Mixed mobile ion effect and relaxation dynamics in fluoride glasses

Abstract

Fluoride glasses containing single alkali ions as modifiers are fluoride and/or alkali ion conductors, depending on the concentration and nature of alkali ions. Addition of a second type of alkali ions into a single alkali glass changes a variety of properties in a nonlinear fashion. These changes are often referred to as the mixed mobile ion effect. We provide several evidences in favour of this effect in fluoride glasses where anions also participate in the diffusion processes in addition to cations, unlike in mixed alkali oxide glasses and crystals. A series of fluoride glass compositions formed with ZrF₄ as the network formers and LiF and NaF as two alkali fluorides are synthesised. The electrical measurements are carried out using a precision RLC meter in the frequency range of 10 Hz-2 MHz in the temperature range from room temperature to just below glass transition temperature. A mixed mobile ion effect in these glasses has been observed. This phenomenon is associated with the low dimensionality of the diffusion pathways in the mixed alkali fluoride glasses compared to single alkali glasses. From different scaling approaches it is observed that the relaxation dynamics in these glasses is temperature independent.