Akihide Kuwabara

The electric field induced ferroelectric phase transition of AgNbO3

Coexistence of two phases of AgNbO3 is shown to explain the experimentally observed polarization–electric field hysteresis loop better than either phase in isolation, based on detailed first-principles calculations of the structural changes and stabilities of different phases of this compound. Calculations confirm a ferroelectric phase transition, whereby the symmetry of the AgNbO3 crystal switches from antiferroelectric Pbcm to ferroelectric Pmc21, under an electric field of 9 MV/cm. The calculated spontaneous polarization (0.61 C/m2) under this field compares well with the experimental value of 0.52 C/m2. After transforming, the structure remains in the ferroelectric state even after the electric field is removed, despite the structure being energetically metastable. As the energy difference between the antiferroelectric and ferroelectric phases is only +0.5 meV/f.u. and the potential energy barrier between them (∼40 meV/f.u.) is comparable to thermal fluctuation energies, it is possible for these two p...

Machine-learning-based selective sampling procedure for identifying the low-energy region in a potential energy surface: A case study on proton conduction in oxides

In this paper, we propose a selective sampling procedure to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest. The proposed sampling procedure is based on a machine-learning method called the Gaussian process, which is used to construct a statistical model of the PES for identifying the region of interest in the configuration space. We demonstrate the efficacy of the proposed procedure for atomic diffusion and ionic conduction, specifically, the proton conduction in a well-studied proton-conducting oxide, barium zirconate

Local condensation around oxygen vacancies in t-LaNbO4 from first principles calculations

({\mathrm{BaZrO}}_{3})

Ferroelectricity in wurtzite structure simple chalcogenide

. The results of the demonstration study indicate that our procedure can efficiently identify the low-energy region characterizing the proton conduction in the host crystal lattice and that the descriptors used for the statistical PES model have a great influence on the performance.

Effect of oxygen vacancy segregation in Au or Pt/oxide hetero-interfaces on electronic structures

First principles calculations reveal that formation of a fully ionized oxide vacancy leads to local condensation of coordination polyhedra forming Nb3O11(7-) in t-LaNbO4.

Theoretical and experimental studies of formation and migration of oxygen vacancies in BaMxTi1−xO3(M = Zr, Ge)

The possibility of the new class ferroelectric materials of wurtzite structure simple chalcogenide was discussed using modern first-principles calculation technique. Ferroelectricity in the wurtzite structure (P63mc) can be understood by structure distortion from centrosymmetric P63/mmc by relative displacement of cation against anion along c-axis. Calculated potential surface of these compounds shows typical double well between two polar variants. The potential barriers for the ferroelectric polarization switching were estimated to be 0.25 eV/f.u. for ZnO. It is slightly higher energy to the common perovskite ferroelectric compound PbTiO3. Epitaxial tensile strain on the ab-plane (0001) is effective to lower the potential barrier. The potential barrier decreased from 0.25 to 0.15 eV/f.u. by 5% ab-plane expansion in wurtzite structure ZnO. Epitaxial ZnO thin film with donor type defect reduction should be a possible candidate to confirm this ferroelectricity in wurtzite structure simple chalcogenide.

Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

We investigated the effects of oxygen vacancy segregation on electronic structures in the vicinity of hetero-interfaces between noble metals (Au and Pt) and yttria stabilized zirconia (YSZ) by performing first-principles calculations and Bader analysis. The density of states (DOS) of the Au/YSZ interface around the Fermi level is less than that of the Pt/YSZ interface, resulting from the lower density of Au-s and -p orbitals around the Fermi level compared with the higher density of Pt-d orbitals. Metal layers adjacent to the interface become negatively charged when the bonding oxygen layer contains a high concentration of oxygen vacancies. These results indicate that segregation of oxygen vacancies to Au or Pt/YSZ hetero-interfaces increase the electronic conductivity of Au or Pt atom layers at the interfaces.

A combined conductivity and DFT study of protons in PbZrO3 and alkaline earth zirconate perovskites

The formation and migration energies of oxygen vacancies in pure BaTiO3, and BaM x Ti1− x O3 (M = Zr, Ge) are calculated by first-principles calculations to understand the effect of doping on the reliability of multilayer ceramic capacitors (MLCCs). The formation and migration energies of oxygen vacancies are found to be larger in BaZr x Ti1− x O3 than in BaTiO3. This finding could be one of the possible reasons behind the improved reliability of Zr-doped MLCCs materials. On the other hand, by substituting Ge, the migration energy of BaGe x Ti1− x O3 becomes larger than that of BaTiO3. This is despite the smaller oxygen vacancy formation energy in BaGe x Ti1− x O3 than in BaTiO3. Even though Zr and Ge are tetravalent in BaM x Ti1− x O3, their valence states are different after the formation of oxygen vacancies, providing an explanation for the differences in vacancy formation and migration energies between BaZr x Ti1− x O3 and BaGe x Ti1− x O3. Our theoretical results are further confirmed by experiments on these model systems.

ZnTaO2N: Stabilized High-Temperature LiNbO3-type Structure

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P63mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P63/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (Ec) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in loweri...