Yoshitaka Uratani

First-principles predictions of giant electric polarization

Giant electric polarization of more than 150 µC/cm2 is predicted for PbVO3 and BiCoO3 on the basis of the first-principles Berry-phase method. The stable crystal structure is tetragonally distorted with a large c/a ratio and significant ionic displacements breaking centrosymmetry. In PbVO3, the key factor that stabilizes such a highly distorted structure and realizes an insulating electronic structure, leading to giant polarization, is the coexistence of ferro-orbital and antiferro-spin orderings in the V4+ d1 configuration. It is shown that the same electronic mechanism works also for BiCoO3 with Co3+ d6 configuration.

First-Principles Study of Lead-Free Piezoelectric SnTiO3

We perform first-principles electronic structure calculations for the lead-free perovskite SnTiO3. Structural optimization is carried out to get the stable tetragonal P4mm structure. Frozen-phonon calculation shows the lattice instability at the centrosymmetric tetragonal P4/mmm structure and the local stability of the P4mm structure. By using the Berry-phase method, the electric polarization and the piezoelectric coefficients are evaluated and found to be comparable with those of PbTiO3.

First-principles study on the electronic structure of bismuth transition-metal oxides

The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO 3 as a typical system in perovskite Bi transition-metal oxides (BiMO 3 ) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn eg orbitals is actually realized in BiMnO 3 , being consistent with crystallographic data, and plays a crucial role in the appearance of ferromagnetism. Total-energy calculation shows the ferromagnetic state is indeed stabilized. Electrical polarization of BiMnO 3 is also estimated based on the Berry phase theory. Lattice instability to off-centred displacement, which is driven by strong covalent bonding between Bi 6p and O 2p states, is found to be rather common in the BiMO 3 series.

First-Principles Study on the Magnetic Anisotropy in Multiferroic PbVO3 and BiCoO3

Magnetic anisotropy energies (MAE) of multiferroic PbVO 3 and BiCoO 3 are evaluated from first-principles density functional calculations. Even though both oxides have similar crystal and electronic structures, calculated easy axes of spin are different: [110] in PbVO 3 and [001] in BiCoO 3 . To explain the difference, the origin of MAE is discussed with a perturbation theory by taking into account of the electronic structure obtained by the first-principles calculations.