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 Ca1–xHoxF2+x solid solution), a tysonite phase (the T-phase, Ho1–yCayF3–y solid solution), and a phase having the orthorhombic β-YF3 structure (the R-phase, HoF3). The ionic conductivity σdc(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 (σ500 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 Ca1–xHoxF2 + x (0 < x ≤ 0.35) nonstoichiometric F-phase, the interstitial mechanism of electrical conductivity occurs. The σ500 K increases as the HoF3 concentration increases to reach 4 × 10−5 S/cm at x = 0.35. The Ho1–yCayF3–y (y = 1 − x, x = 0.77) nonstoichiometric T-phase has σ500 K = 2 × 10−4 S/cm, which is five and 40 times as high as the electrical conductivity of the Ca0.65Ho0.35F2.35 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.