Isaac Abrahams

Oxide ion dynamics in the type-II phase of Bi₃NbO₇have been studied using a series of DFT molecular dynamics calculations.

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.

The solid state system Bi₂O₃:ZrO₂ has been investigated up to 28 mol% ZrO₂ using X-ray and neutron powder diffraction, ac impedance spectroscopy and differential thermal analysis. X-Ray powder diffraction has shown that a solid solution is formed with general composition Bi_2 − xZr_xO_3 + x/2 (0.05 ≤ x ≤ 0.17) for samples calcined at 850 °C. Members of the solid solution adopt a new β-Bi₂O₃ type structure termed β_III, which is closely related to Pb₂F₂O. The defect structure of a sample of composition x = 0.15 has been determined by combined high-resolution neutron and X-ray powder diffraction. High temperature powder X-ray diffraction has been used to confirm the structure of the high temperature polymorph, which is a cubic δ-Bi₂O₃ analogue. In the β_III-phase, solid solution formation proceeds through an anion interstitial mechanism with respect to the hypothetical end member β_III-Bi₂O₃, with interstitial oxide ions located in channels parallel to the c-axis. The defect structure involves a pair of Zr atoms coordinated to interstitial ions in the channels. A possible conduction mechanism is proposed. Differential thermal analysis and ac impedance measurements indicate a complex phase transition to a highly conducting polymorph (σ₇₀₀ = 1.02 S cm⁻¹) at temperatures above ca. 690 °C on heating. Crystal parameters: T = 298 K, Bi_1.85Zr_0.15O_3.075, M = 449.49, tetragonal, P4₂/nmc, a = 7.7206(8), c = 5.6370(6) Å, Z = 4, U = 336.0(1) ų, D_c = 8.889(3) g cm⁻³; T = 1023 K, cubic, Fmm, a = 5.6277(4) Å, U = 178.24(4) ų, Z = 2, D_c = 8.379(2) g cm⁻³.