Takaya Akashi

Solid-state reaction kinetics of LaCrO3 from the oxides and determination of La3+ diffusion coefficient

LaCrO{sub 3} is a p-type semiconductor, and it is stable in both oxidizing and reducing atmospheres at elevated temperatures. LaCrO{sub 3} doped with Ca or Sr exhibits high electrical conductivity. Due to these properties, alkaline earth-doped LaCrO{sub 3} is used as an interconnector for solid oxide fuel cells (SOFC), which are attractive because of their high energy conversion efficiency and clean exhaust gas. Since the interconnector is used at elevated temperatures with a large difference of oxygen pressure between anode and cathode, it is extremely important to clarify its transport properties to understand demixing and creep. In several oxides with the perovskite structure, oxide ion diffusivity has been reported. On the other hand, cation diffusivity, which probably governs the demixing and creep processes, has been scarcely reported. The parabolic rate constant for the solid-state reaction of 1/2La{sub 2}O{sub 3} + 1/2Cr{sub 2}O{sub 3} = LaCrO{sub 3} was measured at temperatures between 1,483 and 1,688 /K in air and at 1695 K in oxygen pressures between 2.1 {times} 10{sup 4} and 1.5 {times} 10{sup {minus}5} Pa.

A ferroelectric barium titanate, BaTi2O5

The crystal structure of monobarium dititanium pentaoxide, BaTi(2)O(5), synthesized by a floating-zone method, was studied by X-ray diffraction. Previous reports describe the structure as being in the monoclinic centrosymmetric space group C2/m. We have recently found that this material exhibits ferroelectricity, and therefore BaTi(2)O(5) should have lower symmetry. The crystal structure of BaTi(2)O(5) was refined in space group C2, revealing a displacement of the Ti atoms along the b axis. This result is consistent with the fact that the ferroelectricity of BaTi(2)O(5) was only observed along the b axis.

Temperature dependence of electron and hole mobilities in heavily impurity-doped SiGe single crystals

Heavily impurity-doped single crystals of SixGe1−x alloy with the composition 0.84<x<1 and large-grained polycrystals with x=0.80 were grown by the Czochralski technique. The Hall-coefficient measurements of the electron and hole mobilities in the grown crystals were carried out in the temperature range of 300–1000K and compared with those in undoped SiGe. The Hall mobilities of electrons and holes in SiGe with a carrier concentration of 1019–1020cm−3 both show a Tn, n∼1, temperature dependence up to elevated temperatures. This indicates that the carrier transport process is mainly rate controlled by charged impurity scattering. In single-crystal SiGe free from grain-boundary effects, the hole mobility increases with decreasing Si content at least up to 0.84 and the electron mobility is greater than in Si and polycrystalline SiGe. These results suggest that scattering processes in alloy semiconductors are more complicated than previously thought.

Incommensurate crystallographic shear structure of BaxBi2-2xTi4-xO11-4x (x = 0.275)

The title compound generates diffraction patterns which are indexable within the framework of the higher-dimensional description of incommensurate structures. However, it is difficult to discriminate the main reflections from the satellite ones. This paper has clarified that the structure can be treated as a strongly modulated structure with sawtooth-like modulation functions and is classified as an incommensurate crystallographic shear (CS) structure. The structure consists of domains isostructural to beta-Bi(2)Ti(4)O(11) and domain boundaries composed of TiO(6) octahedra. Ba and Bi ions are accommodated in the cavities between TiO(6) octahedra in the domain. Domain boundaries are aperiodically inserted, in contrast to the usual CS structures, forming an incommensurate structure.

Anisotropy of electrical conductivity for alkaline earth β-alumina single crystals prepared by a floating zone method

Abstract Sr and Ba β-alumina single crystals were prepared by a floating zone (FZ) method. Their electrical conductivities in air were measured by ac impedance spectroscopy in the temperature range between 873 and 1573 K. Both single crystals cleaved along the (0004) plane. The electrical conductivity in parallel to the cleavage plane was 10–100 times greater than that of perpendicular to the cleavage plane. Sr β-alumina single crystal had higher electrical conductivity than Ba β-alumina single crystal.