G. Mairesse

The bimevox series: A new family of high performances oxide ion conductors

Abstract A new family of oxygen anionic conductors, exhibiting high performances at low temperature, has been prepared and characterized. They derive from Bi4V2O11 by partial substitution of vanadium by other metallic ions (BIMEVOX). Results dealing with the copper substituant (BICUVOX) are detailed.

Electrochemical characterization of BIMEVOX oxide-ion conductors

Partial substitution of metals such as Cu or Zn for vanadium in Bi4V2O11 leads to materials exhibiting high anionic conductivities at temperatures as low as 500 K. These materials have been characterized electrochemically for use as potential solid electrolytes in potentiometric and amperometric oxygen sensors. Their electronic conductivities are significant.

Structural and computational studies of Bi2WO6 based oxygen ion conductors

A combination of neutron powder diffraction and computer simulation techniques was performed on undoped and doped Bi2WO6 Aurivillius type compounds to clarify some of the factors controlling oxygen transport in these materials. Oxygen vacancies in doped compounds are randomly distributed within the perovskite-like slab. The most favourable dopants are predicted to be NbVand TaV on WVI and LaIII on BiIII in accordance with the experimental results. The calculated migration energy of 0.63 eV is in agreement with the values deduced from impedance spectroscopy data for Ta and Nb doped Bi2WO6 at T>550 °C. At lower temperatures, pair clusters are predicted to form with a 0.25 eV mean binding energy, leading to a 0.88 eV activation energy for oxygen vacancy migration, in good correlation with experimental values. Finally, consideration of possible oxygen ion migration pathways in the structure showed that energy barriers to migration are lowest between adjacent apical and equatorial sites of WO6 oxygen octahedra.

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.

Modelling the crystal structures of Aurivillius phases

Abstract Computer simulation techniques are used to model the structures of several Aurivillius phases (with the general formula (Bi 2 O 2 )[A m −1 (B) m O 3 m +1 ]) and related compounds. The methods are based upon interatomic potentials and efficient energy minimisation procedures. The results indicate general agreement between the simulated and experimental structures. From recent diffraction data of the Bi 2 GeO 5 system, a new refinement of the crystal structure has been obtained. A discussion of trends related to the Bi lone-pair parameters of these Aurivillius phases is also presented.

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.

From Bi4V2O11 to Bi4V2O10.66: the VV–VIV transformation in the aurivillius-type framework

Bi4V2O11, the parent compound of the BIMEVOX family, has been studied by TEM and HREM, at room temperature and at high temperatures. Under the experimental conditions, Bi4V2O11 undergoes a VV–VIV reduction, ultimately leading to the formation of Bi6V3O16(Bi4V2O10.66) with VIV/VV= 1/2. This transformation is confirmed by in situ X-ray diffraction studies of Bi4V2O11 under reducing atmospheres up to 330 °C. The symmetry of the α-Bi4V2O11 polymorph, stable at ambient temperature, is dependent on the chemical purity of V2O5 used for the synthesis.

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.