Evolutions of geometry and electronic state introduced by oxygen vacancy for CaMnO3 compound

The cell structures, locally micro-structures, combinations, electronic states as well as carrier transports induced by Oxygen vacancy for the CaMnO3 compound have been systematically investigated by pseudo-potential plane wave functions within the framework of density functional theory calculational method. The results reveal that the Oxygen vacant CaMnO2.75 has larger formation enthalpy than that of the intrinsic CaMnO3 and the Oxygen vacancy formation energy of CaMnO3 is as large as 7.10 eV. The Oxygen vacant CaMnO2.75 is harder to form than the intrinsic CaMnO3. The cubical orthorhombic structure tends to be rhombohedral. The lattice parameters a, b and c decrease simultaneously. The α and γ shrinks, the β increases. The lattice shrinkage induced by oxygen vacancy is anisotropic. The MnO6 octahedrons is twisted and it is shrinked along c and expanded along a. The bond strengths of Mn-O are all enhanced. There are six energy bands and the band gap is narrowed, the Fermi level is heightened and the electrons tend to occupy high energy bands. At the same time, the work function of electrons is lowered from 4.69872 eV to 4.00048 eV. The conductivity should be enhanced for Oxygen vacant CaMnO2.75, the O p and Mn d state electrons contribute most to the upper valance bands as well as the lower conduction bands, they jointly realize the conduction process.