J.C. Boivin

The dependence of polymorphism on the ionic radius of Ln3+ (LnLaEr, Y) in the oxide ion conductors Bi0.775Ln0.225O1.5

Abstract This paper describes the polymorphic transitions of Bi0.775Ln0.225O1.5 (LnLaEr and Y), which is a representative composition of a rhombohedral solid solution over a limited range inside the 20–30 mol% Ln2O3. Three kinds of polymorphs are found: a hexagonal low-temperature modification (β2) isomorphous with Bi0.765Sr0.235O1.383 (BiSrO) type, a hexagonal intermediate (β1), and the well known high-temperature modification (δ) isomorphous with the fluorite type. The way the β2 phase behaves on heating can be divided into two groups depending on the Ln3+ ion: (1) one or two reversible transitions from β2 to β1, or from β2 to β1, then from β1 to δ for La, Pr, Nd, Sm, Eu and Gd; and (2) an irreversible transition from β2 to δ for the others. The thermal behaviour in group (1) closely resembles the BiSrO-type solid solution in the systems Bi2O3MO (MCa, Sr and Ba).

Electrical and structural investigations on a new bismuth lead vanadium oxide solid electrolyte

Three types of solid solutions derived from Bi4V2O11 have been prepared and characterized: Bi2V1−xPbxO (11−3x)2 with x up to 0.10, Bi2−yPbyVO(11−y2with y up to 0.20 and Bi9Pb0.1+xV1−xO(10.9−3x)/ 2 with x up to 0.12. A single-crystal X-ray structure determination was performed on a sample corresponding to the formula Bi1.9Pb0.2V0.9O5.3. The material was shown to be isostructural with BICUVOX.10 (γ-Bi4V2O11). The ability of substitution of V by Pb was confirmed. Conductivity measurements led to values of σ close to that of BICUVOX. 10 at high temperature, but to significantly lower performances at low temperature. This was related to a decrease of the stability of the γ form in the lead-substituted phases.

Anionic conduction properties of BiCaPb mixed oxides

Abstract This study of the Bi 2 O 3 CaOPbO system has determined the equilibrium domains of existence of three solid solutions which belong respectively to the fcc (δ-Bi 2 O 3 ), rhombohedral (BiSrO) and bcc (anti-α-AgI) structural types. The first two solid solutions can be preserved at room temperature in a metastable or/and a stable form within a significant range of composition. They have been investigated by impedance spectroscopy and transport number measurements. In all cases, the substitution of Bi 3+ by the divalent cations leads to a decrease of the conductivity. However, for a constant M 2+ concentration, the conductivity increases slightly with the Pb 2+ /Ca 2+ ratio without significant modification of the activation energy. Transport number measurements, performed particularly on the δ-phase, have shown, at low temperature (≤400 °C), a very attractive quick response to any P O2 variation. The corresponding Nernst potentials were shown to be significantly higher than expected from the classical O 2 /O 2- electrode equilibrium.

Characterisation of the electrode-electrolyte BIMEVOX system for oxygen separation. Part I. In situ synchrotron study

Abstract BIMEVOX membranes for electrochemical oxygen separation were characterised under operating conditions using X-ray synchrotron radiation. Three compositions were studied: a slightly Bi-enriched non-doped phase, BIBIVOX.02, a BICOVOX and a BICUVOX phase. Both anodic and cathodic behaviours were investigated at 620 °C for current densities up to 1 A/cm 2 . These experiments demonstrated the self-ability of these electrolytes to dynamically transform into electrode materials under polarisation. This transformation is due to a slight reduction of the material under bias. The transformation is reversible. The initial phase was recovered when the current was turned off. Even low current densities were sufficient to induce this electrolyte–electrode adaptation.

Anionic conduction properties of the fluorite-type phase in the Bi2O3-Y2O3-PbO system

Abstract The Bi2O3-Y2O3-PbO system study has evidenced a fluorite structure phase easily air-quenchable in a large domain of composition. The investigation of its thermal behaviour by X-ray diffraction, impedance spectroscopy and transport number measurements has shown the existence of two modifications of this fluorite phase, both of which having high oxide conductivities. The isothermal conductivity decreases when Bi3+ is substituted by Pb2+ (or Y3+) but increases as Pb Y+Pb ratio also increases.

Thermal dependence of conduction properties in the Bi2O3Pb2OF2 system

Abstract Conduction properties of bismuth-lead oxyfluorides (Bi,Pb) 2 (O,F) 3 have been investigated by complex impedance analysis. A special attention has been drawn to a hexagonal solid solution Bi (1− x ) Pb x O (1.5− x ) F x (0.45⩽ x ⩽0.741 at 770 K), which is isostructural with ALa 2 O 3 . Conductivity values range from 3 × 10 −5 to 3 × 10 −2 (ω cm) −1 at 585 K. A minimum associated with a maximum in activation energy is observed for the composition x = 0.5 i.e. BiPbO 2 F.

A new multiple perovskite family Sr3(Bi1−ySby)O5.5−δ(0≤y≤1)

Abstract The solid state reaction of an Sb 2 O 3 -SrCO 3 mixture (Sr/Sb=3/1) gives a mixed valency antinomy compound that can be written Sr 3 SbO 5.5−δ . Several structural modifications have been characterized by high temperature X-ray diffraction, TGA and DSC. The room temperature form is isostructural to (β)Sr 3 BiO 5.5 ; a transformation into a cubic 2ap×2ap×2ap superstructure is noted between 285 and 350°C. Over 350°C, the tetragonal pattern is isostructural to (α)Sr 3 BiO 5.5 , with a=8.460(2) A , c=16.493(6) A - both forms correspond to a maximum Sb v content. At 440°C the β form is again obtained. When annealed at 950°C, it yields a multiple perovskite superlattice. We have identified two solid solution domains Sr 3 (Bi 1− y Sb y )O 5.5− δ : for 0≤ y ≤0.25 the Sr 3 BiO 5.5 structural evolution versus temperature is kept and a cubic perovskite superlattice is stabilized for 0.3≤ y

Evidence of a new bismuth vanadium zinc oxyfluoride with the BIMEVOX structure

Abstract Substitution of oxygen by fluorine in a bismuth vanadium zinc oxide belonging to the BIMEVOX family has allowed the synthesis of a mixed anion (O, F) phase which can be considered as the first member of a new BIIMEVOF family. As for the corresponding BIZNVOX phase, three different forms (α, β and γ) have been identified, depending upon the composition and the temperature. The high temperature γ form can be stabilized down to room temperature for a lower Zn content than in the oxide derivatives. X-ray diffraction, thermal analysis and density measurements have been used to confirm that fluorine partly substitutes oxygen leading to the formation of the solid solution: Bi 2 V 1− x − y Zn x Bi y O 5.5−2.5 x − y F 2 x . The evolution of electrical conductivity with fluorine ratio has been investigated by impedance spectroscopy.

Phases stability and electrical properties in the Bi2O3CdOPbF2 system

Abstract The Bi 2 O 3 CdOPbF 2 system has been investigated at temperatures between 600 and 750°C by quenching from the melted Two solid solutions have been evidenced corresponding respectively to the anti α-AgI (I) and ALa 2 O 3 (H) structural types. A significant vitreous domain (G) has also been delimited. Among the two solid solutions, the H phase exhibits the highest σ value (σ=6×10 −5 S cm −1 for Bi 0.580 Cd 0.497 Pb 0.923 O 1.367 F 1.846 at 170°C). Conductivity measurements in the vitreous phase lead to σ values close to 3×10 −4 S cm −1 at 170°C.