Copper pyrites CuS₂ and CuSe₂ as anion conductors

CuS₂ and CuSe₂ with pyrite structures were systematically studied by transport, magnetization, and specific-heat measurements. In remarkable contrast to other 3d transition-metal pyrites, a clear indication of strong electron correlations was absent in the electronic properties of Cu pyrites. We interpret this as a consequence of the dominant chalcogen p character rather than copper d character at the Fermi level. Photoemission results indeed support this picture, indicating that the Cu is predominantly monovalent. We therefore conclude that Cu pyrites, CuS₂ and CuSe₂, can be viewed as anion conductors.