M. Malys

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.

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.

Structural and electrical consequences of high dopant levels in the BIMGVOX system

Abstract The influence of high Mg2+ dopant levels in BIMGVOX, Bi2MgxV1−xO5.5−3x/2 (0.05≤x≤0.40) on structure and conductivity has been investigated using X-ray powder diffraction and a.c. impedance spectroscopy. Four, compositionally dependent, structural ranges are observed at room temperature, with emergence of a new orthorhombic phase at high dopant levels. Generally the Arrhenius plots of conductivity show two linear regions the limits of which are compositionally dependent. The results have been correlated to the stability ranges of various polymorphs within the system.

Phase stability, structure and electrical conductivity in the system Bi2ZrxV1−xO5.5−(x/2)−δ

Abstract The BIMEVOX system, Bi 2 Zr x V 1− x O 5.5− x /2 , has been investigated using ac impedance spectroscopy and X-ray powder diffraction, in order to examine the effects of vacancy and dopant cation concentration on γ-phase stability and ionic conductivity. Four crystallographically distinct phases are observed at ambient temperature over the composition range 0.05≤ x ≤0.50. Below x =0.10, the orthorhombic α-phase is seen. Between x =0.10 and 0.16, the β-phase is stabilised. The stabilisation of orthorhombic α and β phases at lower compositions is typical for BIMEVOXes and results from an ordering of oxide ion vacancies. At x =0.19, the data were modelled on a tetragonal γ-phase cell, while at compositions where x ≥0.22 a mixture of the tetragonal γ-phase and Bi 8 V 2 O 17 is observed. The electrical conductivity is correlated with the stabilisation of the various polymorphs within the system. The observed γ-phase stabilisation region and the solid solution limit are discussed with respect to the defect structure.

Total scattering analysis of cation coordination and vacancy pair distribution in Yb substituted δ-Bi2O3

Reverse Monte Carlo (RMC) modelling of neutron total scattering data, combined with conventional Rietveld analysis of x-ray and neutron data, has been used to describe the cation coordination environments and vacancy pair distribution in the oxide ion conducting electrolyte Bi3YbO6. The thermal variation of the cubic fluorite unit cell volume, monitored by variable temperature x-ray and neutron experiments, reveals significant curvature, which is explained by changes in the oxide ion distribution. There is a significant increase in tetrahedral oxide ion vacancy concentration relative to δ-Bi2O3, due to the creation of Frenkel defects associated with the Yb(3+) cation. The tetrahedral oxide ion vacancy concentration increases from room temperature to 800 °C, but little change is observed in the vacancy pair distribution with temperature. The vacancy pair distributions at both temperatures are consistent with a favouring of [100] vacancy pairs.

Oxide ion distribution, vacancy ordering and electrical behaviour in the Bi3NbO7-Bi3YbO6 pseudo-binary system

Oxide ion distribution, vacancy ordering and electrical conductivity has been examined in the Nb/Yb double substituted bismuth oxide based system Bi3Nb1−xYbxO7−x, using X-ray and neutron powder diffraction, reverse Monte Carlo modelling of total neutron scattering data and a.c. impedance spectroscopy. Transference number measurements confirm the system to be predominantly ionically conducting above ca. 450 °C. Niobium rich compositions show incommensurate ordering of the fluorite subcell, while increasing ytterbium content results in a commensurate fluorite, with fully disordered cation and anion sublattices. Oxide ion distribution shows both compositional and thermal dependencies. The latter is discussed with respect to its effect on the thermal variation of cubic lattice parameter. Substitution of bismuth by niobium and ytterbium in the cation sublattice of bismuth oxide leads to the creation of Frenkel interstitial oxide ions, which increase the tetrahedral vacancy concentration. The high vacancy concentration is confirmed in both Rietveld and RMC analyses of neutron data. Examination of vacancy ordering, in the x = 0.6 composition, indicates a favouring of 〈100〉 vacancy pair alignment.

Phase stabilisation in the pseudo-binary system Bi2MgO4–Bi2VO5.5−δ

Abstract The structure and electrical conductivity of compositions in the pseudo-binary system Bi 2 MgO 4 –Bi 2 VO 5.5− δ were investigated beyond the BIMEVOX stabilisation region using X-ray powder diffraction and ac impedance spectroscopy. Compositions of the general formula Bi 2 V 1− x Mg x O 5.5−3 x /2 were studied and several discrete phases were identified, viz: x =0.30, orthorhombic BIMGVOX; x =0.50, Bi 8 V 2 O 17 ; x =0.60, Bi 12 V 2 O 23 ; x =0.80, a δ-Bi 2 O 3 -type phase; and x =0.90, a γ-Bi 2 O 3 -type phase. In between these compositions, mixtures of phases are observed. High-temperature diffraction reveals phase separation at x =0.30 and 0.90. The structural data correlate well with variations in activation energy and conductivity.

Structural and electrical characterisation of BICOCUVOX

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.

Structural and electrical behaviour in Bi14YO22.5

Structure and electrical conductivity in the oxide ion conducting compound Bi14YO22.5 have been investigated by powder X-ray and neutron diffraction, a.c. impedance spectroscopy, measurements of transference number and ab initio molecular dynamics (MD) simulations. Phase behaviour was studied using variable temperature X-ray diffraction and differential thermal analysis. The structure at room temperature is of the βIII tetragonal-type, details of which are discussed, including its relationship to both β and δ phases of Bi2O3. Bi14YO22.5 undergoes a reversible phase transition to a cubic δ-Bi2O3 type phase at high temperatures. This phase exhibits very high electrical conductivity, with transference numbers indicating that this conductivity is almost purely ionic in nature. MD simulations confirm 3-dimensional oxide ion transport in the βIII-phase.