Electronic and Structural Anomalies in Lead Chalcogenides

The rocksalt-structure PbS, PbSe, and PbTe semiconductors and their alloys exhibit a series of electronic-structure anomalies relative to the II-VI system, including the occurrence of direct gaps at the L point, anomalous order of band gaps and vakebce-band maxium energies versus anions, negative optical bowing, and negative band-gap pressure coefficints. We show that these anomalies result fromthe occurrence of the Pb s band below the top of the valence band, setting up coupling and level repulsion at the L point. Furthermore, we find that the topology of the frustrated octahedral structure leads to the occurrence in the random alloy of two distinct bonds for each anion-carrier pair and to the predicted stabilization of bulk ordered Pb2STe CuPt-like phase.