Incommensurate structure of melilite-type electrolyte : synthesis, crystal chemistry and ion conduction

• Main Author: Wei, Fengxia
• Other Authors: Timothy John White
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Material testing and characterization; DRNTU::Engineering::Materials::Ceramic materials
• Online Access: http://hdl.handle.net/10356/55774
• Institution: Nanyang Technological University

Melilites ([A2]2[BI]2[BII2O7]2) are layered oxides containing distorted pentagonal rings, constructed from BO4 tetrahedra, whose basic structure can be ‘modulated’ to accommodate misfit between the interlayer spacing and the bridging A cations. While prototypically regarded as possessing a tetragonal unit cell metric with atoms distributed in accord with space group , these materials are often incommensurate and are best described in higher dimensional superspace. The focus of this thesis are gallate melilite varieties, that when compositionally tuned, show substantial oxide ion percolation and are potential solid oxide fuel cell (SOFC) electrolytes. In particular, these studies are concerned with the manner in which modulation facilitates the incorporation of extra-stoichiometric oxygen in melilites, and those topological adaptations that open low activation energy diffusion paths. To these ends, melilite powders were synthesized by solid state sintering and single crystals grown using the optical floating zone method. X-ray, neutron, synchrotron and electron diffraction were employed to determine crystal structures, define modulation vectors, and locate interstitial oxygen as a function of temperature. Strong anisotropic oxygen displacements accompany modulation, and forcing gallate melilites to 3D symmetry leads to non-physical crystallographic parameters. However, superspace models employing two dimensional modulation, achieve bond lengths and bond angles that preserve realistic bond valence sums throughout the structure. Moreover, systematic variations in AOn (n = 6, 7, 8) coordination polyhedra, and BO4 tetrahedral distortions are observed, and where melilites contain multiple A site cations, compositional modulation appears. Finally, the materials used to determine average structures from X-ray and neutron diffraction are found by transmission electron microscopy to contain nanoscale domains characterised by pseudo periodic modulation waves along the basal plane diagonal directions. Ion conduction in oxides is related to defects, and the common mobile species are oxygen vacancies, as observed in yttrium stabilized zirconia (YSZ) which is a widely used fuel cell electrolyte. An alternate mechanism is via super stoichiometric oxygen (interstitials), that also yield high conductivity in several oxide families including apatites, fergusonites and melilites. In every case, a flexible framework that can accommodate and stabilize interstitials is a prerequisite to deliver electrolytes with high oxygen mobility. A key advantage of ionic conductors where transport is mediated by excess oxygen is relatively low activation energy. In melilites, quantitative crystallographic analyses, inclusive of higher dimensional features, are crucial to describe the nature of oxygen diffusion. It is especially valuable to map the large anisotropic displacement parameters of oxygen to predict possible diffusion pathways, that can subsequently be validated through molecular dynamic simulation. In the present studies, the key advance was the availability of large melilite single crystals that not only enable modulated structure characterization, but also allow the quantitative evaluation of ion percolation, and showed the preferred oxygen diffusion path was perpendicular to the c axis. For example, in [La1.5Ca0.5]2[Ga]2[Ga2O7.25]2 melilite the ionic conductivity was 0.04 Scm-1 at 800ºC and comparable to YSZ (0.043 Scm-1 at 800 ºC). Importantly, at < 800 ºC melilites outperform YSZ, opening the prospect of constructing efficient fuel cells that operate at intermediate temperatures to reduce fabrication costs and extend the life of these devices. This research, by demonstrating the role of incommensuration in accommodating mobile oxygen, allows tailoring of melilite chemistries to enhance oxygen ion conductivity and optimize their performance as solid oxide fuel cell electrolytes.