Structural and dynamical correlations in Ag2Se: A molecular dynamics study of superionic and molten phases

Abstract

We study the structural properties, single-particle dynamics, and the charge transport in superionic and molten Ag2Se using the method of molecular dynamics. The calculations are based on a model of interionic potentials in which the ions interact through Coulomb interaction, steric repulsion, and charge–dipole interaction due to the large electronic polarizability of the selenium ions. In the superionic phase the Ag ions diffuse through a stable bcc lattice of Se atoms. Structural and dynamical correlations are studied at five temperatures in the superionic phase and three temperatures in the molten phase. Among the structural correlations the results are presented for partial pair distribution functions, coordination numbers, partial structure factors, bond angle distributions, and the wave vector and temperature dependence of the Bragg intensities. Detailed comparison with the neutron and x-ray single crystal diffraction results are made whenever possible. Diffuse neutron and x-ray scattering is calculated and investigated in detail in the vicinity of q0=(1.6,1,0). It is shown that the anisotropic disks of intensity arise entirely due to the collective motions of silver ions and that these correlations manifest in the q space at a point where the Se–Ag partial structure factor is nearly zero. The calculated temperature dependence of the self-diffusion constant of silver is in good agreement with the tracer diffusion measurements. The spectra of velocity autocorrelation functions and the frequency dependent ionic conductivity are calculated. The Haven’s ratio, derived both from the calculated self-diffusion and zero frequency limit of the ionic conductivity, is in good agreement with the experimental results of Okazaki. In the molten phase the calculated neutron structure factor is compared directly with the neutron diffraction experiments of Susman et al. The results for self-diffusion of silver and selenium ions in the molten phase and the frequency dependent ionic conductivity are also discussed.

Ionic conduction in the solid state

“…The normal to superionic transition as well as many other structural and dynamical aspects of Ag 2 S has been reproduced by simulations 96 in good agreement with experimental studies. The frequency dependent conductivity as well as the diffusion coefficient calculated in the simulation study 130 of Ag 2 Se are in good agreement with experiments. The study 130 suggests that the Ag + ions largely occupy the tetrahedral sites of the bcc Se sublattice with only a small fraction occupying the octahedral sites.…”

“…The frequency dependent conductivity as well as the diffusion coefficient calculated in the simulation study 130 of Ag 2 Se are in good agreement with experiments. The study 130 suggests that the Ag + ions largely occupy the tetrahedral sites of the bcc Se sublattice with only a small fraction occupying the octahedral sites. The simulation study 131 on α-Ag 2 Te suggest that the conduction channel of Ag + ions is the one connecting a tetrahedral site to neighbouring octahedral site.…”

“…The works of Vashishta et al 58, 94 on AgI have been a great source of motivation to the researchers working in the field of computer simulation of superionic solids. Following their strategy silver ion conductors of the formula Ag 2 X (where X = S, 96,128,129, Se 130 and Te 131 ) have also been investigated through computer simulations. As in α-AgI, the 'framework' ions in α-Ag 2 S and α-Ag 2 Se forms a thermally agitated bcc lattice while in α-Ag 2 Te they (Te ions) are arranged in an fcc lattice like in the fluorite structure.…”

An interaction potential for barium sulfide: A molecular dynamics study

“…The same functional form of the effective interaction potential has been used to describe several other materials, such as super ionic conductors [24,25], oxides [26][27][28], and perovskites [29,30]. Based on previous simulations [26], the exponents in the steric repulsion term were chosen to be 11, 9, and 7 for Ba-Ba, Ba-S and S-S interactions, respectively.…”

Diffusion Mechanism of Ag ions in Superionic Conductor Ag2Se from Ab Initio Molecular-Dynamics Simulations

“…[7][8][9][10] However, within these models, the investigation on the microscopic origin of the superionic behavior is limited, since these empirical potentials lack the ability to describe the change of chemical bonding accompanied with the diffusive motion of the mobile ions. It should be mentioned that some attempts to describe high ionic mobility in Ag-ion conductors based on electronic band structure calculations have been reported, [11][12][13] although hypothetical crystal structures have been used in these studies.…”