Shogo Miyoshi

Lattice expansion upon reduction of perovskite-type LaMnO3 with oxygen-deficit nonstoichiometry

Abstract Volume variation of dense LaMnO 3+ δ specimens was measured at elevated temperatures and in stepwise changes of oxygen partial pressure. The specimen expanded upon reduction at fixed temperatures, and its dependence on temperature and oxygen partial pressure conformed to that of oxygen-deficit nonstoichiometry, which was formulated on the basis of a defect model by Mizusaki et al. [Solid State Ionics 129 (2000) 163]. The measured expansion was in almost linear relationship with the oxygen nonstoichiometry. In consideration of reported behavior that the unit cell shrinks upon Sr-doping and upon formation of cation vacancy, the origin of the expansion upon reduction is regarded as the increase in the size of B-site cation, which accompanies the charge compensation for the formation of oxide ion vacancies. The expansion rate that was estimated from the variation in averaged ionic radius of Mn was higher than the experimentally observed one; it is supposed that the effect of the increase in B-site radius is partly offset by the increase in the crystallographic distortion.

Lattice creation and annihilation of LaMnO3+δ caused by nonstoichiometry change

Abstract In order to identify the majority ionic defect of La 1− x Sr x MnO 3+ δ in the oxygen-excess region, a dilatometry was made on La 0.96 MnO 2.94+ δ at a fixed temperature (1273 K) by varying the ambient oxygen partial pressure. The specimen of La 0.96 MnO 2.94+ δ expanded with increasing P O 2 in the region of δ >0, while the unit cell volume was known to decrease with increasing P O 2 . The morphological evolution was also observed on the surface of the La 0.96 MnO 2.94+ δ specimen after annealing in an oxidizing atmosphere, indicating that the new lattices were created. From these results, it is concluded that the majority ionic defect of the perovskite oxide, La 1− x Sr x MnO 3+ δ , is cation vacancies in the oxygen-excess region.

X-Ray absorption, photoemission spectroscopy, and Raman scattering analysis of amorphous tantalum oxide with a large extent of oxygen nonstoichiometry

The electronic structure and modification of the local interatomic structure of a reactive sputtered amorphous tantalum oxide (a-TaO(x)) thin film with the variation of oxygen nonstoichiometry, x in a-TaO(x) have been investigated by X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), Raman scattering spectroscopy, and Rutherford back scattering spectroscopy. A parallel chemical shift of Ta4f(7/2) and O1s core levels observed with the variation of x indicates the Fermi level shift by reduction and oxidation in the framework of the rigid band model. Extended X-ray absorption fine structure (EXAFS) suggests both the increase of average coordination number of the first Ta-O shell in polyhedra and a considerable reduction of the average Ta-O bond length with the increase of x. The relative intensity of Raman shift peaks at 670 cm(-1) and 815 cm(-1), corresponding to Ta-O stretching of TaO(6) octahedra and TaO(5) probably with a pyramidal form, respectively, drastically changes between x = 2.47 to 1.86, suggesting the change in the predominant polyhedron from TaO(6) to TaO(5) with a modification in multiplicity of oxygen by the reorganization of the polyhedral network.

Nonstoichiometry-Induced Carrier Modification in Gapless Type Atomic Switch Device Using Cu2S Mixed Conductor

Both ac impedance and dc polarization have been measured to investigate the switching property of a Cu2S gapless type atomic switch with two-probe cells composed of asymmetric configuration of ion blocking and reversible electrodes in addition to symmetric one with reversible electrodes. Strong nonlinear p-type conductivity in current–voltage (I–V) characteristics and a marked change of overall ac impedance with the variation of applied dc bias voltage observed are attributed to the nonstoichiometry induced carrier modification in a Cu2S thin film modulated by the Cu vacancy migration under the presence of an electrical potential field.

Room temperature redox reaction by oxide ion migration at carbon/Gd-doped CeO2 heterointerface probed by an in situ hard x-ray photoemission and soft x-ray absorption spectroscopies

Abstract In situ hard x-ray photoemission spectroscopy (HX-PES) and soft x-ray absorption spectroscopy (SX-XAS) have been employed to investigate a local redox reaction at the carbon/Gd-doped CeO2 (GDC) thin film heterointerface under applied dc bias. In HX-PES, Ce3d and O1s core levels show a parallel chemical shift as large as 3.2 eV, corresponding to the redox window where ionic conductivity is predominant. The window width is equal to the energy gap between donor and acceptor levels of the GDC electrolyte. The Ce M-edge SX-XAS spectra also show a considerable increase of Ce3+ satellite peak intensity, corresponding to electrochemical reduction by oxide ion migration. In addition to the reversible redox reaction, two distinct phenomena by the electrochemical transport of oxide ions are observed as an irreversible reduction of the entire oxide film by O2 evolution from the GDC film to the gas phase, as well as a vigorous precipitation of oxygen gas at the bottom electrode to lift off the GDC film. These in situ spectroscopic observations describe well the electrochemical polarization behavior of a metal/GDC/metal capacitor-like two-electrode cell at room temperature.