The local structure evolution as a function of composition and temperature, and the resulting electrical conductivities were studied in samples with general composition (Yb1-xNdx)2Zr2O7, using Reverse Monte Carlo (RMC) modelling of total neutron scattering data and Impedance Spectroscopy. At low neodymium content (0.00 ≤ x ≤ 0.50), this system crystallizes in the fluorite structure with space group Fm3̄m, where-as at high neodymium content (0.75 ≤ x ≤ 1.00), a pyrochlore structure with space group Fd3̄m is observed. For compositions with high neodymium content prepared at lower temperatures, a fluorite structure is seen, which shows gradual evolution of pyrochlore ordering with increasing calcination temperature. Cation anti-site disorder was present throughout the compositional range. Total conductivity increases with increasing Nd content, up to a maximum at x = 0.75 and then decreases. The conductivity of these oxides is of the order of 10-4 S cm−1 at 700 °C. Variable temperature neutron powder diffraction experiments were carried out on the (Nd1-xCax)2Zr2O7-x (0.10 ≤ x ≤ 0.50) system, to investigate the local structural evolution as a function of composition and temperature. The entire compositional range studied exhibits a single phase with a pyrochlore-type structure. The variable temperature study showed that the pyrochlore structure is maintained throughout the temperature range studied. No cation anti-site disorder was present in any of the compositions, over the temperature range studied. Total conductivity increases with increasing Ca content, up to a maximum at x = 0.30 and then decreases. The conductivity of these oxides is of the order of 10-3 S cm−1 at 700 °C. Structural evolution in oxidising and reducing atmospheres was investigated in cerium zirconate, Ce2Zr2O7+δ. A number of transformations were revealed, depending on the atmosphere (O2 vs CO) and temperature. Upon heating the pyrochlore phase in flowing O2, three distinct phases were observed, with transitions at ca. 140 C and 410 C. At around 140 C, a previously un-identified phase formed, which was fitted with a triclinic model that contained 10 Ce and 10 Zr sites, each with a distorted cubic coordination. Above 410 C, the triclinic phase transformed to a cubic phase, corresponding to the κ-phase (space group P213). RMC modelling of the local structure of the -phase, revealed an ordered, pyrochlore-like cation sub-lattice, with a disordered anion sub-lattice. Upon re-heating the κ-phase in CO, the pyrochlore phase re-appeared at around 260 °C. At 290 °C, the pyrochlore phase was phase pure.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:766149 |
| Date | January 2018 |
| Creators | Shehu, Ali |
| Publisher | Queen Mary, University of London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://qmro.qmul.ac.uk/xmlui/handle/123456789/36699 |
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