Dynamical approach to the atomic and electronic structures of the ductile semiconductor Ag2S.

Silver sulfide in monoclinic phase (α-Ag2S) has attracted significant attention owing to its metal-like ductility and promising thermoelectric properties near room temperature. However, first-principles studies on this material by density functional theory calculations have been challenging as both the symmetry and atomic structure of α-Ag2S predicted from such calculations are inconsistent with experimental findings. Here, we propose that a dynamical approach is imperative for correctly describing the structure of α-Ag2S. The approach is based on a combination of ab initio molecular dynamics simulation and deliberately chosen density functional considering both proper treatment of the van der Waals interaction and on-site Coulomb interaction. The obtained lattice parameters and atomic site occupations of α-Ag2S are in good agreement with experimental data. A stable phonon spectrum at room temperature can be obtained from this structure, which also yields a bandgap in accord with experimental measurements. The dynamical approach thus paves the way for studying this important ductile semiconductor in not only thermoelectric but also optoelectronic applications.

Thermoelectric Ag2Se: Imperfection, Homogeneity, and Reproducibility.

Ag2Se is a narrow band gap n-type semiconductor with high carrier mobility and low lattice thermal conductivity. It has high thermoelectric performance near room temperature. However, there is a noticeable data discrepancy for thermoelectric performance in the reported literature studies, which greatly hinders the rational understanding and potential application of this material. In this work, we comprehensively studied the homogeneity, reproducibility, and thermal stability of bulk Ag2Se prepared by melting and mechanical alloying methods followed by spark plasma sintering. By virtue of the atom probe topology technique, we revealed nanosized Ag- or Se-rich precipitates and micropores with Se-aggregated interfaces that have not been detected previously. The samples prepared by melting and spark plasma sintering exhibit the best homogeneity and repeatability in thermoelectric properties despite abundant nanoprecipitates. Moreover, the thermoelectric performance of Ag2Se is greatly improved by introducing a slight amount of excess selenium. The average zT can steadily reach 0.8-0.9 in the range of 300-380 K, which is among the highest values reported for Ag2Se-based materials. This work will rationalize the evaluation of the thermoelectric performance of Ag2Se.