Local structure/property relationships in functional materials

Thesis

It is increasingly being realised that localised deviations from the average structure can play an important role in a material's properties, and hence an understanding of these deviations is essential when constructing a coherent picture of a system. In this thesis, both neutron and X-ray total scattering data have been collected and used to reveal information on three canonical systems: the high-temperature superconductor YBa₂Cu₃O_7-x; the parent compound of the colossal magnetoresistive manganites, LaMnO₃; and the oldest known magnetic material, Fe₃O₄. Reverse Monte Carlo refinements-using the RMCProfile implementation of the algorithm-have been used as the principal analysis technique, and the functionality of the RMCProfile program has been extended to allow the refinement of magnetic systems involving substitutional disorder. For YBa₂Cu₃O_7-x, the focus of this thesis is on the apical Cu{O bond length. This is shown to have a bimodal distribution, but correlations in the displacements of both atoms disguise this fact in the average structure, thus resolving the apparent controversy that had existed between local- and average-structure probes. LaMnO₃ displays (what was thought to be) a simple order{disorder transition that results in the Jahn-Teller distortion becoming invisible in the average structure above ~ 750 K. Here it is shown that in fact the transition is more complicated, and involves a change in the symmetry of the Jahn-Teller distortion, whereby the long Mn-O bonds move from being opposite one another in the octahedra to being adjacent to one another. This new distortion still breaks the degeneracy of the system, and is consistent with a wide range of existing observations. Finally the low temperature structure of Fe₃O₄ is examined using the updated RMCProfile code. The refinements show sensitivity to local structure variations, producing a bimodal Fe atom bond valence distribution. In addition, the refined magnetic spin configuration is presented, providing the first detailed description of the low-temperature magnetic structure. It is found to be a canted ferrimagnet, and appears to be consistent with Cc symmetry.

Authors: Young, CA

• Publication date: 2014
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: manganites; canonical systems; superconductors; magnetism
• Subjects: Crystallography; Chemistry & allied sciences; Chemical crystallography