Ionic Conductivity of the Three Solid Phases in the CaF2–HoF3 System: A Comparative Analysis

A comparative analysis of ion transport mechanisms in crystals was carried out for three phases formed in the CaF₂–HoF₃ condensed system: a fluorite phase (the F-phase, CaF₂ and Ca_1–xHo_xF_2+x solid solution), a tysonite phase (the T-phase, Ho_1–yCa_yF_3–y solid solution), and a phase having the orthorhombic β-YF₃ structure (the R-phase, HoF₃). The ionic conductivity σ_dc(T) fundamental data gained in experiments on single-crystal samples was used to derive ionic conductivity versus composition and activation enthalpy of ion transfer versus composition dependences. A comparison of the properties of the components of the system under study shows that the conductivity of the HoF₃ R-phase (σ_{500 K} = 5 × 10⁻⁶ S/cm at 500 K) is five orders of magnitude that of the stoichiometric CaF₂ F-phase. In the region of the Ca_1–xHo_xF_{2 + x} (0 < x ≤ 0.35) nonstoichiometric F-phase, the interstitial mechanism of electrical conductivity occurs. The σ_{500 K} increases as the HoF₃ concentration increases to reach 4 × 10⁻⁵ S/cm at x = 0.35. The Ho_1–yCa_yF_3–y (y = 1 − x, x = 0.77) nonstoichiometric T-phase has σ_{500 K} = 2 × 10⁻⁴ S/cm, which is five and 40 times as high as the electrical conductivity of the Ca_0.65Ho_0.35F_2.35 F-phase and HoF₃ R-phase, respectively. The reasons for the rapid anionic transport in the nonstoichiometric T-phase are the ion vacancy electrical conductivity and extensive heterovalent isomorphism of cations.