The Optical, Electronic And Magnetic Properties Of Multiferroic Materials And Semiconductor Photocatalyst

Recently, multiferroic materials have received immense attention because of both the point of fundamental materials physics and their potential applications in spintronic devices with informational storage and processing. Energy crisis and environmental pollution on a global scale are issues of increasing concern in today's society. Semiconductor catalysis has been considered to be an effecitive method for solving the two problems. However, there are so few multiferroic materials and the effect of magnetic electric coupling is vey weak; semiconductor photocatalyst has has low activity and only use of ultraviolet radiation, which are limited multiferroic materials and semiconductor catalysis in practical application. Thus, the critical problem solving and finding new multiferroic and semiconductor photocatalytic material is currently a hot research field.In this thesis, using the materials design codes of WIEN2K and CASTEP, we explored the magnetism and electronic structure of the surfaces and superlattice of multiferroic materials; the charge-orbital-spin ordering in valence-mixed multiferroic materials; the geometrical structure and the photocatalytic acticity of semiconductor photocatalysis based on anatase TiO2 and SrTiO3. The main research work and contenta are listed as follows:(1) We present and discuss the electronic properties of the (001) surface of cubic BaMnO3 with BaO- and MnO2-terminations. Based on the results of the calculated surface energies and stability, we obtain that only the BaO-terminated surface can exist in the (001) surface of cubic BaMnO3. From the analysis of their band structure, we can see that the BaO-terminated surface has obvious half-metallic character, compared with the bulk materials and the MnO2-terminated surface.(2) We explored the electronic structure and electrical transport properties of the cubic barium manganese (BaMnO3) and the BaMnO3/BaTiO3 superlattice. The results show that the construction of a BaMnO3/BaTiO3 superlattice could stabilize the cubic metastable phase of the BaMnO3 and provide a stable half-metallic material for potential applications in spintronic devices.(3) Charge, orbital, and spin ordering of multiferroic BiMn2O5 and the relation between charge-orbital ordering with ferroelectric polarization are investigated. The results reveal that the partial ferroelectric polarization (Pele) originates from the charge ordering,(4) The energy band structure and optical properties for C-monodoped and C/H-codoped anatase TiO2 are investigated. The obtained results indicate that C/H-codoping is more stable than C-monodoping. For C-doped anatase TiO2, the high visible-light catalytic ability is origin from the isolated C 2p states above the valence-band maximum. For C/H-codoping produces significant band gap narrowing, and results in higher visible-light photocatalytic efficiency than that of the C-monodoping.(5) The effect of C-doping on the band gap states and the photocatalytic activity of three possible substitution C-doped SrTiO3 (C@X, X=Sr, Ti, and O) are studied. The obtained results indicate that C impurity prefers to replace the Ti ion. For C@Ti, some C 1s gap states appear near the bottom of the conduction band, which results in the decreased electron transition energy and thus the visible light absorption observed in the experiment.By the above calculation, we get a series of important theoretical results, which provide a solid theoretical foundation for further synthesis of new multiferroic materials, the production of new multiferroic devices and improving photocatalysed efficient of semiconductor photocatalyst.