Yu Kumagai

Anti‐ferrodistortive‐Like Oxygen‐Octahedron Rotation Induced by the Oxygen Vacancy in Cubic SrTiO3

New insights into the atomistic and electronic structure of the oxygen vacancy in SrTiO(3) are presented through first-principles calculations. The oxygen vacancy induces a local anti-ferrodistortive-like oxygen-octahedron rotation, even in the cubic phase. This feature leads to localized electronic states in the bandgap, giving an excellent explanation to the thermal ionization and optical transition observed experimentally.

Crystal and electronic structure and magnetic properties of divalent europium perovskite oxides EuMO3 (M = Ti, Zr, and Hf): experimental and first-principles approaches.

A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).

All-electron CI calculations of 3d transition-metal L2,3 XANES using zeroth-order regular approximation for relativistic effects

X-ray-absorption near-edge structures (XANES) at 3d transition-metal (TM) L(2,3) edges are computed using the all-electron configuration interaction (CI) method. Slater determinants for the CI calculations are composed of molecular orbitals obtained by density functional theory (DFT) calculations of model clusters. Relativistic effects are taken into account by the zeroth-order regular approximation (ZORA) using two-component wavefunctions. The theoretical spectra are found to be strongly dependent on the quality of the one-electron basis functions. On the other hand, a different choice of the exchange-correlation functionals for the DFT calculations does not exhibit visible changes in the spectral shape. Fine details of multiplet structures in the experimental TM L(2,3) XANES of MnO, FeO and CoO are well reproduced by the present calculations when the one-electron basis functions are properly selected. This is consistent with our previous report showing good agreement between theoretical and experimental TM L(2,3) XANES when four-component relativistic wavefunctions were used.