The formation energies of hydrogen (H) defects at tetrahedral and octahedral interstitial sites in a pyrochlore-type yttrium-titanium oxide (Y2Ti2O7) were calculated using density functional theory. The results were compared with the corresponding energies of H and helium (He) defects in iron (Fe) with a body-centered cubic (bcc) crystal structure and Y2Ti2O7 interstitial and substitutional sites in the matrix. The formation energy of a H defect at a tetrahedral site in Y2Ti2O7 (2.87 eV), which is lower than that at an octahedral site (3.01 eV), was larger than the corresponding energies (0.11, 0.24, and 0.41 eV) at tetrahedral interstitial, octahedral interstitial, and substitutional sites in Fe, respectively. This result indicates that H atoms in nanostructured ferritic alloys, constructed by only bcc Fe containing nanometer-sized Y2Ti2O7 nanofeatures, prefer to occupy tetrahedral interstitial sites in Fe, whereas He atoms prefer to occupy octahedral interstitial sites in Y2Ti2O7. The electron-density distribution and density of states of interstitial H in Y2Ti2O7 reveal an essentially attractive chemical interaction resulting from charge transfers from neighboring O2- anions and surrounding Y3+ and Ti4+ cations, whereas those of interstitial He reveal a repulsive interaction. Therefore, the doping H and He atoms prefer to occupy smaller tetrahedral and larger octahedral sites, respectively.
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