Migration of Ag in low-temperature Ag2 S from first principles

Using the density-functional theory combined with the nudged elastic band method, we have calculated migration pathways and estimated the activation energy barriers for the diffusion of Ag ions in low-temperature Ag2 S. The activation energy barriers for four essential migrations for an Ag ion, namely, from a tetrahedral (T) site to an adjacent T vacancy (VT), from an octahedral (O) site to an adjacent O vacancy (VO), from T to VO, and from O to VT, are estimated as 0.461, 0.668, 0.212, and 0.318 eV, respectively, which are comparable to experimental values. This means that diffusions of Ag ions between nonequivalent sites are preferable to those between equivalent sites, and that direct T- VT and O- VO diffusions are less likely to occur than indirect T- VO -T and O- VT -O diffusions. These diffusion behaviors between nonequivalent sites have also been supported by ab initio molecular dynamics simulations, in which the diffusion pathways are directly observed.