First-principles calculation of formation energies and electronic structures of hydrogen defects at tetrahedral and octahedral interstitial sites in pyrochlore-type Y₂Ti₂O₇ oxide

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 O^2- 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.