Optimization of the Compositions of Solid Electrolytes Cd1 – xRxF2 + x (R = La–Lu, Y) with a Fluorite-Type Structure for Conductivity and Thermal Stability

Abstract

Optimization of the compositions of Cd_1–xR_xF_2+x nonstoichiometric phases (CaF₂ type, R is a rare-earth element) for the ionic conductivity and thermal stability is based on the temperature measurements of the electrical conductivity of single crystals depending on the R³⁺ ionic radii and RF₃ content and on the investigation of phase diagrams of the CdF₂–RF₃ systems. It is shown that the conductivity of 20 compositions out of 30 studied Cd_1–xR_xF_2+x crystals exceeds the conditional limit σ_500K = 10^–5 S/cm, below which the use of solid electrolytes in electrochemical devices is believed to be undesirable because of low conductivity. The maximum conductivities σ_500K = (3.0–3.2) × 10^–4 S/cm and σ_293K = (1.5–2.3) × 10^–8 S/cm are observed for solid electrolytes with R = Ho, Er, Tm, or Yb and x = 0.22–0.27.