Ionic Conductivity Measurements of Barium Fluoride Single Crystals

We have measured the a. c. conductivity of pure BaFz single crystals up to 1 336 K. At the higher temperatures we observed polarisation effects possibly due to a reaction of the crystal with the atmosphere in the cell. The defect energies derived from the low temperature region agree well with those obtained by Barsis and Taylor [27, 281. The effect of the high temperature phase transition on the conductivity is discussed. Introduction. Investigations of the alkaline earth fluorides, CaF,, SrF,, BaF,, have shown that the Fion has a relatively high mobility and the predominant point defects are of the anti-Frenkel type [I]. The relative concentrations of Fion vacancies and interstitial Fions can be altered in the usual way by doping with aliovalent ions. An unusual feature of these materials, and this appears to be a peculiarity of compounds with the fluorite structure, is that they will tolerate high concentrations of interstitial anions. Thus trivalent metal fluorides can be dissolved in them in large quantities (up to 50 mole per cent at the melting point [2]), the cation being incorporated substitutionally and the excess anions interstitially. Another special feature of the fluorite compounds is that they exhibit a broad specific heat anomaly at high temperatures which is similar to the lambda-type behaviour associated with order-disorder transitions [3, 41. This has been interpreted as a transition from the normal fluorite lattice, AB,, at low temperatures to a disordered state in which the 3 ion is distributed between normal anion and interstitial sites [3]. The transition has also been investigated by light-scattering [5] and neutron diffraction [6] techniques. One cause of fast ion conduction in solids, sometimes termed superionicity, is a highly disordered lattice [7]. Thus there is interest in the fluorites as model systems in which to study this phenomenon [8]. Information on ionic transport, i. e. defect formation energies, migration energies and diffusion mechanisms, is usually obtained from measurements of the ionic conductivity and self-diffusion in single crystals over a wide temperature range [9-111. For the alkaline earth fluorides a major problem in conductivity measurements is the high reactivity with oxygen and water vapour [12, 131. Conductivity measurements on alkaline earth fluorides are also hampered by two additional factors ; the sample resistance is low and the sublimation rate is high. On the whole the quality of the reported conductivity for the alkaline earth fluorides has not been sufficiently good to use the computer-fitting methods of obtaining defect parameters developed for the alkali halides (see for example reference [14]). Self-diffusion measurements of Fions in alkaline earth fluorides are beset by special difficulties. The radio-isotope 18F has an inconveniently short half-life of 1.7 hours and has only been used in one study of these materials [15]. Another technique for studying diffusion is nuclear magnetic resonance N. M. R. relaxation time measurements. The potential of this technique has been increased by recent theoretical developments in the methods of evaluating precise diffusion coefficients from the measured relaxation times [16]. However, as the recent study of BaF, indicated there are still problems of detail with the N. M. R. which have to be resolved 1171. The N. M. R. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976778 C7-338 V. M, CARR, A. V. CHADW ICK AND D. R. FlGUEROA activation energies in alkaline earth fluorides doped with paramagnetic trivalent metal fluorides are unreliable 118, 19, 201. The pulsed field gradient N. M. R. measurements of Fdiffusion in BaF, reported at this conference shows great promise for the study of alkaline earth fluorides [21]. The specific heat anomaly due to the transition peaks at about 1 235 K in BaF, [4] and there has only been one conductivity study that exceeds this temperature [22]. Conductivity studies have been made across the transition in CaF,, SrF, [23] and in two other materials with the fluorite structure, SrCl, [23] and PbF, 1241. We have extended our earlier conductivity measurements in BaF, to 1 336 K in an attempt to obtain precise results in the region of the transition. The results of these experiments and some of the experimental difficulties we encountered are reported in this