J. Schoonman

Conductivity enhancement in fluorite-structured Ba1−xLaxF2+x solid solutions

The ionic conductivity of single crystals of the fluorite-structured solid solutions Ba1−xLaxF2+x(10−3 <×<0.45) has been studied as a function of temperature and composition in the range 300–900 K. Three regions can be discerned in the concentration dependence of the ionic conductivity: a dilute concentration region (x<10−3), where classic relations between solute content and ionic conductivity hold; an intermediate concentration region (10−3<x≲5×10−2), where large changes occur in the conductivity activation enthalpy and the magnitude of the conductivity; and a concentrated solid solution region (x≳5×10−2) characterized by enhanced ionic motion. In the dilute region the migration enthalpy for interstitial fluoride ions is determined to be 0.714 eV, while a value of 0.39 eV is found for the (LaBaFi)X association enthalpy. The defect chemistry in the intermediate concentration region is shown to be controlled by a superlinear increase of the concentration of mobile defects, while in the concentrated solid solution region a composition-independent amount of ≈1 mole% of interstitial fluoride ions with enhanced mobility, carry the current.

Solid electrolyte properties of LaF3

The small-signal ac response of cells with LaF3 or the solid solutions La1-xBaxF3-x and ionically blocking electrodes has been measured in the frequency range 0.1-3 × 104Hz, and for temperatures from 220 to 650 K. The bulk electrolyte conductivity of LaF3 crystals is anisotropic up to 415 K. For polycrystalline samples grain boundary effects markedly influence the frequency dispersion. Above room temperature these effects mask the bulk electrolyte conductivity in the accessible frequency range. The composition dependence of the bulk electrolyte conductivity of La1-xBaxF3-x solid solutions is compared with 19F NMR data, and used to discuss recently reported association effects in LaF3. Reported anomalies in the electrical properties of LaF3 are used to exemplify the need to study the frequency dispersion over a wide frequency range.

Retarded ionic motion in flourites

Metals halides with the fluorite structure attain conductivity values typical of ionic melts far below their melting points, and also go through a second-order transition. Conductivity data for the fluorites are reviewed, and it is shown that the anion vacancies have a large and unique mobility value at the transition temperatures Tc. At Tc only small concentrations of defects are present, which strongly contradicts the concept of anion-sublattice melting. The conductivity mechanism above Tc is discussed in relation with the enhanced ionic motion model developed for concentrated anion-excess solid solutions based upon flourites. As a consequences of unusually large defect mobilities at Tc it is noted that retard ionic motion occurs above Tc.

Hebb—Wagner polarization of PbI2 with conducting iodine electrodes

Abstract The electrochemical cell I 2 /PbI 2 /C has been utilized to study electronic conduction in PbI 2 . Several iodine-rich mixtures of iodine and poly-2-vinylpyridine (P2VP) or 2-ethylpyridine (2EP) have been tested as iodine reversible electrodes. At equilibrium, under ion blocking conditions, PbI 2 exhibits electron-hole conduction. The polarization current-potential curves have been used to calculate specific electron-hole conductivities in PbI 2 . Deviations from the Wagner dc polarization theory have been observed and are explained. A reported critical potential model (17) to account for such deviations has been revised.

Disorder and electrical properties of fluorites

Abstract The defect structure of fluorite-type anion-excess solid solutions M 1−x RE x F 2+x is reviewed with emphasis on thermal depolarization, ionic conductivity, and to some extent on electronic conductivity and solid-electrolyte application. The composition dependence of the ionic conductivity as obtained from ITC and electrical conductivity studies reveals distinct changes in the conduction mechanism. Besides a discussion of these mechanisms, the influence of solute content on the Faraday transition is considered.

Conductivity enhancement in Ba1−xLaxF2+x solid electrolytes

Abstract The small-signal a.c. response of Ba1−xLaxF2+x solid solution crystals has been studied as a function of temperature and composition (10−3 ×10−2 enhanced ionic motion of a solute independent number of F-interstitials occurs. In addition, the dielectric response of concentrated solid solutions is determined by broad distributions of dipole-like relaxations. The mobility enhancement and the concept of a critical mobility offer a plausible explanation for the fall in the Faraday transition temperature in solid solution crystals.

The small-signal A.C. response of β-PbF2

Abstract The small-signal a.c. response of symmetrical cells with nominally pure or bismuth-doped β-PbF 2 as solid electrolyte and Pt or Au electrodes has been studied at controlled oxygen pressures over the frequency range 10 −4 Hz to 50kHz for temperatures ranging from 25°C to 450°C. The results indicate that oxygen is present as a mobile impurity in β-PbF 2 and in Pb 1−x Bi x F 2+x and participates in an electrode adsorption-reaction sequence. The data have been interpreted with the aid of three different, but related, equivalent circuits, one involving a constant-phase-angle element, the second a single finite-length Warburg element, and the third two finite-length Warburg elements. Nonlinear-least squares analysis of the complex admittance data yields the dependence of the circuit elements on temperature and oxygen pressure.

Small-signal ac response of Ba1−xLaxF2+x crystals

Abstract The small-signal ac response of single crystalline Ba 1− x La x F 2+ x solid solutions has been studied in the temperature region 300–820 K, and in the frequency range 4×10 −2 –5×10 4 Hz. At low temperatures the frequency dispersion is dominated by processes in the bulk. Apart from the bulk conductance these processes include a dielectric response determined by broad distributions of dipole-like relaxations. The increase of the static dielectric constant with increasing solute content is explained. At higher temperatures the frequency dispersion reflects interfacial phenomena. In addition to an almost ideal interface capacitance, phenomena are observed which indicate the growth of a surface layer due to a reaction between the electrolyte, and residual oxygen and/or water vapour from the ambient.

The small-signal ac response of La1−xBaxF3−x solid solutions

Abstract The small-signal ac response of single crystals of La 1− x Ba x F 3− x (0⩽ x ⩽0.104) with ionically blocking electrodes has been measured in the frequency range 10 −2 to 10 5 Hz, and for temperatures from 293 to 1300 K. At low and moderate temperatures the ac bulk response can be modeled with a Debye circuit. The circuit parameters do not have the temperature dependence associated with dipolar reorientation but rather the opposite, as is characteristic of the behavior of free charge carreirs. We have developed a model,in which the theoretically expected dependence of the circuit parameters is concordant to the behavior found experimentally. Thermally stimulated depolarization current (TSDC) spectra exhibit five dielectric relaxation peaks, in addition to a space-charge peak. Two peaks have been analyzed in detail. The reorientation enthalpies are concordant with the conduction activation enthalpies.

19F NMR and conductivity of Ba1−xLaxF2+x solid solutions

Abstract A combination of experimental techniques, viz., 19 F NMR relaxation, electrical conductivity, and Ionic Thermocurrent (ITC) has been employed to study the dynamical behavior of fluorine in Ba 1−x La x F 2+x (1×10 −3 19 F relaxation times T 1 and T 2 , ΔH(T i ), decrease with solute content. For large concentrations ΔH(T i ) values are much smaller than the ΔH(σ) values. ITC spectra of the concentrated solutions reveal a broad distribution of dipolar relaxations. Using fractional polarizations activation enthalpies for dipole reorientations in the range 0.15 to 0.45 eV are obtained. The dynamical behavior is described in terms of localized and diffusive F-interstitial jumps, the former being predominat in 19 F NMR relaxation and ITC.