Ionic and electronic transport in the fast Ag+ conductor α*-Ag3SI

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 alpha* phase of Ag3SI at a given temperature. beta-Ag3SI, synthesized by a mechanochemical approach combined with a subsequent annealing step, transforms into the alpha form at high temperatures. Quenching this phase leads to (metastable) alpha*-Ag3SI that is characterized, at 20 degrees C, by a conductivity sigma of 38 mS cm(-1). The corresponding activation energy turned out to be 200 meV. Storing alpha*-Ag3SI at 30 degrees C for 140 h causes sigma to drop to ca. 15 mS cm(-1) (30 degrees C). A more drastic decrease is seen for the sample when annealed in situ at 60 degrees C resulting in 9 mS cm(-1) (60 degrees C). For comparison, the thermodynamically stable beta form is characterized by 3 mS cm(-1) (20 degrees C). This high Ag+ ion dynamics is confirmed by Ag-109 NMR which yields a single, sharp line at room temperature. Importantly, we also measured electronic conductivities sigma(eon) to corroborate that sigma is predominantly governed by ionic contributions. As an example, for the alpha* form room-temperature potentiostatic polarisation measurements yield sigma(eon) = 2.7 x 10(-8) S cm(-1), which is by a factor of 10(6) lower than its total conductivity of 38 mS cm(-1). For beta-Ag3SI the maximum electronic conductivity turned out to be approximate to 2.0 x 10(-7) S cm(-1).