Solid State Sciences | 2021
Ionic and electronic transport in the fast Ag conductor α*-AgSI
Abstract
Abstract 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\xa0°C, by a conductivity σ of 38\xa0mS\xa0cm−1. The corresponding activation energy turned out to be 200\xa0meV. Storing α*-Ag3SI at 30\xa0°C for 140\xa0h causes σ to drop to ca. 15\xa0mS\xa0cm−1 (30\xa0°C). A more drastic decrease is seen for the sample when annealed in situ at 60\xa0°C resulting in 9\xa0mS\xa0cm−1 (60\xa0°C). For comparison, the thermodynamically stable β form is characterized by 3\xa0mS\xa0cm−1 (20\xa0°C). This high Ag+\xa0ion dynamics is confirmed by 109Ag NMR which yields a single, sharp line at room temperature. Importantly, we also measured electronic conductivities σeon to corroborate that σ is predominantly governed by ionic contributions. As an example, for the α* form room-temperature potentiostatic polarisation measurements yield σeon\xa0=\xa02.7\xa0×\xa010−8\xa0S\xa0cm−1, which is by a factor of 106 lower than its total conductivity of 38\xa0mS\xa0cm−1. For β-Ag3SI the maximum electronic conductivity turned out to be ≈ 2.0\xa0×\xa010−7\xa0S\xa0cm−1.