A COMPARATIVE ANALYSIS OF IONIC CONDUCTIVITY OF THREE SOLID PHASES IN THE SYSTEM CaF-HoF

A comparative analysis of ion transfer mechanisms for crystals of three phases with the structures of fluorite (F-phase, CaF and solid solution CaHoF), tysonite (T-phase, solid solution HoCaF) and orthorhombic modification b-YF (R-phase, HoF) in the condensed system CaF - HoF was carried out. Based on the fundamental data on ionic conductivity s(T), obtained using single-crystal samples in experiments, the dependences "ionic conductivity - composition" and "enthalpy of activation ion transfer - composition" are constructed. A comparison of the properties of the components of the system under study shows that the conductivity of the R-phase of HoF (s = 5 × 10 S/cm at 500 K) exceeds the conductivity of the F-phase of the stoichiometric composition CaF by 5 orders of magnitude. In the region of the F-phase of non-stoichiometric composition CaHoF (0 < x ≤ 0.35) the interstitial mechanism of electrical conductivity is realized. With increasing concentration of HoF the values s increase, reaching 4 × 10 S/cm at x = 0.35. The conductivity of the non-stoichiometric T-phase HoCaF (y = 1-x, x = 0.77) s = 2 × 10 S/cm exceeds the electrical conductivity of the crystals of the F-phase CaHoF and R-phase HoF by 5 and 40 times, respectively. The reasons for the rapid anionic transfer in the non-stoichiometric T-phase are the vacancy mechanism of electrical conductivity and the wide heterovalent isomorphism of cations.