R.N. Vannier

Composition dependence of oxide anion conduction in the BIMEVOX family

Abstract Partial substitution for vanadium in Bi 4 V 2 O 11 can lead to the stabilization at room temperature of one of the three polymorphs α, β, γ exhibited by the mother compound. A tentative classification of the BIMEVOX phases based on these structural polytypes is carried out, and a review of the different identified ways to stabilize the γ-type phases, the most performant ones in term of oxide anion conduction, is proposed. The experimental results clearly indicate that the predominant parameter is not the size or the valence state of the dopant, but rather the structural parameter correlated with the ability for the dopant to regularize the diffusion slab between the Bi 2 O 2 sheets in these layered materials.

Structure and conductivity of Cu and Ni-substituted Bi4V2O11 compounds

The partially Cu- or Ni-substituted compounds (Bi4V2(1−x)M2xO11−3x;M=Cu, Ni) are highly oxygen-conducting. Three phases (α, β, γ) are observed in the unsubstituted compound; α is the low-conducting room temperature phase and γ the high-conducting phase at high temperature. Structure and conductivity are studied as a function of the substitution on the vanadium sites. Between 0 and 6% at room temperature, the Cu compound remains in the orthorhombic α phase and its ionic conductivity increases. A strong anisotropic conductivity is observed. For 0.07≤x≤0.12, the average structure is tetragonal (γ-type) at room temperature. The conductivity is very high and does not vary very much over this substitution range. Impedance spectroscopy measurements have also been carried out on the x=0.07 Ni-substituted compound. Commensurate or incommensurate superstructures are observed for all of these compounds.

Crystal structure determination of α, β and γ-Bi4V2O11 polymorphs. Part I: γ and β-Bi4V2O11

Abstract Using combined X-ray single crystal and neutron powder thermodiffraction data, the crystal structure of the high temperature γ -form of Bi 4 V 2 O 11 was confirmed and accurately refined in the I4/mmm space group and that of the β -form was entirely determined in the centrosymmetric Amam space group. The two-fold superlattice characterising the β structure is the result of an ordering process involving corner-sharing V–O tetrahedra and disordered trigonal bipyramids. A possible scheme for the γ ↔ β phase transition is proposed.

Crystal structure determination of α-, β- and γ-Bi4V2O11 polymorphs. Part II: crystal structure of α-Bi4V2O11

The crystal structure of α-Bi 4 V 2 O 11 was solved in the A2 space group and refined using combined X-ray single crystal and neutron powder diffraction data in the following unit cell a = 16.5949(3) A (3 x a m = 5.5316 A), b = 5.6106(1) A, c = 15.2707(3) A, γ = 90.260(2)°. It is built upon [Bi 2 O 2 ] 2+ layers spaced with vanadium-oxygen slabs where the vanadium atoms exhibit three different oxygen environments. The main characteristic of these V-O slabs is a well defined dimeric unit with two trigonal bipyramids sharing one edge and connected to two VO tetrahedra. These rigid blocks extend along [100] and are spaced with a disordered area where different V-O trigonal bipyramids are interconnected. A possible scheme to explain the actual 6a m superlattice is proposed. It also accounts for the diffusion lines systematically observed along [100] by SAED. A relationship between the crystal structures of the three Bi 4 V 2 O 11 polymorphs and their corresponding conductivity is tentatively suggested.

Thermal behaviour of Bi4V2O11 : X-ray diffraction and impedance spectroscopy studies

Abstract Bi 4 V 2 O 11 powdered samples and single crystals were studied by high temperature X-ray diffraction and impedance spectroscopy to characterize the phase transitions. From high temperature X-ray diffraction on powders and single crystals, the α ⇆ β and β ⇆ γ reversible phase transitions were observed. The β ⇆ γ one is ferroelastic ⇆ paraelastic but surprisingly the α ⇆ β transition also exhibits a ferroelastic character, with a 90 ° switching of the a and b axis on cooling and/or, more scarcely, on heating. Impedance spectroscopy measurements were carried out using platelet shaped single crystals with well developed (001) faces. The corresponding σ ∥ (001 plane) and σ ⊥ ( c direction) bulk conductivities were obtained and compared with values from ceramic pellets, σ ∥ values are close to those characterizing the pellets, and the anisotropy of the conductivity is evidenced by σ ∥ values about 2 orders of magnitude larger than σ ⊥ ones. Slope changes observed in Arrhenius plots are in agreement with the phase transitions.

W-substituted Bi4V2O11

The introduction of tungsten in the [V] sites of Bi4V2O11 gives rise to a Bi2V1 − xWxO(11 + x)2 solid solution for x up to 0.25 on air quenched compounds and up to 0.125 on slow cooled ones. This produces the stabilization of either α, β or γ forms depending on the substitution ratio. These new phases have been characterized by X-ray diffraction, thermal analyses and conductivity measurements. Comparison between Mo- and W-substituted compounds is made.

Oxide ion conductivity in Bi2W1−xMExO6−x/2 (ME = Nb, Ta)

Oxygen deficient Bi2WO6 phases have been synthesized by substitution for WVI with NbV and TaV, in this m = 1 member of the Aurivillius series. The composition range of the solid solutions is limited to 0 < x < 0.15 according to a formula Bi2W1−x(Nb or Ta)xO6−x/2. The maximum oxygen ion conductivities are obtained when x = 0.05 for both phases with moderate activation energies. The performances are intermediate between those of the high conductor BIMEVOX phases and those of doped higher members of the Aurivillius series m = 2, 3 and 4. These results provide the first experimental evidence of the predominant role of the Bi3+ 6s2 lone pair in the oxide ion diffusion process within these layered phases.

Double substitutions in Bi4V2O11

Abstract Several double substitutions have been checked to compare the performances of these compounds with those of ICUVOX.10, which is used as a reference: (i) the introduction of two kinds of atoms in [V] sites (CuNi, CuZn, NiZn, CuMo); (ii) simultaneous substitution for [Bi] and [V] (BiPb and VCu, BiPb and VMo); (iii) substitution of Cu for [V] in the 2Bi 2 O 3 ·V 2 O 5 solid solution. In all cases the oxide anionic conductivity was not improved with respect to singly substituted V/Cu BICUVOX.10.

Preparation, characterization and oxide ion conductivity in U-substituted Bi4V2O11

Abstract A Bi 2 V 1 − x − y U x Bi y O 5.5 + 0.5 x − y solid solution derived from Bi 4 V 2 O 11 has been prepared and characterized with x up to 0.125 for y = 0. Partial substitution of U 6+ for V 5+ in Bi 4 V 2 O 11 leads to the stabilization at room temperature of the high-oxide ion conducting γ-phase, in contrast with other M 6+ dopants which stabilize the β-phase. The lower conductivity in U substituted system compared with BICUVOX.10 is attributed to its higher activation energy. Conductivity values and activation energies of the U substituted phases compare well with Bi 2 UO 6 .

Characterisation of the electrode–electrolyte BIMEVOX system for oxygen separation: Part II. Thermal studies under controlled atmosphere

Abstract The reduction of Bi4V2O11 and BIMEVOX powders was investigated using two methods: thermogravimetric analyses and high-temperature X-ray diffraction under hydrogen flow. The chemical reduction, due to a VV–VIV reduction, was compared to the evolution of cathodes of dense BIMEVOX membranes under current bias which had previously been characterised using in situ X-ray synchrotron diffraction. This correlation enables to quantify the level of reduction reached on Bi4V2O11 membranes under operating conditions. This explains the oxygen transfer in the materials, at least at the cathode under bias.