We studied electronic transport properties of hematite (α-Fe${}_2$O${}_3$) at room temperature under cycling of high pressure up to ∼22 GPa. The original samples and those recovered after high-pressure experiments were examined by x-ray diffraction and Raman and optical absorption spectroscopy. At ambient pressure the original samples were also characterized by temperature measurements of electrical and galvanomagnetic properties. Upon compression, the original single crystals underwent a sluggish structural deconfinement starting above 5 GPa into a nanometric state. Above 5–7 GPa, the nanostructured hematite showed a reversible transition to a state with enhanced electrical conductivity and moderate values of thermoelectric power (Seebeck effect) of about −150 μV/K. This electronic phase corresponds to neither conventional trivalent oxidation state of the iron ions in hematite nor metallic conductivity. Analysis of the electronic transport data in the frameworks of two models, of polaron hopping, and of intrinsic semiconductor conductivity, revealed a change from the electron conductivity to two-band electrical conductivity and suggested that the observed enhancement of the electrical properties in nanocrystalline α-Fe${}_2$O${}_3$ above 5–7 GPa is related to the mixed-valence state of the iron ions. Since α-Fe${}_2$O${}_3$ is believed to undergo a “spin-flop” (Morin) transition near 2–5 GPa at room temperature, we discuss potential contributions of magnetoelastic and other effects to the observed high-pressure properties of hematite.

• Received 9 January 2012

DOI:https://doi.org/10.1103/PhysRevB.86.205131