Hidesada Natsui

Nanosized hexagonal Mn- and Ga-doped BaTiO3 with reduced structural phase transition temperature

Nanosized hexagonal BaTiO3 particles were synthesized at 1373 K, grain growth was prevented by reducing the temperature of the cubic–hexagonal phase transition. Crystal structures of nanosized hexagonal Mn- and Ga-doped BaTiO3 were studied by high-energy synchrotron radiation powder diffraction at BL02B2 of SPring-8. Substituting the trivalent cations of Mn and Ga for Ti weakened the Mn–O and Ga–O bonding forces and stabilized the hexagonal phase, which reduced the phase transition temperature. These nanoparticles can be used in the manufacture of electronic components, owing to the properties of hexagonal BaTiO3.

Structure of Glassy and Metastable Crystalline BaTi2O5 Fabricated Using Containerless Processing

We succeeded in synthesizing a bulk BaTi2O5 glass whose refractive index is extremely higher than conventional silicate glasses. Advanced synchrotron radiation, neutron measurements, XANES analyses and computer simulations revealed the formation of unusual TiO5 network with very random dense packed distribution of oxygen around the barium in BaTi2O5 glass. The 3-dimensional atomic structure of glassy BaTi2O5 (g-BaTi2O5), simulated by Reverse Monte Carlo (RMC) modelling on diffraction data, shows that extremely distorted TiO5 polyhedra interconnected with both corner- and edge-shared oxygen, formed a higher packing density structure than that of conventional silicate glass. We are sure that such an unusual structure is the origin of excellent optical properties.

Dielectric Properties of La Doped-Hexagonal BaTiO3 Single Crystal Grown by FZ Method

The oxygen-deficient hexagonal BaTiO3 shows huge dielectric constant. Because of the oxygen-deficient induced to change the valance from Ti4+ to Ti3+ and formed the charge imbalance in a lattice. However, this dielectric constant is easily disappeared when the oxygen-deficient is compensated. And it is difficult to control the amount of oxygen-deficient in hexagonal BaTiO3. In this study, the oxygen deficient is substituted by La to keep the charge imbalance. It was successful to retain the huge dielectric constant of La doped hexagonal BaTiO3. La3+ could induce to change the valance from Ti4+ to Ti3+. And it shows almost the same behavior on the complex plane of Z by an impedance measurement. La doped hexagonal BaTiO3 has a semiconductive behavior.

Diffusion Behavior of Oxygen and Dielectric Properties in the Hexagonal BaTiO3

The oxygen diffusivity of hexagonal barium titanate by the tracer method and its dielectric constants have been evaluated. The oxygen diffusivity of hexagonal barium titanate does not show any direction dependency along [0001] and [1000] directions in lattice. Because of the face-shared position is more favorable position of oxygen vacancy than the corner-shared position. The dielectric constant along [1000] direction is larger than the [0001] direction. The difference of dielectric constant between both directions was gradually decreased with increasing the annealing temperature owing to reduction of the oxygen vacancies.

Diffusion Behavior of Oxygen in the Hexagonal BaTiO3 Single Crystal Grown by FZ Method

Although the hexagonal barium titanate has been widely studied, the oxygen diffusivity has not been yet. The oxygen diffusivity is very important property, because the electrical properties are dominated by the amount of oxygen vacancy and the diffusivity. In this study, it is measured the diffusivity of oxygen by the tracer method and its activation energy was obtained from the Arrhenius plot. The oxygen diffusivity of hexagonal barium titanate is smaller than the cubic one and its activation energy, 1.73 eV is also smaller. Because of the formation energy of oxygen vacancy in the hexagonal structure is 0.884eV smaller than the cubic structure. These are depended on the distance of nearest O-O bonding, respectively. The formation energies were calculated by the first principle calculation using VASP code.

Crystal structure analysis of manganese and gallium doped hexagonal BaTiO 3 by synchrotron radiation powder diffraction

Crystal structures of manganese doped and gallium doped metastable hexagonal barium titanate were investigated by high energy synchrotron radiation powder diffraction at BL02B2 in SPring-8. The Rietveld analyses reveal that the lattice of gallium doped sample is expanded along the c-axis, while the lattice is not changed in the manganese doped sample, although the ionic radius of gallium is almost same as that of titanium, and besides, that of manganese is fairly smaller. These results imply that the lattice is essentially expanding by the doping. It is considered that the lattice expanding is significant in the stabilization of the hexagonal phase.