Mineo Sato

Enhancement of discharge capacity of Li3V2(PO4)3 by stabilizing the orthorhombic phase at room temperature

Abstract Li 3 V 2 (PO 4 ) 3 and solid solutions of Li 3−2 x (V 1− x Zr x ) 2 (PO 4 ) 3 were prepared by a solid state reaction. A high temperature orthorhombic phase of Li 3 V 2 (PO 4 ) 3 with a β-Fe 2 (SO 4 ) 3 -type was successfully stabilized at room temperature by substituting Zr for V with substitution ratios beyond x =0.05. The pure material of Li 3 V 2 (PO 4 ) 3 exhibited a cathode performance with two well defined regions of plateau at around 3.7 and 4.1 V vs. Li/Li + upon charging and 3.6 and 4.0 V vs. Li/Li + upon discharging, respectively, suggesting two types of phases produced upon the charge/discharge process. On the other hand, the cathode performance of the orthorhombic stabilized materials showed almost the same charge/discharge voltages as those of the pure material, but, with two plateaus slightly sloping, showed a considerably improved charge/discharge cycle performance compared to that of the pure material. Such improvement on the charge/discharge cycle performance is suggested to come from the disordered lithium ion arrangement in the orthorhombic phase.

New Silicate Phosphors for a White LED

We focus on the development of new silicate phosphors for a white LED. In the europium doped silicate system, four LED phosphor candidates-Li 2 SrSiO 4 :Eu 2+ , Ba 9 Sc 2 Si 6 O 24 :Eu 2+ , Ca 3 Si 2 O 7 :Eu 2+ and Ba 2 MgSi 2 O 7 :Eu 2+ were found. Luminescent properties under near UV and visible excitation were investigated for the new Eu 2+ doped LED silicate phosphors. These new phosphors have a relatively strong absorption band in a long wavelength region.

Transport studies of La1.92Sr0.08Cu1-xMxO4(M=Ni and Zn) and Nd2-yCeyCuO4 up to about 900 K

Abstract Various transport quantities have been measured for La1.92Sr0.08Cu1-xMxO4 (M=Ni and Zn) and Nd2-yCeyCuO4 over a wide temperature region to study the crossover-like change of the electronic state previously found in high-Tc oxides. The change shows a formation process of the anomalous low carrier concentration state near the Mott metal-insulator phase boundary. The Zn doping produces only a weak change of the T-dependence of the transport properties, except for electron localization effects at very low temperatures, while the Ni doping introduces a rather significant change of their T-dependence. Results of the measurements on Nd2-yCeyCuO4 have revealed that a rough relation of the electron-hole symmetry with respect to the “half-filled” Mott-insulating phase holds in the transport behaviors.

Photocatalytic water splitting on hydrated layered perovskite tantalate A2SrTa2O7.nH2O (A = H, K, and RB)

A series of layered perovskite tantalates, A2SrTa2O7 (A=H, Li, K, and Rb), were prepared as novel photocatalysts for photocatalytic water splitting into H2 and O2 under UV irradiation. The layered perovskite tantalates with hydrated interlayer space, A2SrTa2O7·nH2O (A=H, K, and Rb), showed higher H2 formation rate than anhydrous layered tantalate, Li2SrTa2O7, and anhydrous perovskite tantalate, KTaO3. H2SrTa2O7·nH2O and K2SrTa2O7·nH2O showed high activity for overall splitting of water without loading co-catalysts. The reaction over H2SrTa2O7·nH2O proceeded steadily more than 70 h, demonstrating a high durability of the catalyst. Effects of hydrated interlayer space on the catalytic activity were discussed on the basis of the results of photoluminescence spectra and the hydrogen evolution from aqueous solution of n-butylamine as a test reaction. The results indicate that the availability of interlayer space of layered tantalate as reaction sites is an important factor to improve the photocatalytic activity of Ta-based semiconductor materials.

Crystal structure determination and ionic conductivity of layered perovskite compounds NaLnTiO4 (Ln = rare earth)

Abstract The layered perovskite compounds NaLnTiO4 (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) were synthesized by the solid state reaction. The crystal structure of these compounds were determined by the Rietveld analysis. Single phases of the compounds (except for (Ln = Lu) could be prepared only under very restricted preparation conditions. The composition of NaLuTiO4 was not a single phase under the conditions employed in this study. Compounds NaLnTiO4 have a tetragonal symmetry for Ln = La−Nd, while an orthorhombic symmetry is observed for Ln = Sm−Lu. The stabilization of NaLnTiO4 is discussed on the basis of the relative sizes of the rare earth and the alkali metal ions. The lowering of the symmetry is considered to be introduced by the mismatch between TiO2 and LnO2 layers. Ionic conductivities attributed to the interlayer sodium ions were observed at high temperatures. The magnitude of ionic conductivity of NaLaTiO4, with a single perovskite layer, was much higher than that of Na2La2Ti3O10, with a triple perovskite layer. This high ionic conductivity is due to the weak interaction between the perovskite layer and interlayer sodium ions.

CONCENTRATION QUENCHING OF THE Eu3+-ACTIVATED LUMINESCENCE IN SOME LAYERED PEROVSKITES WITH TWO-DIMENSIONAL ARRANGEMENT

Abstract The quenching of Eu 3+ -emission in the layered perovskite compounds SrRE 2 Al 2 O 7 , BaRE 2 Ti 3 O 10 , RETa 3 O 9 and RbRETa 2 O 7 (RE=rare earths) with two-dimensional arrangement of rare earth ions was investigated. The highest luminescence intensity was observed in both tantalum compounds, RETa 3 O 9 and RbRETa 2 O 7 . The concentration quenching found in these compounds can be explained by a percolation model with two-dimensional interactions between the Eu 3+ sites in the host lattice. The energy transfer between the Eu 3+ ions takes place by the multipole–multipole interaction among the nearest-neighbor sites in the rare earth sublattice.

Structure and ionic conductivity of MLaNb2O7 (M K, Na, Li, H)

Abstract MLaNb 2 O 7 (M  K, Na, Li, H) compounds were synthesized by a solid state reaction for M  K and by ion exchange for M  Na, Li and H, respectively, and their crystal structures were analysed by a Rietveld method. In the structures for all the compounds, the main framework is built up of a double layer of perovskite units (LaNb 2 O 7 ) and an alkali ion or proton is located at the sites between the layers. Ionic conduction attributed to the interlayer ion was observed above 200 °C for all the compounds.

Luminescence properties of layered perovskites activated by Eu3+ ions

Abstract Dependence of emission intensity on Eu 3+ concentration in Na 2 Gd 2(1− x ) Eu 2 x Ti 3 O 10 with a triple perovskite layer and NaGd 1− x Eu x TiO 4 with a single perovskite layer was studied. Critical values for concentration quenching are x = 0.40 for Na 2 Gd 2(1− x ) Eu 2 x Ti 3 O 10 and x = 0.25 for NaGd 1− x Eu x TiO 4 . The difference in the critical concentration between both compounds is discussed in terms of their crystal structures determined by the powder X-ray diffraction patterns using the Rietveld method. The concentration quenching found in these compounds can be explained by means of a percolation model with nearly two-dimensional interactions between Eu 3+ sites in the host lattice.

Cu-site doping effects, transport and magnetic properties of high-Tc oxides and their hole concentration dependence

Abstract Doping effects have been studied by using La 2− y Sr y Cu 1−χ M χ O 4 (M = Zn, Ni and Mg) and YBa 2 (Cu 1−χ Zn χ ) 3 O 6+ y . The rate of T c -suppression, |d T c /dχ|, is found to be inversely proportional to the carrier concentration p . The disappearance of the superconductivity with Cu-site doping seems to be primarily due to the loss of the itinerant nature of the holes, which is supported by the variation of the spectral function χ(ω) of the magnetic excitations with temperature, studied by neutron scattering on YBa 2 (Cu 0.965 Zn 0.035 ) 3 O 6.75 . The peak of Hall coefficient versus T curves and the “spin gap” effect found in the T -dependence of the resistivities ϱ become less significant with Zn-doping.

Photostimulated Luminescence and Structural Characterization of Ba5 ( PO 4 ) 3Cl : Eu2 + Phosphors

Intense luminescence whose wavelength is ca. 435 nm is observed under the excitation of 680 nm in Ba 5 (PO 4 ) 3 Cl:Eu 2+ phosphors irradiated by x-rays. From ESR measurements, typical F-centers are created in the phosphors on x-ray exposure. These results clearly indicate photostimulated luminescence (PSL) occurring in these phosphors. The substitution sites of Eu 2+ ions as well as the concentration of vacancies on the Cl - sites were determined by Rietveld analysis for powder x-ray diffraction patterns