| Topological insulators have raised a new upsurge of research activities in condensed matter sciences because of their special surface properties.These properties are determined by the topological properties of the electronic states which abide by time reversal symmetry.Topological insulators may have great applications in spintronics and quantum computation.YBiO3,as a theoretically predicted topological insulator,has attracted extensive attentions for its large gap character and thus less possible entanglement between surface and bulk states.In this thesis,the Bi-Y-O system has been investigated by X-ray powder diffraction,electron diffraction,UV-vis and IR experiments.In order to obtain high quality YBiO3 powder,the reaction conditions such as the sintering temperature,the sintering time,the heat treatment process,and the starting ratios of reactants have been studied.The optimized preparation conditions have been obtained.A metastable cubic high temperature phase of BiYO3 with fluorite-type structure has been structurally characterized for the first time and shows a large band gap of?5.9 eV.A unified description for the numerous structural variants discovered in the Bi-Y-O system is established within the symmetry breaking approach.This rich structural phenomenon makes the Bi-Y-0 system a promising candidate in the search for new topological insulators for applications.On this basis,a long standing controversy on the phase diagram of the Bi-Y-O system has been solved.The determined phase boundary of this phase at the Y2O3 rich side lies between 50%/mol and 50.25%/mol.Our DFT calculations deny the prediction that "provskite structure YBiO3" is a new topological insulator and predict a high pressure phase for BiYO3 with perovskite(ABO3)structure and ordering of Bi and Y on the A and B sites,respectively.Our analysis of the nature of the low energy electronic structure shows that this phase is not a suitable candidate for a topological insulator. |
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