Papers by Keyword: Layered Perovskite

Abstract: This study investigated the effects of Ba substitution on protonic conductive properties in the Fe doped Sr₂TiO₄ layered perovskite. We synthesized sintered samples of (Ba_xSr_2-x)(Ti_0.90Fe_0.10)O_4-δ (x= 0.05, 0.10) (BSTF05, BSTF10). The result of X-ray diffraction suggests that solid solute limitation of Ba is between x= 0.05 and 0.10. BSTF05 at 600 °C shows proton and oxide-ion conductivities as well as elecronic conductivity. It means that BSTF05 is a triple conductor at 600 °C under oxidation atmosphere. The proton conductivities in BSTF05 are lower than that in Ba un-doped Sr₂(Ti_0.9Fe_0.1)O₄-_δ evaluated in our previous work, suggesting that the effect of the Ba substitution on proton defect generation is small. The redox reaction of Fe ions is more important for creation of proton defects in the layered perovskites.

Abstract: Technetium-99 is a key radioisotope from a nuclear waste perspective because of its long half-life (t_1/2= 2.13x10⁵ years) and its abundance in used nuclear fuel. As such, it is targeted in separation strategies such as UREX+, for isolation and encapsulation in solid waste forms for final disposal in a nuclear repository. We report here results regarding the incorporation of Tc-99 into ternary oxides of different structure types: pyrochlore (Nd₂Tc₂O₇), perovskite (SrTcO₃), and layered perovskite (Sr₂TcO₄). The objective was to determine synthesis conditions of these ceramic waste forms to immobilize Tc-99 as Tc (IV) and to harvest crystallographic, thermophysical and hydrodynamic data. The fabricated ceramic technetates exhibit good crystallinity and lattice parameters and atomic coordinates could be refined with high accuracies. Thermophysical properties of the oxides, such as the critical temperature (T_c) for superconductivity, were analyzed using AC magnetic susceptibility measurements. In our efforts to compare hydrodynamic properties of the ceramic waste forms with those to Tc-bearing borosilicate glass, we applied the standard test method ASTM C1220-10. Hereby matrix corrosion and Tc-leaching of Tc-99 containing LAWE4 type borosilicate glass and ceramic specimens were compared by static leaching experiments.

Abstract: Grafting reactions between an n-decoxy derivative of ion-exchangeable layered perovskite HLaNb₂O₇·xH₂O and polyethylene glycols (PEGs) with various molecular masses were investigated. X-ray diffraction analysis showed that a reflection corresponding to the interlayer distance of the n-decoxy derivative of HLaNb₂O₇·xH₂O (2.73 nm) disappeared and that new diffraction lines ascribable to smaller interlayer distances, which increased with increases in the molecular mass of PEGs, appeared. The solid-state ¹³C CP/MAS NMR spectra of the products indicated that ethylene oxide chains were covalently bound to the interlayer surface of HLaNb₂O₇∙xH₂O. The signal assignable the carbons bound to OH groups suggests that the grafting reactions for the formation of Nb-O-C bonds typically involve one of the terminal OH groups in PEGs.

Abstract: Oligothiophene derivatives, 4,4’-bis-(2-aminoethylthienyl)-3,4-ethylenedioxythiophene dihydrohalide (AET-EDOT·HX: X = I, Br), were synthesized by Grignard coupling reactions. The hybrid films of oligothiophene and lead halide were fabricated by casting and spin-coating methods. The effect of halogen species on the optical properties of the perovskites was investigated. The X-ray diffraction profiles of (AET-EDOT)PbX₄ cast films indicated that the layered structures were formed by the self-organization. After hybridization of AET-EDOT·HX with PbX₂, new absorption peaks due to the confined excitons were observed at 400 nm for (AET-EDOT)PbBr₄ film, and at 395 nm for (AET-EDOT)PbI₄ film. These results indicated that the diamino-type oligothiophenes were successfully incorporated into the organic-inorganic layered perovskites. AFM images of (AET-EDOT)PbX₄ cast films showed polycrystalline grains, indicating that the polycrystalline structures covered on the substrate surface.

Abstract: To understand a stability of the orbital-modulated (OM) state in Sr2-xRxMnO4 (R = Pr, Nd), the crystallographic features of oxide samples with 0.20 < x ≤ 0.50, prepared by the coprecipitation technique, have been investigated by transmission electron microscopy. When the Pr content x corresponding to the eg-electron concentration increased from x = 0, the OM state appeared in 0.20 < x ≤ 0.40 as a neighboring state of an orbital ordered (OO) state around x = 0.20. The interesting features are that around x =　0.44 there is a nanometer-sized coexisting state consisting of the OM and disordered tetragonal (DT) states, and that an average size of OM-state regions in the coexisting state decreases with increasing the eg-electron concentration. In addition, the so-called CE-type antiferromagnetic state found in Sr1.5La0.5MnO4 is absent in the Nd and Pr cases. This suggests that the stability of the OM state is not directly associated with charge ordering of eg electrons, but basically due to only their orbital ordering.

Abstract: Lithium-ion conducting properties were investigated for a layered perovskite oxide Li2SrTa2O7 (LST) and defect-controlled LST, synthesized via solid state reactions. The ionic conductivities of A-site solid solutions Li2[Sr-(La2/3□1/3)-(La1/2Li1/2)]Ta2O7 (□ denotes vacancy.) suggested that lithium ions migrate in the Li-layer. The conductivity of Li-deficient (Li2-z□z)(LazSr1-z)Ta2O7 increased dramatically from 4.2 × 10-6 S cm-1 (z = 0, LST) to 1.6 × 10-3 S cm-1 (z = 0.2) at 400°C with increasing Li-vacancy concentration. This result obviously indicates that the conductivity of LST originate from the Li migration through vacancies in the Li-layer.

Abstract: Sr4Bi4Ti7O24 (SBT7) ceramics were prepared. X-ray powder diffraction indicated that single layered perovskite ferroelectrics were obtained. The ferroelectric and dielectric properties of SBT7 ceramics were investigated. The results showed that Sr4Bi4Ti7O24 is piezoelectric material. The dielectric properties were measured as a function of temperature and two peaks were observed in the curve.

Abstract: Reduced niobates, MLixCa2Nb3O10 (M = Rb, K and Na) and KNaxCa2Nb3O10 were synthesized by intercalation reaction with n-butyllithium or sodium azide. Ruddlesden-Popper type structure of sodium intercalation compound, KNaCa2Nb3O10, differ from that of the parent compound and lithium intercalation compound, KLiCa2Nb3O10. The magnetic susceptibility measurements showed that the sodium intercalation compound became superconductor with transition temperatures below 3.5 K. New lithium intercalation compound Na0.1Li0.8Ca2.45Nb3O10, for an ion exchange compound, Na0.1Ca2.45Nb3O10, with KCa2Nb3O10 type structure show the diamagnetic signals below 5 K.

Abstract: Rare earth Yb-doped bismuth titanate Bi4-xYbxTi3O12 (BYT) nanocrystalline powders were obtained by a modified sol-gel method. TG-DTA, FI-IR and XRD were used to determine the lowest heat treatment temperature. Phase changes in the formation of BYT crystalline powders were discussed by XRD. The effect of Yb3+ cation substitution for Bi3+ cation on the microstructure of BYT was also developed with XRD. The grain size of BYT nanopowders is about 40 nm determined by TEM.