Mobile silver ions and glass formation in solid electrolytes

Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1-7) are of interest for their use as additives in network glasses(8-12), such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures-an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the 'base glass'), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 degreesC. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.