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Structural investigation of layered compounds based on the perovskite SrTiO?.

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Bowden, Mark Edward 


Changes of stoichiometry in the perovskite SrTi03 have been investigated in the SrOLa203-Ti02 system, which result in the formation of layered structures within the perovskite matrix. When present in low concentrations, the layers are distributed in a disordered fashion and therefore cannot be detected by diffraction techniques. They may, however, be observed using high-resolution electron microscopy (HREM) , which revealed a tendency for the layers to group together at low concentrations. As the frequency of layers increased, they were observed to form ordered arrays, and led to three different layered families of stoichiometric compounds which may be defined by the layer structure and the interlayer spacing. These are summarised in the following table, where the layer orientation given refers to planes in perovskite: Composition change Addition of SrO Addition of 0 Removal of Ti Layer orientation {100} {110} {111 } Compositional series Sr n+1 Tin03n+1 Sr n-4La4 Tin0 3n+2 Sr n-3La4 Tin0 3n+3 Ordered members (7=1,2 and 3 (7=4, 4.5 and 5 (7=3,4 and 5 Previous studies have not agreed regarding the stability of the n=3 {lOO} layered compound. In this report, the well-ordered nature of this material has been verified using HREM, and the structures of all of the {lOO} and {Ill} layered phases refined by the Rietveld method. The n=4.5 and n=5 {11 O} layered materials have not been reported previously, although isostructural compounds are known. A new compound with the composition Sr3La2Ti201Q has been identified, and its structure determined using a combination of HREM and Rietveld analysis. It contains slabs of the n=l {lOO} layered structure, resulting in ribbons of Ti06 octahedra running parallel to the b axis. In the electron microscope, the surfaces of crystals were found to be covered by a rim of disordered material. This was observed to crystallise into the perovskite structure under the influence of the electron beam. ii For many of the compounds described above, HREM revealed defects in the layer stacking and layer structure. The nature of these defects has been investigated using computer-based models and, where appropriate, simulation of HREM images. Much of the behaviour of the layered compounds has been discussed in terms of elastic strain. The layer structures of all of the compounds had a different size than the corresponding perovskite dimension, and so the cell parameters converged toward the perovskite value as the thickness of the perovskite lamellae increased. Consideration of elastic strain can explain this behaviour, as well as the stability ranges of the layered families. Experiments designed to synthesise an n=3 {lOO} layered compound containing copper were not successful. However, three solid solution ranges were found, including complete solid solution between Sr2Ti04 and La2Cu04.





Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge