Density functional study of Ag in Ge2Se3

We present density functional calculations of isolated Ag atoms in a crystalline model for Ge₂Se₃. We present defect levels for interstitial and substitutional sites. Absent other defects, we show that interstitial Ag donates an electron to the conduction band of Ge₂Se₃ because the highest occupied atomic level is well above the Kohn-Sham conduction band edge. Thus, we predict that Ag is always positively charged, although its charge state is fractional and significantly below +1. This results accounts for both the conductive properties of Ge₂Se₃:Ag and the persistence of field-driven Ag motion. The presence of three fold-coordinated Ge alters the physics considerably. The midgap Ge states are electron sinks for the conduction electrons donated by Ag. Finally, we present evidence that Ag will displace Ge into interstitial sites, creating both dangling Ge orbitals and substitutional Ag with a strong Ag-Ge bond.