Phase Transition and Liquid-like Superionic Conduction in Ag₂S

Abstract

Superionic conductivity in certain polymorphs of Ag2S has inspired numerous concepts for materials applications, but the relationship between the structure and the mobility of silver ions remains poorly explored. Here, we report ab initio molecular dynamics simulations for low- (acanthite) and high-temperature (argentite) Ag2S polymorphs that reveal the dynamical processes giving rise to the superionic behavior in the latter. Similarities between their sulfur sublattices enable simulations of silver ion diffusivities and pathways on essentially an equal footing. For the higher temperature polymorph, calculated temperature-dependent mean square displacements and activation energies by the nudged elastic band method show good correspondence with expectations from the experiment. In the superionic state, silver atoms diffuse in a liquid-like behavior with no preferred diffusion pathways, within the relatively stable body-centered cubic sulfur framework. In contrast, conduction in acanthite appears to depend more on the mobilities of electronic charge carriers.

Modeling and Characterization of Stochastic Resistive Switching in Single Ag2S Nanowires

“…Figures 3 (c Interestingly, fitted values were found close to the actual values reported in the literature. In particular, the mobility µ was found between 1×10 3 to 8×10 3 µm 2 /(sV) and was only slightly below the mobility experimentally observed for the cubic Ag 2 S-β phase µ=15×10 3 µm 2 /(sV) at T=450 K (for comparison the Ag + mobility in the monoclinic phase of Ag 2 S-α, is µ~1 µm 2 /(sV) at T=300 K) 23,43 . The difference could be attributed to the presence of different charge carriers, namely electrons and Ag + ions, and a mixture of acanthite and argentite phases 35 .…”

“…Thus in the model high concentration of scattered nano-islands will produce a low R ON value and a low effective length L of the wire for fixed mobility µ found in the literature 23,43 . On the other hand, a low concentration of scattered islands will result in a high magnitude of R ON and effective length of the wire L e close to observed L.…”

“…Due to the fluctuations caused by Ag + ion redox exchange and metastable atomic positions, the conductive channel's thickness will also fluctuate unpredictably 19,42 , and hence can be modeled with a stochastic process described by Equations 5 and 6 with normally distributed noise with standard deviation parameter σ . Metallic Ag nano-clusters are formed and reorganized spontaneously within the volume of the Ag 2 S phase due to the heat and electric field-induced relocation of Ag + ions in 3D volume 23,35 . Therefore, percolation theory is used to connect the conductivity in the ON state via the R ON variable with the volume fraction of Ag nano-islands in the mixture of Ag 2 S-α and Ag 2 S-β phases and thus is described by the power-law in Equation 5.…”

“…The evidence of the Ag nano-crystal formation in Ag 2 S NW is also supported by comparing switching times in thin films and long wires. In an acanthite, Ag 2 S-α wire model with an effective length of 16 µm, switching between ON/OFF states would take approximately 50 seconds at 3 V driving voltage potential due to relatively low mobility of Ag + ions in the phase, namely µ=0.5 µm 2 /(sV) 23,43 . However, in the experiment and the best-fitted parameters of the model to the experimental data of a single Ag 2 S NW (Figure 3 (d) and Table 1), switching occurs within tens of seconds.…”

“…For example, acanthite Ag 2 S-α is a low-temperature polymorph with a monoclinic crystal structure that is stable up to ~450K 22 . Above 450K and up to ~860K Ag 2 S is in argentite Ag 2 S-β phase with an ordered bcc lattice of sulfur atoms and Ag + ions that partially occupy tetrahedral and octahedral sites that gives it excellent ion mobility and increased electrical conductivity [22][23][24][25] .…”