Bonds, bands, and band gaps in tetrahedrally bonded ternary compounds: The role of group V lone pairs Original Research Article

An interesting class of tetrahedrally coordinated ternary compounds has attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I3–V–VI4, contain three monovalent-I (Cu, Ag), one nominally pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II–VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II–VI) and replacing four type II atoms by three type I and one type V atoms. We find that nominally pentavalent-V atoms are effectively trivalent and their lone (ns2) pairs play an active role in opening up a gap. The lowest conduction band is a strongly hybridized anti-bonding combination of the lone pair and chalcogen (VI) p-states. The magnitude of the gap is sensitive to the nature of the exchange interaction (local vs non-local) and the V–VI distance. We also find that the electronic structure near the gap can be reproduced extremely well within a local theory if one can manipulate the position of the filled d bands of Cu and Ag by an effectively large U.