STABILIZATION OF SUPERIONIC ALPHA-AGL AT ROOM-TEMPERATURE IN A GLASS MATRIX

SINCE the discovery 1 that the high-temperature phase of silver iodide (alpha-AgI) has an ionic conductivity comparable to that of the best liquid electrolytes, solid electrolytes have attracted wide interest. Possible applications of these materials range from solid-state batteries to electrochromic displays and sensors 2. Although alpha-AgI displays conductivities of more than 10 S cm-1 (ref. 3), owing to the almost liquid-like mobility of Ag+ ions, the crystal transforms below 147-degrees-C to the beta-phase with a conductivity of only approximately 10(-5) S cm-1 at room temperature. Efforts to achieve good conductivities at lower temperatures have focused on the addition of a second component to AgI to form solid solutions or new compounds such as RbAg4I5 and Ag2HgI4 (refs 4-7). Here we report our success in depressing the alpha --> beta-transformation temperature so as to stabilize alpha-AgI itself at room temperature. We use a melt-quenching technique to prepare crystallites of alpha-AgI frozen into a silver borate glass matrix. The quenched material showed diffraction peaks characteristic of alpha-AgI and displayed ionic conductivities of about 10(-1) S cm-1. Further development of these glass/crystal composites may make the high ionic conductivity of alpha-AgI available for room-temperature solid-state applications.