F − ion conductivity and diffusion properties in ZrF 4-based fluoride glasses with various NaF concentrations (0 ⩽ xNaF ⩽ 0.45)

Abstract

Selecting two series of ZrF 4-based fluoride glasses containing at the same time the BaF 2 and NaF network modifiers and a third series with NaF only, an investigation of electrical and diffusion properties has been undertaken as a function of x NaF in a wide concentration domain (0 ⩽ x NaF ⩽ 0.45). The ac conductivity data have been analysed by the complex modulus formalism and a hopping mechanism for transport in these materials is suggested. The 23Na and 19F NMR investigations have shown that Na + ions do not participate as charge carriers in the conduction mechanisms and the diffusion properties are due only to motions of mobile F − ions. Mobility and number of F − ions mobile in the NMR time scale increase when x BaF 2 increases, result in agreement with the composition dependence of electrical properties at long range. Experimental points representative of log σ 473 K and ΔE σ as a function of x NaF for the glasses studied are located on both sides of “master” curves involving a conductivity minimum and an activation energy maximum for x NaF ∼- 0.30. Variation of the decoupling index, R τ ( T g), with x NaF is in agreement with that of σ 473 K in the whole of the studied composition. On the contrary, the variation of the Kohlrausch parameter, β, is more difficult to analyse and may be explained by the coupling model only for x NaF ≲ 0.15. The variation of various electrical parameters is discussed as a function of glass composition.