Fluoride, rare-earth conductors

Ionic Conductivity (see Ionic Conductors). Pluoride anionic conductivity is observed mainly in derivatives of fluorite (Cap2) and tysonite (Lap3). If Cap2 is doped by a tervalent rare-earth metal ion, the additional fluoride ions are positioned in interstitials where they become mobile by a hopping mechanism. 

Rare-earth fluorides are good fluorine-ion conductors (see sect 5.2). Addition of aliovalent cations significantly increases the fluorine-ion conductivity even fiulher. For example, single crystals of Lap3 doped with Eup2 are widely used in commercial apphcations of ion-selective electrodes (ISE) as specific electrodes (Frant and Ross 1966). In the field of ISE, only the pH-sensing glass electrode is more widely used. Fluoride-ion detection is important in sea water, water minerals, rocks, fossils and minerals, biomedical applications, potable water and plant and animal metabolism. 

Rare-earth elements are vital constituents of several prominent high-temperature solid electrolytes ranging from oxygen- or fluoride-ion conductors in the fluorite structures to protonic conductors in the doped perovskite phases and trivalent-ion conduction in Sc2(W04)3 and 3-alumina-type compounds. Solid electrolytes are considered as important for scientific studies and technological applications in vital areas such as fuel cells, batteries, sensors, process control and environmental protection. 

Another common fluoride ionic conductor is calcium fluoride. La order to increase the fluoride ionic vacancies, an aliovalent cation has to be mixed with CaF2. In this case, trivalent rare-earth cations such as La +, ate used as the mixing ion. La addition to these fluoride electrolytes, P-PbF2 also shows a good F" ionic conduction. The details of these two fluorine conducting electrolytes are described below. 

The detailed properties of other common rare-earth fluoride conductors are described below. 

One of the main applications of the fluorine conductor in the sensor field is the measuring of the fluorine content in test solutions. In most cases, the operating temperature is around room temperature because solutions are aqueous. Therefore, the candidate electrolytes for the sensor are the rare-earth fluorides which retain the hexagonal structure at room temperature. In addition, the fluoride electrolytes should be water stable... 

One other rare-earth application is their direct use as a base material of the sensor. One typical application is with rare-earth fluorides. The fluorides are excellent fluorine ionic conductors even at room temperature and also very stable in an aqueous solution. These features make the commercial use in fluorine sensing in a solution possible. A practically used material is lanthanum fluoride. Other rare earths also show similar... 

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