Structure, dynamics, and electronic structure of liquid Ag-Se alloys investigated byab initiosimulation

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics, and electronic properties of the liquid alloy ${\mathrm{Ag}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Se}}_{\mathit{x}}$ at 1350 K and at the three compositions x=0.33, 0.42, and 0.65. To provide a point of reference, calculations are also presented for the equilibrium structure and the electronic structure of the α-${\mathrm{Ag}}_2$Se crystal. The calculations are based on density-functional theory in the local-density approximation and on the pseudopotential plane-wave method. For the solid, we find excellent agreement with experiment for the equilibrium lattice parameters and the atomic coordinates of the 12-atom orthorhombic unit cell, and we present an analysis of the electronic density of states and density distribution. The reliability of the liquid simulations is confirmed by detailed comparisons with very recent neutron-diffraction results for the partial structure factors and radial distribution functions (RDF) of the stoichiometric liquid ${\mathrm{Ag}}_2$Se. Comparison with the predictions of an empirical interaction model due to Rino et al. is also given for l-${\mathrm{Ag}}_2$Se. The ab initio simulations show a dramatic change of the Se-Se RDF with increasing Se content. This change is due to the formation of Se clusters bound by covalent bonds, the Se-Se bond length being almost the same as in pure c-Se and l-Se. The clusters are predominantly chainlike, but for higher x there is a significant fraction of threefold coordinated Se atoms. It is shown that the equilibrium fractions of Se present as isolated atoms and in clusters can be understood on a simple charge-balance model based on an ionic interpretation. The Ag diffusion coefficient in the simulated stoichiometric liquid is consistent with experimental values measured in the high-temperature superionic solid. The Ag and Se diffusion coefficients both increase with Se content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than 1 ps. The electronic density of states (DOS) for l-${\mathrm{Ag}}_2$Se strongly resembles that of the solid. Some of the changes of DOS with composition arise directly from the formation of Se-Se covalent bonds. Results for the electronic conductivity σ obtained using the Kubo-Greenwood approximation are in adequate agreement with experiment for l-${\mathrm{Ag}}_2$Se, but for the high Se contents the simulation results for σ are 3–4 times greater than experimental values. Possible reasons for this are discussed. © 1996 The American Physical Society.