First-principles Calculations On The Metal Fluorides And Metal Phosphides

In this dissertation, first principles calculations based on density function theory have been carried out to investigate the metal fluorides (BiF3and Fe-F compounds) and metal phosphides (Co-P compounds, ZnP2, GaP and InP), which have been used as cathode material and anode material in the Li-ion batteries, respectively. Firstly, the effects of O and Te doping on the structural, magnetic and electronic structure of BiF3have been studied. Secondly, the structural property of FeF2and the effect of Co doping on the FeF3have also been studied. Besides, the structural stability and electronic structure of Co-P compounds have been calculated and compared. Finally, the elastic properties and electronic structure of ZnP2, GaP and InP have also been discussed. The major works are summarized as follows:(1) The effects of O doping on the structural, magnetic and electronic structure of BiF3have been investigated. Based on the calculated cohesive energies, O impurities prefer substituting F atom at tetrahedral sites (0.25,0.25,0.25) and Bi4OF11(Ⅱ) is formed. And the volume of BiF3changes little due to similar radius of O and F atoms. By analyzing density of states of Bi4OF11(Ⅱ), it has been found that Bi4OF11(Ⅱ) presents magnetic character and half-metallic state, implying its enhanced conductivity and potential applications in Li-ion batteries. The character of bond in Bi4OF11(Ⅱ) was discussed by analysis of the charge density and bader charge. The results show that O doping weakens ionic bond in BiF3.(2) The effects of Te doping BiF3on the crystal structure, magnetic and electronic structure of BiF3have been investigated. By calculating lattice constants, formation energy and cohesive energy, it is found that the volume of BiF3shrinks and the structural stability of Te doped BiF3becomes worse and the band gap of BiF3decreases with the increase of Te doping concentration.(3) First-principles GGA+U calculations have been used to investigate the magnetic properties, electronic structure, and bonding mechanism of FeF2. By calculating lattice constants and magnetic moment as a function of effective Hubbard U (Ueff), it has been found that the optimum value of Ueff is equal to4eV, the magnetic moment is1.752μB and the value of c/a is0.704, which are in good agreement with experimental results. The calculated band gap of FeF2is2.565eV. Besides, the electronic structure and bonding mechanism of FeF2are investigated by the analysis of electron localization function (ELF), Bader charge and charge density. The results show that bonding behavior between Fe and F atoms exhibits a mixture of ionic and little covalent character.(4) The theoretical studies of three types of doped models (CoFe5F18, Co2Fe4F18and Co3Fe3F18) have been calculated. It is found that the stability of Co-doped FeF3decreases with the increase of Co-doping concentration. And that Co doping has little impact on the crystal structure of FeF3. But Co-doping can transfer G-type anti-magnetic structure of FeF3to metamagnetism structure. Besides, the band gap of FeF3decreases via Co doping.(5) The structural stability and electronic structure of Co-P compounds such as Co2P (Ⅰ)(orthorhombic), Co2P(Ⅱ)(hexagonal),CoP,CoP2and CoP3have been investigated. The stability of Co-P compounds decreases with the increase of P element. By analyzing the electronic structure of Co-P compounds, it is found that Co2P(Ⅰ)(orthorhombic), Co2P(Ⅱ)(hexagonal) and CoP show metallic character, while CoP2and CoP3show semiconductor character.(6) The lattice constants, elastic properties, Debye temperature and electronic structure of poly crystalline tetragonal ZnP2have been calculated. The bulk modulus (B), shear modulus (G), Young’s module (E), and elastic anisotropy of polycrystalline tetragonal ZnP2were evaluated. The results showed that the tetragonal ZnP2has large elastic isotropy in compressibility and small elastic anisotropy in shear. Besides, tetragonal ZnP2shows slightly brittle characteristic.(7) The elastic properties and electronic structure of GaP and InP have been studied systematically. The elastic constants of GaP and InP have been calculated. From the calculated elastic constants, bulk modulus (B), shear modulus (G), Young’s modulus (E) and Poisson’s ratios (ⅴ) of corresponding polycrystalline materials have been deduced, by using Reuss-Voigh-Hill (RVH) approximations. The Debye temperature (TD) of both compounds was finally obtained. The calculated results indicate that InP shows ductile characteristic while GaP shows brittle characteristic.As a result, InP has better plasticity than GaP.