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The reduction of immobile cations La 3+ and Ce 3+ in fluoride-conducting solid electrolytes (FSE) LaF 3 ( Eu 2+ 0.8 mol %), LaF 3 ( Sr 2+ 5 mol %), and CeF 3 ( Sr 2+ 5 mol %) in contact with Ag, Bi, Si, La, Ce, and Sm working electrodes is studied by chronoamperometry and voltammetry with linear potential scan. Discovered is linear dependence of initial segments of potentiostatic transients of cathodic current on t 1/2 at FSE interfaces with Ag, Bi, La, Ce, and Sm. The dependence is due to diffusion-controlled instantaneous nucleation of Ln and Ce. The La 3+ and Ce 3+ reduction at the FSE/Ag interface is reversible in a narrow region. The reduction and oxidation of La 3+ and Ce 3+ (cations of the FSE rigid lattice) at the FSE/Me (Me = La, Ce and Sm, Bi, Si) interface is irreversible and involves a chemical reaction.

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Studies of single crystals of nonstoichiometric fluorides of PrF 3 :Sr 2+ and NdF 3 :Ba 2+ in contact with a Pt electrode were carried out using the method of voltammetry with a linear potential sweep, chrono amperometry, and cyclic voltammetry. The reaction of gaseous oxygen reduction (GOR) at the PrF 3 :Sr 2+ /Pt interface was accompanied by appearance of mobile oxygen forms. The catalytic activity of PrF 3 :Sr 2+ with respect to gaseous oxygen, same as CeF 3 :Sr 2+ , can be explained by variable valency of praseodymium and cerium fluoride, which creates opportunities for using them in oxygen and oxygen-containing gas sensors. The GOR reaction at the NdF 3 :Ba 2+ /Pt interface appeared as a result of anodic polarization. After charging at E = 3.2 V, the NdF 3 :Ba 2+ single crystal was successfully used for fluorination of Sb and Ag. NdF 3 :Ba 2+ is most promising in devices based on reactions with participation of mobile fluoride ion in solid electrolytes, particularly, for generation of gaseous fluorine.