Electrochemical behavior of single crystals of nonstoichiometric cerium(III) fluorides

Abstract

The electrochemical properties of single crystals of cerium fluoride alloyed with bivalent cations Sr²⁺, Ca²⁺, Ba²⁺, Sr²⁺ + Ca²⁺, Sr²⁺ + Ba²⁺, Ba²⁺ + Ca²⁺ and also with La³⁺ and La³⁺ + Ba²⁺ cations are studied using the dynamic voltammetry and impedance spectroscopy. The conductivity of symmetrical cells with Ag electrodes is determined using the method of impedance spectroscopy in the frequency range from 450 to 5 kHz at the temperatures from 20 to 100°C: for CeF₃: Sr²⁺ (0.5 mol %) + Ba²⁺ (0.5 mol %), σ = σ₀ exp[(−0.284 ± 0.005/kT]; for CeF₃:Ca²⁺ (0.5 mol %) + Sr²⁺ (0.5 mol %), σ = σ₀ exp[(−0.292 ± 0.017/kT]. The steady-state and dynamic voltammogams of symmetrical electrochemical cells with nonpolarizable reference electrodes and CeF₃ single crystals alloyed with Sr²⁺, Ca²⁺, and Ba²⁺ bivalent cations exhibited ohmic polarization. For cells with CeF₃ containing La³⁺ as an admixture, a hysteresis was observed, which could not be eliminated by chemical and electrochemical treatment of crystals. In the dynamic voltammetric curves of asymmetric cells with nonpolarizable and silver electrode and CeF₃ crystals alloyed with Sr²⁺, Ca²⁺, and Ba²⁺, a range of ideal polarizability (from 0 to ∼−2.7 V), and also cerium redox processes and silver fluorination-boundary regeneration were observed. In the dynamic voltammetric curves of asymmetric cells with CeF₃ containing La³⁺ admixture, no range of ideal polarizability was observed; however, the reactions of silver fluorination and reduction of solid-electrolyte cerium were well pronounced at the corresponding potentials.