W. Wrobel

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.

First and Second Universalities: Expeditions Towards and Beyond

Abstract Understanding the mechanisms of translational and localised ionic movements in disordered materials has seen intense activity spanning several decades. This article attempts to convey a concise overview of our contribution to this field over the period from 2005 to 2010 and to place it in its broad context.

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.

Phase and electrical behaviour in Bi4NbO8.5.

A study of phase and electrical behaviour in the bismuth niobate, Bi(4)NbO(8.5), using x-ray and neutron powder diffraction, thermogravimetric analysis (TGA), x-ray photoelectron spectroscopy (XPS) and ac impedance spectroscopy is presented. Two polymorphs were identified in this composition, a tetragonal phase (type III), which can appear at temperatures above 800 °C and a pseudo-cubic phase (type II) evident at lower temperatures. The defect structure analysis of the type II phase is consistent with the existence of chains of niobate polyhedra, which facilitate electronic conduction at low temperatures. The appearance of the type III phase is strongly dependent on experimental conditions and TGA and XPS measurements suggest a likely association with change in oxygen stoichiometry.

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 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.