J.M. Reau

Electrical properties of glasses in the Na2O–MoO3–P2O5 system

Abstract A range of coloured electronic or mixed ionic–electronic glasses has been evidenced in the Na 2 O–MoO 3 –P 2 O 5 system. The properties of these glasses have been studied along different composition lines corresponding either to a fixed Na 2 O content or a constant Mo/P ratio. An EPR spectroscopy investigation of these glasses has allowed to determine the Mo 5+ ion percentages in these materials. The electrical properties of these glasses have been studied by impedance spectroscopy, and the electronic and ionic contributions have been evaluated. The properties of these sodium glasses have been compared with those of lithium glasses with the same compositions.

Etude par diffraction neutronique des solutions solides K1−xBixF1+2xetRb1−xBixF1+2x

Abstract Bragg neutron diffraction studies have been carried out on the fluorite type solid solutions K 1− x Bi x F 1+2 x (0.50 ⩽ × ⩽ 0.70) and Rb 1− x Bi x F 1+2 x (0.50 ⩽ × ⩽ 0.60). The distribution of the fluorine atoms between normal and interstitial sites is given as a function of substitution rate. A substition mechanism is proposed. Electrical and NMR results on one side and structural data on the other side are correlated. A study of the background as a function of temperature has allowed to determine the static origin of its modulation. By inelastic neutron diffusion, it has been shown that the number of carriers is weak, which involves a high mobility.

New Li+ ferroelectric conductors with tetragonal tungsten bronze structure

Abstract The evolution of conductivity in tetragonal tungsten bronze (TTB) oxides belonging to the Pb 5(1− x ) Li 10 x Ta 10 O 30 solid solution was studied using two techniques: a.c. complex impedance spectroscopy and the four-probe method. The results obtained from both techniques allowed the correlation of the evolution of conductivity with composition. Indeed, the conductivity mainly appears to be governed by the total number of Li + ions in the structure. Furthermore, it seems that when the number of Li + ions in the structure is small, two separated regions appear: • a low temperature region (LT), characterized by activation energy Δ E LT , a very small number of mobile ions and a low value of the prefactor σ 0 ; • a high temperature region (HT), characterized by activation energy Δ E HT (Δ E HT >Δ E LT ), a larger number of charge carriers and a higher value of the prefactor σ 0 . nThe existence of these two regions is explained in terms of extrinsic–intrinsic behavior. Dielectric measurements were also performed, with the substitution 2Li + –Pb 2+ leading to a decrease of the Curie temperature which is explained in terms of chemical bonding and antagonist effects.

Relaxations in new ferroelectric tantalates with tetragonal tungsten bronze structure

Abstract Ceramics belonging to the Pb5xK6(1−x)Li4(1−x)Ta10O30 solid solution were prepared and characterized by dielectric and a. c. impedance complex measurements. Two types of relaxations were observed: a relaxor behavior and an impedance relaxation. Characteristics of these two relaxations were studied and explained for a particular composition Pb3.75K1.50LiTa10O30.

Advances on fluorine ion conductors, basic and applied research

Abstract Fluorides which are insulators, often possess a high anionic conductivity due to weak chemical bonding of the F - ion. The examination of the known ionic conductors allows us to point out some general criteria characteristic of high mobility of F - . The mechanisms of conduction determined on the basis of neutron diffraction are described. The possibility of utilization of such materials for thermodynamic measurements, specific electrodes, solid state batteries, electrochromic devices and gas sensors are examined.

The cubo-octahedral cluster in the fluorite-type lattice: A theoretical approach

Abstract A cubo-octahedral cluster within the anionic sublattice is proposed as an extended defect in the AM 3 F 10 phases ( A = K, Rb; M = Y, Bi). A computer simulation technique is used starting from crystallographic data to determine their validity and to evaluate some physical properties. A simulation study of new point defects in alkaline-earth fluorides doped with tetravalent cations is developed. The cubo-octahedral cluster is shown to be stable when it is introduced within the fluorite lattice. A mechanism is proposed for its formation on the basis of the aggregation of simpler defects.

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.

Structural and electrical studies of the PbF2-BiOF system

Abstract An investigation of the PbF 2 -BiOF system at 600°C has allowed isolation of a Pb 1−x Bi x O x F 2−x solid solution (0 1−x Bi x O x F 2−x are for x ⩾ 0.50 as good as those of the best fluorides.

On a low-temperature form of KBiF4

Abstract The existence of a low-temperature α form of KBiF 4 has been shown by differential thermal analysis and microcalorimetry as well as by electrical measurements. Properties of α-KBiF 4 are compared to those of K 1− x Bi x F 2 x +1 (0.50 ≤ x ≤ 0.70) with fluorite-type structure and KBi 3 F 10 .

Anionic conductivity of some bismuth fluorides with fluorite-type structure

Abstract The transport properties of fluorite-type Na 1−x Bi x F 1+2x (0.60⩽ x ⩽0.70), K 1−x Bi x F 2+x (0.50⩽ x ⩽0.70), Rb 1−x Bi x F 1+2x (0.50⩽ x ⩽0.60) and Pb 1−x Bi x F 2+x (0⩽ x ⩽0.50) solid solutions have been studied. They are all anionic superconductors. Some of them could be used as electrolytes in solid galvanic cells.