Copper pyrites (formula presented) and (formula presented) as anion conductors

(formula presented) and (formula presented) with pyrite structures were systematically studied by transport, magnetization, and specific-heat measurements. In remarkable contrast to other (formula presented) 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, (formula presented) and (formula presented) can be viewed as anion conductors.