible with minimal hysteresis.

• Received 20 August 2009

DOI:https://doi.org/10.1103/PhysRevB.81.104110