F. Krok

Defect chemistry of the BIMEVOXes

Since their discovery in 1988, the BIMEVOXes have been the subject of significant research due to their high oxide ion conductivity at relatively low temperatures. The development of these materials is briefly reviewed. The defect structure of the BIMEVOXes is discussed and used to construct general defect equations for solid solution formation. Two limiting models are proposed by which solid solution formation can occur. In the Equatorial Vacancy (EV) model, vacancies are located exclusively in bridging sites in the vanadate layer. In contrast, the Apical Vacancy (AV) model assumes vacancies are located exclusively in non-bridging apical sites in the vanadate layer. The general defect equations can be used to predict theoretical solid solution limits for all types of substitutions for vanadium in Bi4V2O11−δ. These limits are found to vary not only with the charge of the dopant ion, but also with the coordination number of the metal dopant. In most cases it is found that the EV model yields theoretical solid solution limits close to those observed. The EV model is also used to present a mechanism for ionic conduction in BIMEVOXes, which involves movement of equatorial oxide ions/vacancies between vanadium octahedra and tetrahedra with the formation of a five-coordinate vanadium intermediate.

Effects of inhomogeneity on ionic conductivity and relaxations in PEO and PEO-salt complexes

Electrical properties of purified poly(ethylene oxide)-PEO and PEO with dissolved lithium salts, PEO10:LiCF3SO3 and PEO10:LiN(CF3SO2)2, were investigated by impedance spectroscopy. Changes of the impedance spectra during crystallization and melting were recorded. Semicrystalline polymer films exhibited frequency dependence of conductivity with two limiting values of conductivity at low and at high frequency. It was concomitant with dispersion of the electric permittivity exhibiting the low-frequency values of the order of 200. In the case of PEO with dissolved salt, when the film was quasi-amorphous, the electric permittivity also exhibited pronounced dispersion, which was probably due to the presence of crystalline PEO–salt complexes. The dielectric relaxation caused by rearrangement of dipoles in PEO chain was observed at low temperatures and high frequencies.

Defect structure of quenched γ-BICOVOX by combined X-ray and neutron powder diffraction

Abstract A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi 2 V 1 − x Co x O 5.5 − 3x 2 ( x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi 2 O 2 ] n 2 n+ and [ V 1 − x Co x O 3.5 − 3x 2 ] n 2n− with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral.

A model for the mechanism of low temperature ionic conduction in divalent-substituted γ-BIMEVOXes

Abstract A model for the conduction mechanism in divalent-substituted γ-BIMEVOXes, of general formula Bi2MxV1−xO5.5−(3x/2)−δ (M=divalent cation), is proposed. This model is based on the defect structure derived from combined X-ray and neutron powder diffraction studies, which show both tetrahedral and octahedral vanadium polyhedra in the defect vanadate layer. The proposed defect structure can be used to accurately predict solid solution limits in the divalent-substituted BIMEVOXes. The proposed conduction mechanism involves positional exchange of oxide ions and vacancies in the equatorial plane of the vanadate layer and results in the effective two-dimensional movement of the vanadium polyhedra through the structure. Three main features are identified in the structure of the γ-BIMEVOXes which are fundamental to the exhibition of high oxide ion conductivity shown in these materials. These are, firstly, the polarizibilty and arrangement of the Bi 6s2 lone pairs with respect to vacancies in the vanadate layer; secondly, the well-known variable co-ordination of V in oxide systems; and thirdly, the large vacancy concentration in the vanadate layer.

Defect structure and ionic conductivity as a function of thermal history in BIMGVOX solid electrolytes

The defect structure of the oxide ion conducting solid electrolyte, Mg substituted Bi4V2O11−δ (BIMGVOX), was examined by high-resolution powder neutron diffraction. A detailed explanation of interpretation of the defect structure is presented. The general formula for the BIMGVOX solid solutions Bi2V1−xMgxO5.5−3x/2 assumes complete oxidation of vanadium to VV. Analysis of the neutron diffraction data reveals the defect structure and indicates that there is, in fact, partial reduction of vanadium to VIV. The extent of reduction is dependent on thermal history, with high temperature quenched samples showing a greater degree of reduction than exponentially slow cooled samples. This is correlated with differences in electrical behaviour at low and high temperatures. Differences in ionic conductivity and activation energies between samples with different thermal histories are explained in terms of the balance between charge carrier concentration and the extent of defect trapping effects.

Structural and electrical characterisation of BINIVOX

Electrical conductivity behaviour and unit cell parameter variation have been investigated in the double substituted BIMEVOX system Bi2V0.9CoyCu0.1-yO5.35, 0<y<0.1 by a.c. impedance spectroscopy and X-ray powder diffraction. Variation of the unit cell volume indicates a microdomain structure for the material with Co rich and Cu rich regions. Both low and high temperature conductivities increase with increasing Co content. However, the overall conductivities appear to be lower than the two end members, BICOVOX and BICUVOX, which suggests that some type of defect trapping may be present in these double substituted systems.

Chemical intercalation of magnesium into solid hosts

The use of two organometallic reagents, di-n-butylmagnesium and magnesium bis(2,6-di-tert-butylphenoxide) for the intercalation of magnesium into a range of solid hosts is investigated. New magnesium intercalation compounds based on cubic and layered TiS2 are described.

Electrical and structural study of BICOVOX

Abstract The relationship of structure and conductivity has been investigated in the Co substituted BIMEVOX system Bi 2 V 1 − x Co x O 5.5 − 3x 2 , 0.07 ≤ x ≤ 0.25 (BICOVOX). This system has been characterised using a.c. impedance spectroscopy, X-ray and neutron diffraction. The Arrhenius plots all show two straight line segments, one at low temperatures, below 450 °C, and one at high temperatures, above 550 °C. In the intermediate region, 3 types of behaviour are observed which appears to be composition dependent. All compositions showed differences in conductivity after the first heating cycle. However, this thermal hysteresis persisted in subsequent heating-cooling cycles only in compositions below x = 0.10. All BICOVOX samples in the composition range studied show essentially the tetragonal γ-phase structure, with evidence for a superstructure modulation at low temperatures from the neutron data.

Electrical conductivity dispersion in Co-doped NASICON samples

Impedance spectra of Co-doped NASICON polycrystalline samples (x = 2.0) were measured in the frequency range 0.5 MHz-1.2 GHz and analyzed in terms of the conductivity dispersion. The dispersion of bulk electrical conductivity obeys the power law frequency dependence σ(ω) = σ 0 + Kω n . It is observed from room temperature up to 450 K, i.e. up to the temperature of phase transformation of NASICON. The activation energy of the conductivity of grain interiors and the activation energy of the frequency, at which the dispersion sets on, have the same value of 0.313 eV. The activation energy value of the frequency dependent part of conductivity (0.077 eV) constitutes the proper fraction of the activation energy of the dc conductivity. Similar dispersion is found at lower frequencies for the grain-boundary conductivity. It extends in temperature up to 550 K, well over the temperature of phase transformation of NASICON. The phase transformation, taking place in the crystal grains of the material, does not influence the electrical behavior of grain boundaries. The activation energies for the grain boundary conductivity and for the grain-boundary onset frequency also have equal values (0.440 eV).

Electrical conductivity and structure correlation in BIZNVOX

The structure and electrical conductivity of the fast oxide ion conducting system BIZNVOX, Bi 2 Zn x V 1- O 5.5- (0.05≤x≤0.27), were investigated using X-ray powder diffraction and ac impedance spectroscopy. There is a compositional dependence of the conductivity behaviour as a function of temperature. This behaviour was correlated with the stabilisation of various polymorphs within the system. The x = 0.10 composition was found to have one of the highest low temperature conductivities (σ 300 = 4.09× 10 -3 S cm -1 ) for any BIMEVOX compound.