Study Of The Structure And Electronic Structure Of The Co-based Spinel Oxide Materials

In recent years,Co-based spinel materials have been widely used in many fields.Experimentalist can improve the performance of Co-based spinel materials through doping,designing structural morphology,and creating oxygen vacancies.However,it is difficult from experiment itself to give a clear explanation to the underlying mechanism behind the improved performance,and thus it is necessary to study the structural and electronic structures of these materials from atomic level.In this thesis,we have studied the atomic and electronic structures of the Co-based spinel materials from first-principles calculations,and analyzed the microscopic mechanism behind the macroscopic properties systematically.The main conclusions are summarized below:1)We found that the inverse spinel configurations of MCo₂O₄?M=Ni?Co?Mn?at higher temperatures is more stable than the spinel configurations with better electronic conductivity when the contribution of configurational entropy is considered.In the inverse spinel structures MnCo₂O₄ and NiCo₂O₄,the 3d orbitals of Mn and Ni atoms contribute the main part of the states near the Fermi level.From the crystal field theory,the double degenerate e_g orbitals of Mn and Ni split slightly under the octahedral crystal field,and the resulting two orbitals,whose energy level separation is small,exhibit as conduction and valence bands of the system.As a result,MnCo₂O₄and NiCo₂O₄ have better conductivity comparing with Co₃O₄.It is also found that F doping and O vacancies can further improve the conductivity of MnCo₂O₄.2)We studied the stable spinel NiCo₂O₄,and found that Jahn-Teller?J-T?distortion in the NiO₄ tetrahedron caused the transformation of NiCo₂O₄ from cubic phase to tetragonal phase,and the electronic structure changes from metallic to semiconducting.Due to the covalent bonding interaction between Ni-d_xz,Ni-d_yz,and O-2p orbits,J-T distortion results in the energy release of 0.84 eV?per chemical formula?.3)From energetic point of view,it is demonstrated that P atoms prefer to replace Co atoms,rather than O atoms in the P-doped Co₃O₄.P atoms are likely to act as an electron donor in an oxygen-rich environment,which is consistent with the electron-loss characteristics of Co atoms in Co₃O₄.The electronic properties of P and O in the P-doped Co₃O₄ make P atoms at O-sites being thermodynamically unstable.These results propose new mechanisms on P-doped Co-based spinel materials.