Chemically-invariant percolation in silver thioarsenate glasses and two ion-transport regimes over 5 orders of magnitude in Ag content

Abstract

Ionic conductivity σi measurements of AgY−As2S3 (Y = Br, I) glasses, covering 13 orders of magnitude in σi(x) over 5 orders of magnitude in silver content x, confirm two drastically different ion transport regimes in silver halide thioarsenate glasses. As expected, the ionic conductivity in the critical percolation domain, 20 ppm < x ≤ 2 at.% Ag, follows a power-law dependence, σi(x,T)∝xT0/T, where the critical temperature T0 is related to the connectivity of the glassy host. Moreover, we show that σi(x,T) is chemically invariant in the critical percolation domain. Three silver thioarsenate glass families, AgY- and Ag2S-As2S3, reveal identical ionic conductivity within experimental uncertainty over three orders of magnitude in silver contentx. The ionic conductivity diverges in the modifier-controlled region, x > 7-10 at.% Ag, and the difference in σi(x) between AgI- and Ag2S-As2S3 glasses approaches 4 orders of magnitude. Random distribution of silver in the critical percolation domain, shown by DFT modeling of neutron and high-energy x-ray diffraction data, is a key of the observed conductivity invariance. When silver cation leaves the residence site and travels throughout the glass network, characterized by the average Ag-Ag separation distance of 12 Å or more, the memory of its original chemical form (sulfide or halide) vanishes rapidly with increasing the mean square displacement. A non-random silver distribution in the modifier controlled region implying formation of preferential conduction pathways via direct contacts of edge- and corner-sharing silver chalcogenide or chalcohalide polyhedra rules out this possibility. Chemically-invariant ionic conductivity seems to be a common feature of any disordered system with random distribution of mobile ions having similar size of charge carriers.