Ionic and electronic transport in the fast Ag conductor α*-AgSI

Understanding the manifold origins that lead to fast ion transport in crystalline and amorphous solids is a vital topic in materials science. To advance in this field, the study of ion dynamics in model substances, partly inspired by applications, is a suitable route to identify the most important cause of ultrafast ion exchange processes in selected classes of materials. The ternary compound Ag₃SI, which crystallizes with different space groups, represents such a model substance; its complex Ag-sublattice offers many, only partly occupied and adjoining Ag⁺ sites that guarantee a very high Ag⁺ ion conductivity. Here, we used broadband impedance spectroscopy and studied the temperature stability of the ionic conductivity of the α* phase of Ag₃SI at a given temperature. β-Ag₃SI, synthesized by a mechanochemical approach combined with a subsequent annealing step, transforms into the α form at high temperatures. Quenching this phase leads to (metastable) α*-Ag₃SI that is characterized, at 20 °C, by a conductivity σ of 38 mS cm⁻¹. The corresponding activation energy turned out to be 200 meV. Storing α*-Ag₃SI at 30 °C for 140 h causes σ to drop to ca. 15 mS cm⁻¹ (30 °C). A more drastic decrease is seen for the sample when annealed in situ at 60 °C resulting in 9 mS cm⁻¹ (60 °C). For comparison, the thermodynamically stable β form is characterized by 3 mS cm⁻¹ (20 °C). This high Ag⁺ ion dynamics is confirmed by ¹⁰⁹Ag NMR which yields a single, sharp line at room temperature. Importantly, we also measured electronic conductivities σ_eon to corroborate that σ is predominantly governed by ionic contributions. As an example, for the α* form room-temperature potentiostatic polarisation measurements yield σ_eon = 2.7 × 10⁻⁸ S cm⁻¹, which is by a factor of 10⁶ lower than its total conductivity of 38 mS cm⁻¹. For β-Ag₃SI the maximum electronic conductivity turned out to be ≈ 2.0 × 10⁻⁷ S cm⁻¹.