Na+ ion conducting glasses in the NaCl-Ga2S3-GeS2 system: A critical percolation regime

Sodium chalcogenide and chalcohalide glasses exhibit high ionic conductivity and can be used in all-solid-state batteries, chemical sensors and other devices. However, the available information on NaCl-containing chalcogenide glass systems is relatively poor. In this work we will present conductivity results for (NaCl)x(Ga2S3)0.2-0.2x(GeS2)0.8-0.8x glasses over a wide composition range, changing between x = 10− 4 and 0.3, i.e., 3.5 orders of magnitude. Dc conductivity and ac impedance measurements have been used to study the electric properties of this glassy system. The room temperature conductivity increases by 11 orders of magnitude with increasing sodium concentration, ranging between 10− 17 S cm− 1 (x = 0) and 10− 6 S cm− 1 (x = 0.3, y = 10.1 at.% Na). The activation energy decreases from 0.99 to 0.35 eV, respectively. The composition dependence of the ionic conductivity shows two drastically different transport regimes at low (x ≤ 0.1, y ≤ 3.1 at.% Na) and high (x > 0.1) sodium concentrations. A characteristic feature of the conductivity isotherm for diluted glasses is a power law dependence of the ionic conductivity σi(y) ∝ yT0/T, where T0 is the critical temperature related to connectivity of the host glass. The rapid increase of conductivity in a rather limited concentration range and the power law dependence of σi(y) are thus caused by a percolation-controlled mechanism. Deviations from the critical percolation regime are observed at higher sodium content and presumably related to the change in the ion transport mechanism.