Mobile silver ions and glass formation in solid electrolytes

Solid electrolytes are a class of materials in which the cationic or anioni c constituents are not confined to specific lattice sites, but are essentia lly free to move throughout the structure. The solid electrolytes AgI and A g2Se (refs 1-7) are of interest for their use as additives in network glass es(8-12), such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential application s for batteries, sensors and displays. Here we show that these composite gl asses can exhibit two distinct types of molecular structures-an intrinsic p hase-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 tempe ratures correspond to the solid-electrolyte glass phase and the main glass phase (the 'base glass'), enabling us to show that the glass transition tem peratures for the AgI and Ag2Se phases are respectively 75 and 230 degreesC . Furthermore, we show that the magnitude of the bimodal glass transition t emperatures can be quantitatively understood in terms of network connectivi ty, provided that the Ag+ cations undergo fast-ion motion in the glasses. T hese results allow us to unambiguously distinguish base glasses in which th ese additives are homogeneously alloyed from those in which an intrinsic ph ase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.