Synthesis and characterisation of bivox and binavox materials
This thesis describes the compositional limits, phase stability and electrical properties of several oxide ion conducting phases in the Bi2O3-V2O5 binary and Bi2O3-V2O5-Na2O ternary phase diagrams. Samples were prepared either by the mixed oxide route frm Bi2O3, V2O5, and Na2CO3 reagents or by hydrothermal processing of Bi2O3 and V2O5 using (2-3 M) NaOH. Sample were analysed using a combination of techniques including x-ray (XRD) and electron diffraction (ED), thermal analysis (DTA/TGA), electron probe micro-probe analysis (EPMA), and impedance spectroscopy (IS). The well-known oxide ion conductor, based on the ideal composition Bi4:V = 2:1) was shown to exist as a solid solution phase between 2.15:1 = 2.5:1 at 800 - 850°C. XRD, ED, and EPMA data provide evidence for a new polymorph (d) for Bi-rich compositions based on a defect fluorite d-Bi2O3 cell, in addition to the well-known a, b, and g polymorphs. This solid solution exhibits the highest level of oxide ion conductivity (at 800°C) of all the phases studied, with values of ~ 0.1 Scm-1 at 800°C. Post-annealing samples at 650°C for extended periods show that the Bi-rich solid limit is strongly temperature-dependent and hat compositions 2.4-2.5:1 are metastable and decompose into phase mixtures which include a low temperature triclinic phase based on a face-centred cubic cell with composition Bi2.9VO6.85. The next solid solution phase occurs at 3.5-3.75:1 and not at 4:1 as has been previously believed. Selected area electron diffraction (SAED) revealed complex crystal chemistry with the existence of three different types of patterns all based on a 3-fold multiple of a d-Bi2O3 fluorite-type subcell. Most crystals had incommensurate modulation and also streaking indicative of structural disorder, attributed to variations in the stacking order of V4O10 units within the cell. Samples were stable after post-annealing at lower temperatures, suggesting they are thermodynamically stable, albeit with complex, low crystal symmetry. They are modest oxygen ion conductors with s-5 mScm-1 at 800°C.