Semiconducting BiOCuSe Thermoelectrics and Its Metallic Derivative Bi2YO4Cu2Se2

ZrCuSiAs-type mixed-anion oxychalcogenides and oxypnictides form a massive family of functional materials readily tailored to host diverse transport properties. Here we investigate semiconducting BiOCuSe, which has been considered as a promising candidate for intermediate-temperature thermoelectric applications, and its intergrowth derivative Bi2YO4Cu2Se2, by using X-ray absorption spectroscopy. Stronger anisotropy and bond covalency are revealed for the latter phase, and in contrast to the simple charge-balance scenario of [Bi3+O2-](+)[Cu+Se2-](-) in the former phase, mixed-valence Cu+/Cu2+ with a single coordination sphere is seen in the selenide block of Bi2YO4Cu2Se2, which gives rise to a metallic electrical conductivity.