Study Of Band Structure Of Rare Earth Doped Perovskite-type Oxides

Composite oxide materials of perovskite type are attracted broad attention related fieldsbecause of its unique crystal structure and excellent physical properties. Some of this kind ofmaterials have been widely used in redox catalysts, solid resistance, high temperature heating,etc. In this paper, we study strontium titanate (SrTiO3) and Barium titanate (BaTiO3) materialswith the typically perovskite structure by ion doping and substitution, and obtain themechanisms of the change of the band structure and physical properties. SrTiO3hasadvantages of good thermal stability, low dielectric loss, and high dielectric constant, and hasbeen widely used in the field of ceramic industry and electronic machinery. At the same time,its excellent properties of photocatalytic activity have also wide application in the field ofcatalysis. BaTiO3is a kind of typical ferroelectric materials with perovskite structure, and isthe most widely used ferroelectrics. By the doping SrTiO3and BaTiO3can not only make theCurie temperature of the material increase or decrease, but also change the dielectric constantand conductivity. At present, these two kinds of materials doped systems have been donelarge aggregation of studies, and attained some progress. But we need to deeply understandelectronic structure change and association of electronic structure and properties.In this thesis, the First-principles ultra-soft pseudo-potential approach of the plane wavebased upon the Density Functional Theory using the CASTEP package of Materials Studio6.0has been performed to investigate the doping modification of SrTiO3and BaTiO3. Themain contents are as follows:1. By the detailed investigation of the Band Structure, Density of States of the ideal SrTiO3,Ba single doped SrTiO3, Ba-Gd co-doped SrTiO3, we get the lattice constant and the band gapafter geometry optimization, and compare the Band Structure and Density of States partialDensity of States of the three systems with each other. The calculation results reveal that thelattice constant after geometry optimization is3.95602in the Ba-doped SrTiO3system. TheBand Structure analysis shows that the doped and undoped SrTiO3systems have the samecharacteristics of indirect band gap, and band gap is smaller and the quantity of bands changesbecause of Ba dope. Ba-doped SrTiO3single system still shows as insulation. After structuraloptimization of Ba-Gd co-doped SrTiO3system, the symmetry of original doped structurechanged, and the structure has undergone the distortion. Co-doped system presents a directtransition type Band Structure, the top of the valence band passes through the Fermi level intothe conduction band of the system, which leads to significant increase of the intrinsic carrierconcentration and the conductivity of the material, and improves the conductivity of SrTiO3.2. The cubic phase of BaTiO3is as the base, La, Sm, Gd, Dy, Ho, Yb, and Lu are selected todo as the impurity atoms. We calculate Band Structure, and Density of States of the dopedsystems of the different elements, and mainly analyzes the La, Gd, and Yb doped system. The results show that with the rare earth doped, structure still show the indirect band gap, and theyare about1.7eV~2.2eV. In the energy Band Structure of La-doped Ba0.75La0.25TiO3andGd-doped Ba0.75Gd0.25TiO3systems, bottom of conduction band of the system pass throughthe Fermi surface, and the materials show certain conductivity. In the Yb-dopedBa0.75Yb0.25TiO3system, the Yb4f shell layer is full of electrons, the structure is stable,electrons are difficult to be lost, so Yb-doped Ba0.75Yb0.25TiO3materials show insulation.From the results of the study on comparison with the Density of States of seven rare earthdoped systems, it turn out that the six-doped systems show conductivity except the Yb-dopedsystems. With the increase of atomic number of rare earth element, La, Sm, Gd, Dy, and Hodoped BaTiO3systems have a gradual increase trend of quantity of Density of States at Fermisurface and the conductivity of the system. In Yb-doped system, quantity of Density of Statesbecomes very small, and does not have obvious conductivity. In the Lu-doped system, theDensity of States is bigger than it of La, Sm, Gd, Dy, and Ho, the system present conductivity,which is the same as the previously mentioned five kinds of doped system. The result is inconformity with the relevant experimental results of the literature.