Cation Effects in some series of mixed alkali fluoride glasses

An investigation of electrical properties of two series of fluoride glasses containing simultaneously LiF and NaF has been undertaken. The alkali fluoride contents are respectively x AF = 0.10 and x AF = 0.30 : (ZrF 4 ) 0.58 (BaF 2 ) 0.25 (LaF 3 ) 0.07 (LiF) 0.10‐x (NaF) x and (ZrF 4 ) 0.50 (BaF 2 ) 0.15 (LaF 3 ) 0.05 (LiF) 0.30‐x (NaF) x . In both series, a mixed alkali effect revealed by the existence of a conductivity minimum is apparently observed. It is shown, however, that the minima have a quite different nature and none of them is actually compatible with the classical „mixed alkali effect” of mobile cations. In the series x AF = 0.10, where charge transport is essentially assured by the F − ‐ions, this effect results from the presence of non mobile alkali ions of different nature in the vicinity of the F − ‐ions. In the series x AF = 0.30, the electrical properties of the extreme compositions x LiF = 0.30 and x NaF = 0.30 are due respectively either to Li + ‐ions only or to mobile F − ‐ions only. The variation of the ratio r = Na/(Li + Na) from 0 to 1 corresponds to progressive cross over from a cationic conducting glass to an anionic conducting one. The presence of a minimum of conductivity for 0.50 < r < 0.75 has been explained.