Physical properties of the delafossite LaCuO 2

Abstract

High-quality LaCuO 2, elaborated by solid-state reaction in sealed tube, crystallizes in the delafossite structure. The thermal analysis under reducing atmosphere (H 2/N 2: 1/9) revealed a stoichiometric composition LaCuO 2.00. 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 Cu 2+ 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 ( μ 300 K =3.5×10 −7 cm 2 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 ( V fb=0.15 V SCE) and the hole density ( N A=5.8×10 17 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.