Ionic Conductivity Measurements of Barium Fluoride Single Crystals

Abstract

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].

Retarded ionic motion in flourites

“…Instead it has now been shown convincingly that this positb viation relates to a change from intrinsic conductivity by fluoride ion vacancic intrinsic conductivity by fluoride interstitials [29][30][31]37,40]. It should, how~ be pointed out that Carr et al [29] found evidence for slightly higher conduc ties in the region 1120-1250 K (= Tc) than can be modelled with the vacancy to interstitial conductivity change over. From the concentration data in table 1 it is apparent that vacancy-interstitial interactions [39] can be of importance around T c, and the cause of this deviation in the region 1120-1250 K.…”

“…1 indicate that the occurrence of a negative deviation from Arrhenius behavior is related with a critical mobility for the anion vacancies. The data in table 1 show its value to be about 2. been observed for SrF 2 [38] and BaF 2 [29,38] starting about 400 K below T for/3-PbF 2 starting at about 240 K below T c [7,17,31]. The upward curvatur, often been related with the broad specific heat anomaly, and hence the onset ordering of the anion sublattice.…”

“…Recently, Carr et al [29] and Figueroa et al [30] studied the ionic conductivity and self-diffusion in BaF 2. These studies led to activation enthalpies that are consistent with the data for fluoride ion vacancies reported by Barsis and Taylor [27].…”

Molecular dynamics simulation of the superionic conductor BaF2

“…8, presenting also the calculated values of D6, that the simulated DF is at least in qualitative agreement with experiment. 12,13 Activation energy of conductivity EQ depends on temperature and is E6 = 0.4 eV near the melting point. This value agrees with the value of activation energy of diffusion ED = 0.3 eV, obtained from MD -simulation.…”

PLENARY SESSIONMatter transport in ionic crystals

“…the enthalpies and entropies of defect formation, migration and association. Ionic conductivity runs should generally comprise something between 100 and 200 measurements taken from the precipitation temperature up to as close to the melting point as possible 161 although experimental difficulties caused by sublimation [7], deformation [8], or reaction of the crystal [9] may prevent the full exploitation of this ideal range. It is not feasible to obtain the same high density of data in diffusion measurements.…”