Atomic Ordering and Gap Formation in Ag-Sb Based Ternary Chalcogenides*

Ag-Sb based ternary chalcogenides are important in optical phase change and thermoelectric applications. Although discovered almost 50 years ago and thought to be semiconductors, a fundamental understanding of their electronic structures had been lacking. We report \textit{ab initio} electronic structure studies using density functional theory (DFT) to explain their observed atomic structures, the physics of gap formation and their low-energy properties. Total energy calculations yield theoretical atomic structures which are consistent with experiment. Ag/Sb ordering is found to have a huge impact on the electronic structure near the Fermi energy. It gives pseudogap structure in some ordered structures, and either a pseudogap or a gap in others. For the lowest energy structures, as one goes from Te to Se to S, the (indirect) band gap goes from being negative to positive. Transport properties of AgSbTe$_{2}$ can be understood in terms of a small intrinsic band gap and extremely shallow impurity states. The calculated negative band gap in this compound can be ascribed to the defficiency of DFT.