M. El Omari

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.

Etude des propriétes de conduction ionique des solutions solides ca1-xThxF2+2x et Ca1-xUxF2+2x

Abstract A comparative study of the ionic conductivity properties of the solid solutions Ca1−xThxF2+2x (0 ≤ x ≤ 0.18) and Ca1−xUxF2+2x (0 ≤ x ≤ 0.19) has been undertaken and correlations between the electrical properties and the short-range order within these phases have been established. The model of the processes of clustering proposed for the anion excess CaF2-type solid solutions M2+1−xM′2+αxF2+αx (α = 1, 2, 3) has been applied to the solid solutions Ca1−xM″xF2+2x (M″ =Th, U). The formation of the 1:0:3 clusters is confirmed when x increases in the composition domain (0.01 ≲ x ≤ xL) (xL: upper limit of substitution rate). Two sublattices of vacancies are located inside the 1:0:3 clusters (y1) and in the immediate neighborhood (y2); these are related respectively to the F″ and F‴ interstitial fluoride anions. The Ca1−xUxF2+2x phase, involving the smallest tetravalent cation, has a larger number of vacancies y2 and weaker space constraints for the same substitution rate. It results in better electrical properties for that solid solution than for Ca1−xThxF2+2x. The solid solutions Ca1−xM″xF2+2x (M″ =Th, U) are characterized by the same process of clustering as the solid solutions Ca1−xLnxF2+x (Ln =La, Nd, Gd) involving rare earth substitutional cations of large size.

Investigation by impedance and 19F-NMR spectroscopies of a new oxyfluoride solid solution range in the NaF–BiF3–Bi2O3 system

Abstract A wide range of fluorite-type oxyfluoride solid solutions is exhibited in the NaF–BiF 3 –Bi 2 O 3 system. Starting from the entirely fluoride Na 0.5(1− z ) Bi 0.5(1+ z ) F (2+ z ) solid solution along the NaF–BiF 3 line, the range is extended towards BiOF and the various range compositions can be formulated Na 0.5(1− z − h ) Bi 0.5(1+ z + h ) F 2+ z − h O h . The transport properties were studied as a function of the anion excess ( z ) and the oxygen content ( h ). Correlations were established between F − ion motions at long range determined by impedance spectroscopy and motions visualized over the NMR scale time.

New anion-conducting solid solutions in the NaF–MF3–M2O3 (M=Nd, Bi) systems

Abstract New fluorite-related solid solutions have been prepared by solid state reaction of NaF, MF 3 and M 2 O 3 (M=Nd, Bi). The dependences of cubic lattice parameter and ionic conductivity versus composition are determined. For the NdOF-based solid solution, the hypothesis of anion conductivity has been supported by measurements of ionic transport number by the emf method. The influence of the electronic structure of trivalent cations with different electronic configurations on the composition and electrical properties of these new phases is discussed.

The nature of anionic clusters in anion excess fluorite-type oxidefluoride solid solutions in MF–BiF3–BiOF systems (M = Na, K)

Abstract Various compositions of the range of fluorite-type oxidefluoride solid solution in the NaF–BiF3–BiOF system are investigated by 19 F NMR spectroscopy. Using the following formulation, Na0.5(1−z−h)Bi0.5(1+z+h)F2+z−hOh, this study is undertaken as a function of the anion excess (z) and the oxygen content (h). It results in the existence of two fluoride sublattices in the solid solution range. An approach to the nature of the fluorine–oxygen order is proposed when the compositions become richer and richer in BiOF. The clustering model is then extended to the homologous potassium oxidefluoride solid solutions.

Processus de clustérization au sein des solutions solides Ca1−xLnx F2+x (Ln=La, Nd, Gd) comportant un substituant trivalent lanthanidique de grande taille

Abstract The model of the processes of clusterization proposed for the anion-excess CaF2-type solid solutions M1−x2+M′x2+αF2+αx (α=1, 2, 3) has been applied to the solid solutions Ca1−x LnxF2+x (Ln=La, Nd, Gd) involving rare earth substitutional cations of a large size. The formation of 1:0:3 clusters when x increases in the composition domain (0.02⩽x⩽0.42) has been confirmed. Two sublattices of vacancies located inside the clusters (y1) and in the immediate neighbourhood (y2) are related respectively to the F″ and F‴ interstitial fluoride ions. The Ca1−xGdxF2+x phase characterized by the existence of the 1:0:3 clusters and the presence of the smallest substitute, has, for a given value of x, the highest percentage of vacancies y2 and the weakest space constraints. It results in the best electrical performance for that solid solution.

Corre´lations entre proprie´te´se´lectriques et structurales de solutions solides de type fluorinea`exce`s d'anions: Essai de mode`lisation

Abstract In a fluoride ion excess CaF 2 -type solid solution of formulation M 2+ 1−x M′ 2+α x F 2+αx (α = 1, 2, 3) , the sum of the interstitial fluoride ions n F int and the sum of the vacancies in normal sites n □ can be represented according to general equations y = (mx 3 + λqx)(x 2 + q) : the m and λ parameters depend on structural features, and q is bound to electrical properties. This model allows one to correlate in a continuous way the composition dependence of electrical properties and the progressive extension of clusterization when the substitution rate increases.

Evolution du processus de clusterisation au sein de la solution solide Pb1−xZrxF2+2x

Abstract The formation of double-file monodimensional clusters 2n+2:4n:2 within the solid solution Pb 1−x Zr x F 2+2x (0≤x≤0.18), which become more and more extended when x increases, is proposed from a study of correlations between structural and electrical properties. These clusters, based on the association of (2n+2) anionic vacancies in the normal positions of the fluorite structure, 4n F′ and 2 F″ interstitial anions, offer a close analogy with the clusters 2n+2:3n:2 shown in the solid solution Pb 1−x In x F 2+x (0≤x≤0.25). In both these series of materials, clusters of same size containing n=3 substitutional cations characterize the compositions which have the best electrical performance.

Phase transitions and ionic transport in K3InF6

Abstract Four allotropic forms are shown for the K 3 InF 6 ternary fluoride by various experimental investigations, microcalorimetry, high temperature X-ray diffraction, optical and electrical measurements. The ac conductivity data of K 3 InF 6 are analysed in the Z *, M * and e * formalisms and the conductivity relaxation parameters are determined. Conductivity is of Arrhenius-type on either side of a transition temperature T tr ( T tr =488±10 K) which is very close to the Curie temperature T C ( T C =480±5 K). A conductivity jump of about one decade is observed at T C and that transition is of ionic-superionic type.