Physical properties of the delafossite LaCuO2

High-quality LaCuO2, elaborated by solid-state reaction in sealed tube, crystallizes in the delafossite structure. The thermal analysis under reducing atmosphere (H2/N2: 1/9) revealed a stoichiometric composition LaCuO2.00. The oxide is a direct band-gap semiconductor with a forbidden band of 2.77 eV. The magnetic susceptibility follows a Curie–Weiss law from which a Cu2+ concentration of 1% has been determined. The oxygen insertion in the layered crystal lattice induces p-type conductivity. The electrical conduction occurs predominantly by small polaron hopping between mixed valences Cu+/2+ with an activation energy of 0.28 eV and a hole mobility (μ300 K=3.5×10−7 cm2 V−1 s−1), thermally activated. Most holes are trapped in surface–polaron states upon gap excitation. The photoelectrochemical study, reported for the first time, confirms the p-type conduction. The flat band potential (Vfb=0.15 VSCE) and the hole density (NA=5.8×1017 cm−3) were determined, respectively, by extrapolating the curve C−2 versus the potential to their intersection with C−2=0 and from the slope of the linear part in the Mott–Schottky plot. The valence band is made up of Cu-3d orbital, positioned at 4.9 eV below vacuum. An energy band diagram has been established predicting the possibility of the oxide to be used as hydrogen photocathode.