Lanthanum fluoride ionic conductivity

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

The discovery of the lanthanum fluoride electrode by Frant and Ross (1966) resulted in an appreciable improvement in the detection of fluorine. Metal—fluoride compounds, especially light rare-earth fluorides, show an excellent ionic conduction. The carrier in the solid fluoride is mainly anionic fluorine with some electronic conduction. This means that the material is a fluorine conducting solid electrolyte, a feature which highly enhances the... 

M. Takashima, S. Yonezawa, M. Leblanc, Synthesis and oxide ion conductivity of lanthanum-europium oxide fluoride, La2Eu203Fg, Solid State Ionics, 154-155, 547-553 (2002). 

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