Oxide-Ion Conductors for Energy Applications: Structure and Properties

Abstract

Oxide-Ion Conductors for Energy Applications: Structure and Properties
PhD Thesis Matthew S. Chambers 2019

This thesis describes experiments aimed at understanding the average and local structure of a range of different oxide-ion conductors, with the aim of applying this insight to designing better materials.

The main experimental technique used was neutron total scattering, and Chapter 1 reviews the previous literature concerning the application of this technique to oxide-ion conductors.
Chapter 2 describes the synthetic and characterisation techniques employed to investigate the materials in this study.

Chapter 3 discusses the attempted synthesis of Ge5–xAlx(PO4)6O1–x/2 (x = 0, 0.1, 0.2, 0.3, 0.5, 1, 2) materials, which we hoped would be new examples of mixed-coordination-number oxide-ion conductors. Powder X-ray diffraction and elemental analysis suggested that Al3+ was successfully doped into the parent. The electrical properties of x = 0, 0.2, 0.5 and 1 samples were measured for the first time, and showed an increase in conductivity with increasing x. However, solid-state nuclear magnetic resonance spectroscopy experiments showed that Al3+ was not incorporated in the main phase, but instead formed a likely amorphous aluminium-phosphate phase. These materials have therefore not been pursued further.

Chapter 4 discusses investigations into the long range and local structures of La8R2(GeO4)6O3–y (R = Sr, La, Bi; y = 0, 1) apatite-type oxide-ion conductors using high-resolution powder synchrotron diffraction, high-resolution powder neutron diffraction and neutron total scattering. La8Sr2(GeO4)6O2 was found to adopt P63/m symmetry at all temperatures, whilst La8R2(GeO4)6O3 (R = La, Bi) adopt P-1 symmetry at room temperature and P63/m at 780–800 °C. The excess interstitial oxygen in La8R2(GeO4)6O3 (R = La, Bi) was found to be distributed over three similar sites at room temperature. Local structural studies gave the first detailed picture of the local coordination geometry of the interstitial oxygen atoms, which are important for ionic conductivity. Whilst a range of Ge coordination numbers are present, the dominant feature is the formation of GeO5 units. These are predominantly square pyramidal at room temperature, but become more like trigonal bipyramids at high temperature. This shows that the local structure appears to increase in symmetry as the long-range structure increases in symmetry.

Chapter 5 reports the synthesis of nominal La8R2(SiO4)6O3 (R = La, Bi) apatite-type oxide-ion conductors, the electrical properties and the composition limits. These are commonly referred to as A-10, O-27 compositions in the literature due to their formula of A10Si6O27. The results from Rietveld refinement, energy-dispersive X-ray spectroscopy and solid state nuclear magnetic resonance spectroscopy call into question the existence of “oxygen-excess” or O-27 lanthanum silicate apatites.

Chapter 6 investigates the long-range and local structure of the perovskite-derivative, Ba3NbMoO-8.5, which has been shown to have high oxide-ion conductivity. High resolution powder synchrotron diffraction revealed for the first time that there is a low-temperature phase and a high-temperature phase and that the sample shows structural hysteresis. Total scattering analysis indicates that the most common coordination number for Nb5+/Mo6+ is CN = 5 at both room temperature and 600 °C. The local structure of the O2/O3 sublattice is also highly disordered, forming hexagonal sheets of atomic density even at room temperature. The O2 and O3 sites are more localised at room temperature, but spread across the hexagonal sheets at 600 °C. The results give good insight into the potential conduction pathways in the material.

Chapter 7 reports work done on rhombohedral Bi0.775Ln0.225O1.5 (Ln = La, Dy) phases. High-resolution powder synchrotron diffraction and powder neutron diffraction show that the (0 0 l) reflections are poorly fitted. An investigation into stacking faults was made, but none of the models produced a good fit to the data. Small box PDF analysis shows that the average crystallographic structure is insufficient to describe the local structure. The structural complexities of the system are yet to be fully understood.