J. Senegas

F− ion conductivity and diffusion properties in ZrF4-based fluoride glasses with various NaF concentrations (0 ⩽ xNaF ⩽ 0.45)

Abstract Selecting two series of ZrF 4 -based fluoride glasses containing at the same time the BaF 2 and NaF network modifiers and a third series with NaF only, an investigation of electrical and diffusion properties has been undertaken as a function of x NaF in a wide concentration domain (0 ⩽ x NaF ⩽ 0.45). The ac conductivity data have been analysed by the complex modulus formalism and a hopping mechanism for transport in these materials is suggested. The 23 Na and 19 F NMR investigations have shown that Na + ions do not participate as charge carriers in the conduction mechanisms and the diffusion properties are due only to motions of mobile F − ions. Mobility and number of F − ions mobile in the NMR time scale increase when x BaF 2 increases, result in agreement with the composition dependence of electrical properties at long range. Experimental points representative of log σ 473 K and ΔE σ as a function of x NaF for the glasses studied are located on both sides of “master” curves involving a conductivity minimum and an activation energy maximum for x NaF ∼- 0.30. Variation of the decoupling index, R τ ( T g ), with x NaF is in agreement with that of σ 473 K in the whole of the studied composition. On the contrary, the variation of the Kohlrausch parameter, β, is more difficult to analyse and may be explained by the coupling model only for x NaF ≲ 0.15. The variation of various electrical parameters is discussed as a function of glass composition.

Ionic conductvity and NMR investigation of quaternary glasses in the ZrF4BaF2ThF4LiF system

Abstract The existence of glasses involving large amounts of LiF (up to 60%) within the ZrF4BaF2ThF4LiF quaternary systems has allowed the authors to study the evolution of transport properties with varying LiF content. A minimum of ionic conductivity bound to a maximum of activation energy has been detected when the atomic Li/F ratio is equal to ≅ 0.07. In the Li-low concentration domain, σ increases regularly and ΔE decreases simultaneously when the BaF2 concentration increases; on the contrary in the Li-high concentration region log σ and ΔE are quasi-linear functions, increasing and decreasing respectively, of the LiF rate. A 7Li and 19F NMR investigation has shown that Li+ and F− ions are simultaneously mobile and the temperature dependence of the number of mobile F− ions has been determined. In the Li-low concentration domain transport properties result from mixed contributions of mobile Li+ and F− ions, for high Li concentrations they depends only on the Li+ rate. Glasses with high Li-content have good electrical performnces (e.g. σ175°C ≅ 2.10−4ω−1 cm−1 for Zr0.20Ba0.10Li0.60Th0.10F2).

L'oxyde double Fe2WO6. I. Structure cristalline et filiation structurale

Abstract The authors have found a new structural type, related to α-PbO2, called tri-α-PbO2. The oxide Fe2WO6 is the prototype. It crystallizes in the orthorhombic system with the following cell parameters: a = 4.576 A, b = 16.766 A, and c = 4.967A. The space group is Pbcn. The structure has been determined by X-ray single-crystal methods and refined by least-squares procedures (R = 0.065). The structure consists of zig-zag chains parallel to the c-axis. Each such chain is built up by MO6 (M = Fe or W) octahedra-sharing edges. The chains are linked together by corner sharing. There are two types of chains: one containing only iron atoms, the other being an ordered 1-1 arrangement of iron and tungsten atoms.

Transport properties of ZrF4BaF2LaF3AF (A = Li, Na) glasses

A comparative study of electrical properties of two series of glasses ZrF4(0.62−0.4x)BaF2(0.30−0.5x)LaF3(0.08−0.1x)AF(x) (A = Li, Na) (0 < x < 0.30) has been carried out as a function of AF content. A minimum of ionic conductivity and a maximum of activation energy has been detected when the atomic Li/F ratio is equal to ≅ 0.07. By analogy with the glasses belonging to ZrF4BaF2ThF4LiF system, it is proposed that in the low concentration Li domain Li/F ⪆ 0.07) , they properties result from mixed contributions of mobile Li+ and F− ions and for high Li concentrations (Li/F ⪆ 0.07), they depend only on the Li+ rate. When A = Na, on the contrary, conductivity decreases regularly and activation energy increases with increasing x. The electrical properties result only from the contribution of mobile F− ions. A 23Na NMR study has shown that Na+ ions do not participate in the conductivity.

Alkali fluoride containing fluorozirconate glasses: Electrical properties and NMR investigation

Abstract Electrical properties of glasses inside the ZrF4BaF2ThF4LiF system have been studied as a function of composition by conductivity and NMR measurements. Two series of materials can be distinguished. The transport properties of highly concentrated Li+ glasses seem only to result from the Li+ concentration and to increase with that concentration. On the contrary, a mixed contribution of F− and Li+ ions must be considered for the low Li+ concentration glasses. On the other hand, the modifier cations (Ba2+ at low concentrations of lithium, Li+ at high ones) can greatly influence the transport properties. Glasses with a high Li content exhibit good electrical performance (e.g., σ 175° C ⋍ 2 × 10 −4 Ω −1 cm −1 for the glass of composition Zr0.20Ba0.10Li0.60Th0.10F2).

Determination of the influence of the NaF rate on the charge carriers parameters in some fluoride glasses containing the NaF alkali fluoride

Abstract The bulk impedance parameters of various fluoride glasses series containing a large extent in NaF rate have been determined from an analysis of ac conductivity data measured in a wide temperature range. The charge carriers concentration in each glass studied has been evaluated using the Almond-West formalism and shown to be temperature independent. Transport properties in these materials are due to a hopping mechanism of the fraction of F − ions mobile at long range (3–5%). The variation of different conductivity parameters as a function of the NaF content has been determined. When the NaF rate increases, a conductivity decrease is observed, due mainly to a decrease of the charge carriers mobility.

Evidence of monodimensional clusters in some fluorite-type anion-excess solid solutions: Correlation between vacancies and interstitial fluorine anions

Abstract The formation of two-file monodimensional 2 n : 2: 3 n 2 and 2 n : 2: 4 n : 2 clusters is proposed to account for the fluorine ion conductivity of the Pb 1− x In x F 2 + x and Pb 1 − x Zr x F 2 + 2 x solid solutions on hand of a structural correlation model. The clusters are based on the association of 2 n + 2 vacancies in the normal positions of the fluorite-type structure with 3 n (or 4 n ) F′ and 2F″ interstitial anions, they are compared with the single-file monodimensional cluster n + 1: 2 n : 1 found for the analogous Pb 1 − x Bi x F 2 + x series. Correlations are established on one hand between interstitial F′ ions and vacancies within clusters, on the other hand between interstitial F″ ions and vacancies located on the cluster boundaries. The electrical properties are discussed as a function of concerned clusters for the materials of highest performance in each solid solution.

Corrélations entre propriétés électriques et ordre à courte distance au sein des solutions solides Ca1−xYxF2+x et Ca1−xLuxF2+x

Abstract A model has been proposed to correlate the composition dependence of electrical properties and the progressive cluster growth with increasing x in a fluoride anion excess CaF2-type solid-solution of M2+1−xM′2+αxF2+αx (α = 1,2,3). According to the model, the density of interstitial fluorine ions and the density of vacancies at normal sites are respectively represented by the functions yint. and y□ which depend on three parameters called λ, m, and xe: Y int. = (λ−α)mx 3 +λ(m−α)x 2 e (λ−α)x 2 +(m−α)x 2 e Y □ = (λ−α)(m−α)(x 3 +x 2 e x (λ−α)x 2 +(m−α)x 2 e The λ and m parameters respectively define the clustering conditions for the lowest and highest values of x. The xe parameter is the degree of substitution for which a maximum of conductivity in the composition dependence of the electrical properties is encountered. This model, applied to the solid-solutions Ca1−xYxF2+x (0 ⩽ x ⩽ 0.38) and Ca1−xLuxF2+x (0 ⩽ x ⩽ 0.40), shows the progressive transformation of 4:4:3 monomer clusters into cubooctahedral 8:12:1 clusters when x increases. The percentage of cubooctahedral clusters in these solid-solutions quenched from 950°C has been determined as a function of composition.

Short-range order and diffusion processes in the Na1-xBixF1+2x anion-excess solid solution

Abstract The F − diffusion processes within the anion-excess solid solution Na 1− x Bi x F 1+2 x with fluorite-type structure have been studied by 19 F-NMR investigation. Two types of fluoride ions, F n and F′, are distinguished in the 19 F-NMR spectra of rigid lattice at low temperature ( T =175 K) and the distribution of fluoride ions among the two sites is determined by deconvolution of the experimental spectra. The short-range order in these materials is analogous to the clustering process observed in the K 1− x Bi x F 1+2 x solid solution. It corresponds to an order between cubic entities and cubooctaedral clusters. Two diffusion mechanisms are identified on both sides of a T r temperature. The first one at T T r corresponds to an F − ions exchange inside cubic or cubooctaedral clusters independently, and the second at T > T r to an F − exchange between cubic and cubooctaedral clusters.

Correlations entre proprietes electriques et ordre a courte distance au sein de la solution solide Pb1−xBixF2+x

Abstract The existence of single-file monodimensional clusters n +1 : 2 n : 1, the length of which grows when x increases, is proposed for the solid solution Pb 1− x Bi x F 2+ x (0 ⩽ x ⩽ 0.50). These clusters are based on the association of ( n + 1)vacancies in the normal positions of the fluorite structure, 2 n F ′ and 1 F ″ interstitial anions. The formation of clusters containing n =3 trivalent substitutional cations is shown for the value x e =0.236 of substitution rate, close to x max ⋍0.25 , value for which Pb 1− x Bi x F 2+ x has the best electrical performance. Very extended clusters containing n =10 monomer clusters are proposed for the upper limit Pb 0.50 Bi 0.50 F 2.50 .