Electronic States of Impurities and Their Effect on Material Properties

Abstract

Abstract Electronic structure calculations were performed to study the effect of impurity on material properties by using the DV-Xα molecular orbital method. Firstly, calculations were carried out for halogen atoms (cluster) substituted for oxygen atoms in a Cu-O plane in copper oxides. It was found that the energies of the orbitals belonging to F atoms make a small contribution to the highest occupied orbital, whose energy corresponds to the Fermi energy. A small amount of charge carriers therefore can enter F impurity sites. Thus, it is expected that F impurities will work effectively as pinning centers and enhance the critical current density in superconductors. Secondly, we focused on the role of oxygen impurities in element-selective corrosion of austenitic stainless steels in liquid sodium. Calculation was carried out for an oxygen impurity associated with an Na atom located at the site on top of a Cr atom at the Fe(001) surface in sodium. The result shows that a positively charged Cr atom will be released selectively into sodium by an O anion. Thirdly, we examined the ion-implantation effect on the bond between composite atoms of Si3N4. When Fe, Mo and Hf atoms are supposed to be ion-implanted, d energy levels of these atoms appear in the energy gap or near the gap. A transfer of the Mulliken charge between the implanted atoms and surrounding atoms occurs due to the mixing between the d orbitals of the implanted atom and sp orbitals of surrounding atoms. Due to the reduced ionicity of Si, the attack on ceramics atoms by 0 (oxygen) inliquid sodium becomes weak. At the same time, however, the covalent bond strength betweenSi and N becomes weaker, which may in some cases depress corrosion resistance.