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

Abstract

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics and electronic properties of the liquid alloy 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 presented for the equilibrium structure and the electronic structure of the α-Ag2Se 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 Ag2Se. Comparison with the predictions of an empirical interaction model due to Rino et al. is also given for ℓ-Ag2Se. 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 chain-like, but for higher x there is a significant fraction of 3-fold 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 hightemperature 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-Ag2Se 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-Ag2Se, 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.

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni–Al alloys

“…The VASP code makes use of ultrasoft pseudopotentials [18], and an expansion of the electronic wave functions in plane waves. The approach has been applied widely in AIMD simulations of liquid metals and alloys [19][20][21][22][23][24][25][26][27]. Our simulations were performed using a cubic supercell containing 128 particles with periodic boundary conditions and C-points sampling.…”

Structure and property correlation for Ag deposition on α-Al2O3—a first principle study

“…HF calculations have since been used on several other oxides [13][14][15][16][17]. DFT (density functional theory) methods have begun its venture in oxides somewhat later but progressed in a rapid manner; hence, a number of studies have been reported in the last decade on oxides as well as other materials [18][19][20][21][22][23][24][25]. In the last couple of years, there have been considerable progresses in the area [26][27][28][29][30][31][32] of electronic and structural properties of mixed metal oxides using first principle density functional theory.…”

Preparation and formation mechanism of silver gallium selenide powders prepared via the sol–gel method

“…During the selenization process, selenium powders that are mixed with precursors melt to form liquid selenium. It is reported that the diffusion of Ag over Se increases with the selenium content in a mixture of Ag and Se [36]. The above results suggest that the prepared precursors would react rapidly with the liquid selenium.…”