Evaluation of electronic states of implanted materials by molecular orbital calculation

Saito, Junichi  ; Kano, Shigeki

In order to understand the effect of implanted atom in ceramics and metals on the sodium corrosion, the electronic structures of un-implanted and implanted materials were calculated using DV-X cluster method which was one of molecular orbital calculations. The calculated materials were -SiN, -SiC and -SiC as ceramics, and f.c.c.Fe, b.c.c.Fe and b.c.c. Nb as metals. An Fe, Mo and Hf atom for ceramics, and N atom for metals were selected as implanted atoms. It is known that these metallic elements have exhibited the excellent corrosion resistance against liquid sodium in previous experiments. The summary of results is shown as follows. Energy levels of implanted atom appeared in or near energy band gap in ceramics. The change of ionicities of each element which showed amount of transferred charges and bond order which showed the covalent bond strength between atoms depended on substrates and implanted atoms. The strength of ionic bonding between atoms reduced in -SiN, as ionicities of constituent atom decreased by the implanted atom. The decreases of ionicity depended on implanted atoms and Hf implantation showed the largest decrease in implanted atoms. The bond order in -SiN and -SiC decreased by the implantation. When the implanted atom occupied at substitutional site in -SiC, the ionicities decreased but the bond order increased. Electron state densities of s and p components of implanted N atom appeared lower energy level than that of d component of constituent Fe (or Nb) atoms. The charge transfer took place to N atom from surrounding Fe (or Nb) atoms. Thus the ionicity of N atom was larger than mother metal. The bond order between mother elements reduced by N implantation. In particularly, there were significant decreases of bond order in b.c.c. Fe and b.c.c. Nb. Hence, the bond order in whole cluster decreased largely except for f.c.c. Fe. Consequently, it is ...