First Principles Calculation Of Properties For Iron_based Materials

Based on the density functional theory (DFT) of first principles, using the full potential linearized augmented plane wave method with the improved local orbital(APW+lo) and the generalized gradient approximation (GGA), the lattice structures, energies, electronic bands, density of states (DOS) and magnetism of 26 different iron-based compounds are calculated for the magnetic materials with the ThMn12-type structure, and the superconductor materials with the similar layer structure derived from the FeAs atomic group including four types of composition "1111", "122", "111" and "011". The relationships of the composition to the lattice, energy, electronic bands, DOS, magnetism, and superconductivity are discussed.The progress in the magnetic R(Fe,M)12 and novel iron-based superconductor materials are introduced in the first chapter. The intermetallic compounds R(Fe, M)12 with the ThMn12-type structure are concerned widely for their prefect magnetism, such as the high magnetic saturation, the strong magnetic anisotropy, and the low density of rare earth/ transition elements. In January,2008, the discovery of iron-based materials LaO1-xFxFeAs, with superconducting critical temperature 26 K opened a new round of research on the high-temperature superconducting materials. Such kind of superconductor contains the magnetic Fe atoms which are used to be considered disadvantageous to the superconductivity, thus a challenge rises to understand the nature and mechanism for superconductivity becomes a challenge. The research of lattices, electronic bands, magnetism, and superconductivity of Iron-based materials are the front of the field of superconducor.The brief of the quantum mechanics in solid state is given in the second Chapter. The theoretical methods and approximations of quantum mechanics used in the calculation for solid states are introduced, including the basic framework and progresses of DFT, local density approximation (LDA), GGA, non-local functional, self-interaction correction and some other functional forms are introduced. Some codes used for the crystal structure based on the DFT of first principle are presented. And the calculation methods and functions of the wien2k code used in this dissertation are shown in the end of the chapter.Based on the DFT of first principle, using the methods of APW+lo and GGA, the lattices, energies, electronic bands, DOS and magnetism are calculated in the third chapter for Y, Fe, Cr, V, Y2Fe17, YFe11M(M=Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn), YFe12-x,Crx(x=0-5) and YFei2-xCrx (x=0-5). Taking YFe12-xCrx and YFe12-xVx as examples, the nature of the unstable binary RFe12 system with the ThMn12-type structure and the stabilization effect of the third element M are investigated. The reaults show that the pure binary RFe12 would spontaneously decomposes to a mixed phase Y2Fe17+Fe with the lower energy, the additional third party such as V and Cr elements, the energy of YFe12-xMx would get lower and form the stable ThMn12-type structure. The effect of the third element M for the magnetism, electronic bands, and atomic sites are discussed and compared with experiment.Based on the DFT of first principle, using the methods of APW+lo and GGA, the lattices, energies, magnetism, electronic bands and DOS of fifteen iron-based superconducting compounds including LaOFeAs, Ca1-xKxFe2As2, AeT2Pn2, NaFeAs, LiFeAs, and FeSe are calculated. The results show that, the LaOFeAs system with anti-ferromagnetism is more stable than the system with spin polarization. The negative magnetic moment of LaOFeAs comes from the delocalized electrons near theгpoint. It is concluded that the vibration of the oxygen atoms might couple with the electronic transitions among the bands near theгpoint. It is shown that the doping K atom in Ca1-xKxFe2As2 would increase the lattice parameters a, b, energies of the valent bands and DOS for the spin down electrons, and would decrease the parameter c and the lattice volume, and thus would enhance the magnetism of the system. Based on the analysis of the relationship among the lattices, energy, magnetism, electronic bands, DOS and superconducting transition temperature of AeTT2Pn2 series, five advantageous factors are derived for the superconducting critical temperature. It is shown that the magnetism of AeFeAs and FeSe decreases rapidly with the decrease of their volumes about the equilibriums. A strong correlation between the superconductivity and magnetism of the iron-based materials is indicated by the experimental pressure effect of superconductivity.The conclusions of this dissertation are shown in the final chapter.