STRUCTURE, DYNAMICS, AND ELECTRONIC-STRUCTURE OF LIQUID AG-SE ALLOYS INVESTIGATED BY AB-INITIO SIMULATION

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics, and electronic properties of the liquid allo y Ag1-xSex 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 presente d for the equilibrium structure and the electronic structure of the al pha-Ag2Se crystal. The calculations are based on density-functional th eory in the local-density approximation and on the pseudopotential pla ne-wave method. For the solid, we find excellent agreement with experi ment 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 reliabi lity of the liquid simulations is confirmed by detailed comparisons wi th very recent neutron-diffraction results for the partial structure f actors and radial distribution functions (RDF) of the stoichiometric l iquid Ag2Se. Comparison with the predictions of an empirical interacti on model due to Rino et al is also given for l-Ag2Se. The ab initio si mulations show a dramatic change of the Se-Se RDF with increasing Se c ontent. This change is due to the formation of Se clusters bound by co valent bonds, the Se-Se bond length being almost the same as in pure c -Se and /-Se. The clusters ara predominantly chainlike, bat 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 a toms and in clusters can be understood on 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 diff usion coefficients both increase with Se content, in spite of the Se c lustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than I pst The electronic density of sta tes (DOS) for l-Ag2Se strongly resembles that of the solid. Some of th e changes of DOS with composition arise directly from the formation of Se-Se covalent bonds. Results for es the electronic conductivity sigm a obtained using the Kubo-Greenwood approximation are in adequate agre ement with experiment for l-Ag2Se, but for the high Se contents the si mulation results for sigma are 3-4 times greater than experimental val ues. Possible reasons for this are discussed.