Synthesis and Ionic Conductivity of Fluoride–Oxide Composites in NdF3–Nd2O3 and NdF3–SrF2–Nd2O3 Systems

Abstract

Fluoride–oxide composites 99NdF₃⋅1Nd₂O₃, 98NdF₃⋅2Nd₂O₃, 97NdF₃⋅3Nd₂O₃, 92NdF₃⋅5SrF₂⋅3Nd₂O₃, and 91NdF₃⋅5SrF₂⋅4Nd₂O₃ (mol %) have been fabricated in the NdF₃–Nd₂O₃ and NdF₃–SrF₂–Nd₂O₃ systems by the directional crystallization technique. It is found that the target tysonite-type Nd\(_{{1-x}}\)Sr_x(O,F)\(_{{3-\delta }}\) solid solution is partially decomposed upon cooling with segregation of nonstoichiometric modifications based on the NdO\(_{{1-x}}\)F\(_{{1 + 2x}}\) phase. The electrical conductivity of the composites has been analyzed by impedance spectroscopy in the temperature range of 294–522 K. The σ_dc values at 500 K are 5.2 × 10^–4, 6.5 × 10^–4, and 2 × 10^–3 S/cm for 99NdF₃⋅1Nd₂O₃, 97NdF₃⋅3Nd₂O₃, and 92NdF₃⋅5SrF₂⋅3Nd₂O₃, respectively; after doping of SrF₂ into the composite, the conductivity increases by a factor of 4 due to heterovalent substitutions of cations Sr²⁺ for Nd³⁺ and formation of anion vacancies. It is shown that cationic substitutions are much more efficient (in comparison with anionic substitutions) for the mechanism of ionic conduction in tysonite-type solid solutions. The σ_dc value of oxofluoride composite 92NdF₃⋅5SrF₂⋅3Nd₂O₃ is larger than that for fluoride composite 70NdF₃⋅30SrF₂ by a factor of 50 at room temperature. The composites under study can be applied as medium-temperature solid-state sensors for simultaneous detection of oxygen and fluorine.