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R6sumB. -Nous avons mesure la conductivite avec une tension alternative des monocristaux de BaF2 jusqu'a 1 336 K. Aux temperatures plus 6levCes nous avons observe les effets de la polarization, peut-&re le rbultat d'une reaction des cristaux avec I'atmosphere dans le dispositif. Les energies des defauts derives du domaine de tempbature moins eleve s'accordent bien avec ceux que Barsis et Taylor 127, 281 ont obtenu. L'effet de la transition de phase aux hautes temperatures sur la conductivitb est discute.Abstract. -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 F-ion vacancies and interstitial F-ions 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].

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