Ionic conductivity of fluorite type crystals CaF2, SrF2, BaF2, and SrCl2 at high temperatures

The ionic conductivity of fluorite-type crystals (CaF2, SrF2, BaF2, and SrCl2) has been studied at high temperatures including the melting points. Represented in detail, the technique of measurements is based on the use of a capillary-type cell made of hexagonal boron nitride. Measurements were carried out on polycrystalline specimens obtained by the solidification of corresponding melts. A common feature of all fluorites is that the temperature dependences of conductivity in the superionic region are described by sigmoid-like curves in the Arrhenius plot. The behaviour of the apparent Arrhenius energy and molar conductivity in the superionic region as well as on melting crystals has been characterized. From comparative analysis of these data, it is suggested that the conductivity anomaly is mainly due to a non-uniform course of anion sublattice disordering in crystals up to melting points. The variation of transport properties is well correlated with the relative sizes of cations and anions in the fluorite lattice, that assumes a regularity in the degree of sublattice disorder in these solids near the melting points. (C) 2001 Elsevier Science Ltd. All rights reserved.