F- ION CONDUCTIVITY AND DIFFUSION PROPERTIES IN ZRF4-BASED FLUORIDE GLASSES WITH VARIOUS NAF CONCENTRATIONS (0-LESS-THAN-OR-EQUAL-TO-X(NAF)LESS-THAN-OR-EQUAL-TO-0.45)

Selecting two series of ZrF4-based fluoride glasses containing at the same time the BaF2 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 less than or equal to x(NaF) less than or equal to 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 Na-23 and F-19 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(BaF2) increases, result in agreement with the composition dependence of electrical properties at long range. Experimental points representative of log sigma(473K) and Delta E(sigma) 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) similar or equal to 0.30. Variation of the decoupling index, R(tau)(T-g), with x(NaF) is in agreement with that of sigma(473K) in the whole of the studied composition. On the contrary, the variation of the Kohlrausch parameter, beta, is more difficult to analyse and may be explained by the coupling model only for x(NaF) less than or similar to 0.15. The variation of various electrical parameters is discussed as a function of glass composition.