Takahisa Omata

New ultraviolet‐transport electroconductive oxide, ZnGa2O4 spinel

ZnGa2O4 having the spinel structure was prepared, and optical and electrical properties of the material were measured. By the measurements of diffuse reflectance spectra, ZnGa2O4 was found to have a wider band gap (∼5 eV) than ITO (indium tin oxide). Electrical conductivity of the H2‐annealed ceramic of ZnGa2O4 was 3×101 S cm−1. Thus the ZnGa2O4 spinel was found to be a new UV‐transparent electronic conductor.

Size dependent optical band gap of ternary I-III-VI2 semiconductor nanocrystals

The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells.

New oxide phase with wide band gap and high electroconductivity, MgIn2O4

It was demonstrated that the MgIn2O4 spinel is a very promising material as a transparent electronic conductor. By the measurements of diffuse reflectance spectra, the optical band gap of MgIn2O4 (∼3.4 eV) was found to be wider than that of ITO (indium tin oxide). Electrical conductivity of the sintered sample of MgIn2O4 at room temperature has reached almost 102 S cm−1 with no intentional doping. The conduction was found to be due to electrons introduced from oxygen vacancies.

Crystal structure of metastable κ-CeZrO4 phase possessing an ordered arrangement of Ce and Zr ions

Abstract A pyrochlore-type precursor having less anti-phase domain boundaries was prepared by reducing a single t′-(Ce0.5Zr0.5)O2 phase at 1573 K. Its oxidation at 873 K proceeded leaving the ordered arrangement of Ce and Zr ions. In the powder X-ray diffraction (XRD) pattern of the metastable κ-CeZrO4 phase obtained, many small and sharp superlattice diffractions, characterizing the crystal structure of the κ-CeZrO4 phase, were observed in addition to the diffractions observed in the pyrochlore-type precursor. All the diffraction peaks could be indexed on the basis of the cubic lattice with a=1.05270 nm. The condition limiting possible diffractions for space groups of P23, P213 and P 4 3m agreed with the XRD data and also the space groups permitted the ordered arrangement Ce4+ and Zr4+ ions along with 〈110〉 direction similar to the pyrochlore-type precursor. Finally, it was found from Rietveld analysis that the κ-CeZrO4 phase belongs to the space group of P213. In the ZrO8 polyhedron of the κ-CeZrO4, long and short Zr–O bonds compared with the average Zr–O bond length existed. The long and short Zr–O bonds may relax the compulsive 8-fold coordination of Zr4+ ions. A pair of enantiomorphic coordinates belonging to the same space group of P213 was appeared for the κ-CeZrO4.

New oxide phase with wide band gap and high electroconductivity CdGa2O4 spinel

CdGa2O4 spinel was found to be a promising material as a transparent electronic conductor. By measurements of diffuse reflectance spectra, CdGa2O4 was found to have a wider band gap (∼3.5 eV) than indium tin oxide. The electrical conductivity of the sintered sample of CdGa2O4 was over 102 S cm−1 with no intentional doping.

Generation of electron carriers in insulating thin film of MgIn2O4 spinel by Li+ implantation

Li+ implantation at room temperature of insulating thin films of polycrystalline MgIn2O4 spinel (∼1.3 μm thick) was found to generate electron carriers efficiently. Li+ ions were implanted at 80 keV to a fluence of 1×1016 cm−2 and subsequently at 160 keV to the same fluence. Some implanted films were subjected to a post‐annealing at 300 °C. Depth profiles of implanted Li+ ions measured with secondary‐ion‐mass spectroscopy agreed with that calculated with the trim code. Conductivity at room temperature increased from σ<10−7 to ∼101 S cm−1 upon the Li+ implantation. The generation yield of electron carriers in the as‐implanted film was ∼20% and increased up to ∼40% upon post‐annealing. Two optical‐absorption bands were induced upon the implantation, one at about ∼500 nm and another above ∼1000 nm extending to the IR region, which was attributed to plasma oscillation of electron carriers. The former band faded and the latter absorption increased its intensity upon post‐annealing. He+ implantation, which was ...

Water and hydrogen evolution properties and protonic conducting behaviors of Ca2+-doped La2Zr2O7 with a pyrochlore structure

Abstract Undoped and Ca 2+ -doped La 2 Zr 2 O 7 with a pyrochlore structure, i.e. (La 2 − x Ca x )Zr 2 O 7 − δ , La 2 (Zr 2 − x Ca x )O 7 − δ and (La 2 − x / 2 Ca x /2 )(Zr 2− x /2 Ca x /2 )O 7 − δ , were prepared by a conventional ceramic method. Composition dependencies of a lattice parameter showed a slight decrease, a sharp increase and a slight increase with increasing x in systems of (La 2− x Ca x )Zr 2 O 7− δ , La 2 (Zr 2− x Ca x )O 7 − δ and (La 2− x /2 Ca x / 2 )(Zr 2− x / 2 Ca x / 2 )O 7− δ , respectively. Proton concentrations in these oxides were evaluated by detection of evolved H 2 O gas, which was generated by thermal desorption, using a quadrupole mass spectrometer. Although no evolutions of H 2 O, H 2 or O 2 gases were detected for undoped La 2 Zr 2 O 7− δ , clear evolutions of H 2 O gas were observed for all Ca 2+ -doped samples. The amount of evolved H 2 O per mole of oxide, [H 2 O], was found to be 3.0×10 −2 for (La 1 . 9 2 Ca 0 . 0 8 )Zr 2 O 7− δ , 8.4×10 −3 for La 2 (Zr 1 . 9 6 Ca 0 . 0 4 )O 7− δ and 8.2×10 −3 for (La 1 . 9 8 Ca 0 . 0 2 )(Zr 1 . 9 8 Ca 0 . 0 2 )O 7 − δ . Further, small but clear evolutions of both hydrogen and oxygen were detected only for (La 1 . 9 2 Ca 0 . 0 8 )Zr 2 O 7− δ . Differences in the amount of evolved water, and the starting and maximum temperatures of the water evolution were discussed with reference to characteristics of the pyrochlore structure. E.m.f. measurements for various gas concentration cells with the sintered (La 1.98 Ca 0.02 )(Zr 1.98 Ca 0.02 )O 7− δ pellet as a solid electrolyte showed that the mean transference number of proton was 1 at ≤873 K and 0.82 at 1073 K. Thus, Ca 2+ -doped La 2 Zr 2 O 7− δ with a pyrochlore structure is a promising protonic conductor at high temperature.

Electrical Conduction Properties of Sr-Doped LaPO4 and CePO4 under Oxidizing and Reducing Conditions

Electrical conduction properties of Sr-doped LaPO 4 and Sr-doped CePO 4 under H 2 O/O 2 and H 2 O/H 2 conditions were investigated with conductivity measurements. Conductivities of I mol % Sr-doped LaPO 4 were 10 - 5 . 2 -10 - 3 . 5 S cm - 1 at 500-925°C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO 4 , conductivities were 10 - 2 . 8 -10 - 2 . 0 S cm - 1 at 500-925°C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO 4 under the same conditions. Such high conductivities of Sr-doped CePO 4 seemed attributable to electronic conduction due to partial oxidation of Ce 3 + to Ce 4 + caused by substituting Sr2 + for Ce 3 + . Under wet reducing conditions, however, conductivities of the material decreased to 10 - 5 . 2 -10 - 3 . 4 S cm - 1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO 4 . It was concluded that Sr-doped CePO 4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO 4 and CePO 4 under wet oxidizing and reducing conditions are discussed.

Oxygen release behaviour of Ce(1−x)ZrxO2 powders and appearance of Ce(8−4y)Zr4yO(14−δ) solid solution in the ZrO2–CeO2–CeO1.5 system

Abstract To clarify the existence of metastable phases in the ZrO 2 –CeO 2 –CeO 1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce (1− x ) Zr x O 2 ) with various compositions, x , in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t′ and t″ phases was very complicated. The single κ phases, (Ce (1− x ) Zr x O 2 ) with the composition, x =0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O 2 gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of κ phase may be x =0.45∼0.65. A new tetragonal phase t*, (Ce (1− x ) Zr x O 2 ), which was attained by cyclic redox process together with annealing in O 2 gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x =0.20∼0.65. A metastable solid solution expressed by a chemical formula of Ce (8−4 y ) Zr 4 y O (14− δ ) ( y =0∼1) possessing a CaF 2 -related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce (8−4 y ) Zr 4 y O (14− δ ) solid solution was proposed.

Preparation of MgIn2O4-x thin films on glass substrate by RF sputtering

MgIn2O4-X thin films were deposited onto a silica glass plate by the RF sputtering method. The highest conductivity observed for the film post-annealed under H2 flow was 2.3×102 S/cm, with a carrier concentration of 6.3×1020 cm-3 and a mobility of 2.2 cm2V-1s-1. No distinct optical absorption band was observed in the visible region.