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.
The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO 3 as a typical system in perovskite Bi transitionmetal oxides (BiMO 3 ) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn e g 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.
We perform first-principles electronic structure calculations for the lead-free perovskite SnTiO 3 . 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 PbTiO 3 .
The possible origin of ferromagnetism in PbTiO 3 containing vacancies is investigated by performing first-principles calculations. We demonstrate that O and Ti vacancies both induce ferromagnetism but by different mechanisms: the ferromagnetism driven by the O vacancy originates from the spin-polarized e g state of the nearest Ti atom, whereas that driven by the Ti vacancy is due to the half-metallic p x state of the nearest O atom. The results presented here provide fundamental insights into the design of multiferroics in conventional ferroelectrics.
Possible ferromagnetic and ferroelectric phases are explored for bismuth transition-metal oxides with double-perovskite structure A 2 BB 0 O 6 on the basis of first-principles calculations within the local spin-density approximation (LSDA) and generalized gradient approximation (GGA). It is found that a lattice instability of the cubic to a non-centrosymmetric phase always happens in the all cases of lead and bismuth perovskite oxides with the 3d transition-metal ions at the B site. Placing bismuth ion at the A site in the doubleperovskite structure, several sets of the 3d transition-metal ions are selected according to their total valence sum and the Goodenough-Kanamori rule for the superexchange coupling. Ferromagnetic solutions are actually obtained both within LSDA and GGA for Bi 2 CrFeO 6 , Bi 2 MnNiO 6 and Bi 2 CrCuO 6 . For non-centrosymmetric monoclinic Bi 2 MnNiO 6 , the ferromagnetic and ferroelectric phase has the spin magnetic moment of 5m B and the electric polarization of 28 mC=cm 2 .Condensed matter systems show a macroscopic response, either magnetic, electric or elastic, to the corresponding external field. Each response originates in spin and orbital magnetic moments, electric dipole or lattice strain in the microscopic level. Some systems reveal a spontaneous response associated with a ferro-ordering of the relevant microscopic degree of freedom under certain circumstances. Multiferroic materials have more than one ferro-orderings simultaneously. Among them, ferromagnetic and ferroelectric materials are potentially important for applications to next-generation devices such as spin field-effect-transistor or logic in memory and may involve many unsolved issues to be investigated in basic science. There are only a few known as ferromagnetic and ferroelectric materials, particularly with perovskite-type crystal structure. It is, therefore, quite important and valuable in materials science and engineering to explore possible ferromagnetic and ferroelectric materials.Considering why they are so rare in nature might provide us a clue to the exploration. The reason may be because most of magnetic perovskite oxides (typically denoted as ABO 3 ) have an antiferromagnetic order due to a superexchange coupling between the magnetic ions. It is well known, according to the Goodenough-Kanamori rule for the superexchange coupling [1,2], that certain combinations of the transition-metal ions B and B 0 (d 3 -d 5 and d 3 -d 8 electronic configurations) may possibly give a ferromagnetic coupling for the 180 superexchange mechanism. Note that the bond angle of B-O-B 0 is nearly 180 in the perovskite structure. Using this strategy, Ueda et al. have successfully fabricated a ferromagnetic superlattice stacking LaCrO 3 (d 3 ) and LaFeO 3 (d 5 ) layer by layer along the [1 1 1] direction [3].Ferroelectric materials should be insulating for preventing charge leakage and thus are likely ionic crystals. Furthermore, it is widely recognized that covalent bonds between the cation and anion orbitals are crucial to realize a...
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