Papers by Keyword: Oxygen Deficiency

Abstract: We report a synthesis route to form a stable powder of SrCoO_3-δ which show ferromagnetic component at room temperature. The powder was obtained from brownmillerite SrCoO_2.5 using chemical oxidation. The x-ray diffraction pattern of the oxidized powder was best fitted using a tetragonal structure of I4/mmm with lattice constants of a = b = 10.87 Å, c = 7.664 Å and its oxygen content of 2.77 (δ 0.23) was measure by iodometric titration. In spite of a single-phase structure, the elemental compositions of oxidized powder characterized by the EDX measurement showed the presence of oxidation elements that might contribute to magnetic impurities at low temperatures. The powder nevertheless can be used for magnetic applications at room temperature.

Abstract: Polycrystalline Nd1-xSrxMnO3 was subjected to a thermobaric treatment, that is, a quenching under a high quasihydrostatic pressure of 9 GPa , starting from the temperature 1000 °C. This results in a strong change of the electroresistive and magnetoresistive properties. The changes in the electrical transport might result from the increase of oxygen deficient regions in the boundaries between the nanosized grains.

Abstract: Strontium cobaltite doped with Fe and Nb, SrCo0.6Fe0.2Nb0.2O3−z, with perovskite-like structure, being one of the promising materials for oxygen permeable membrane implementation, has been investigated via in situ high temperature X-ray powder diffraction on Bruker D8 Advance instrument and high-resolution powder diffraction at synchrotron radiation beamline. The oxygen outlet from the lattice has been found to stay in correlation with the structural rearrangement forming two phases with different oxygen deficiency. The activation energy of phase splitting process has been estimated to be Eactivation ≈ 0.63 ± 0.06 eV.

Abstract: Ceramic anodes, made of perovskite-type rare-earth and strontium cobaltites substituted in both sublattices, exhibit a high electrocatalytic activity towards oxygen evolution in alkaline media. This work analyzes the relationships between cation composition, defect structure, electronic conductivity and electrochemical performance for a wide group of perovskite-like cobaltites, including Ln1-yAyCoO3-δ (Ln= Pr, Nd, Sm; A= Sr, Ca; y= 0-0.4), La1-x-ySrxBiyCoO3-δ (x= 0-0.6, y= 0-0.1), La0.7-xSr0.3CoO3-δ (x= 0-0.10), Sr1-xBaxCoO3-δ (x= 0.1-0.2) and SrCo1-yMyO3-δ (M=Fe, Ni, Ti, Cu; y= 0.1-0.6). The materials were prepared by the standard ceramic technique and characterized employing XRD, TGA, iodometric titration, and total conductivity measurements. A relatively high electrochemical performance in alkaline solutions was observed for (La,Sr)CoO3-based compositions with a moderate A-site deficiency. For SrCoO3-based materials, an increase in the oxygen evolution rate was found when co-substituting cobalt with several transition metal cations, such as Fe3+/4+ and Cu2+/3+. The results show that, in general, the key composition-related factors influencing electrochemical activity in alkaline media include the oxygen vacancy concentration, the average positive charge density in the crystal lattice, and possible blocking of active sites on the electrode surface.

Abstract: We investigated the influence of photoexcited nonequilibrium electrons in Bi4Ti3O12-d which contains a high density of oxygen vacancies. As increasing oxygen deficiency, the photocurrent undergoes a rapid enhancement, accompanied by a remarkable change of kinetics. In heavily oxygen deficient Bi4Ti3O12-d (d=0.45), we observed dispersive photoconductivity, a behavior resembles with persistent photoconductivity phenomena in some perovskites. We argue that oxygen vacancies acting as trapping centers for electrons are linked to dispersive diffusion in the perovskite blocks, which control carrier recombination.

Abstract: The effects of grain-size in La0.7Ca0.3MnO3-δ (LCMO) system have been studied in connection with the magnetic and electronic properties. LCMO system prepared by the solid-state reaction was annealed in air at 1200, 1300 and 1400°C. The grain sizes of LCMO samples become larger with increasing of annealing temperature (TA ). The magnetization in LCMO samples increased while the coercive field decreased with increasing TA. The conductivity increased and the metal-insulator transition temperature TM-I decreased with increasing of TA. These physical properties are due to the oxygen deficiency caused by the increase of grain size. Finally, it was found that the grain size and the mechanical connection between grains play an important role in determining the electronic and magnetic properties.