Tohru Higuchi

Protonic-Electronic Mixed Conduction and Hydrogen Permeation in BaCe0.9 − x Y 0.1Ru x O 3 − α

The protonic-electronic mixed conductors are of great interest for their potential applications particularly for the hydrogen separation that is essential for hydrogen production from hydrocarbons. This paper deals with the mixed conduction properties of BaCe 0 . 9 - x Y 0 . 1 Ru x O 3 - α (x = 0-0.1) in which Ru is partially substituted for Ce in the high-temperature proton conductor, BaCe 0 . 9 Y 0 . 1 O 3 - α . Appreciable hydrogen permeation through the Ru-doped materials was observed and is attributed to ambipolar diffusion. The mixed conducting mechanism is discussed in terms of the defect chemistry and electronic structures revealed by the electrochemical and spectroscopic measurements.

Atomic-resolution spectroscopic imaging of oxide interfaces

Perovskite oxides show a rich variety of electronic phases in bulk. The prospect of combining these phases at the atomic scale has fueled the interest in oxide heterostructures. The existence of interfaces can, however, greatly affect the macroscopic properties of these structures, leading on the one hand to exotic new phases stabilized at the interface or on the other hand to the degradation of bulk-like properties. With recent advances in electron microscopy the composition and bonding at such buried interfaces can now be imaged with atomic resolution. The bonding information is obtained from the near-edge fine-structure of core-level electron energy loss spectra. Here, we discuss the near-edge fine-structure focusing on the O-K and transition metal L 2,3 edges which are of particular importance for the family of perovskite oxides. Spectroscopic imaging of a vanadate/titanate heterostructure demonstrates the capability of the technique to characterize atomic-scale interdiffusion at interfaces. Resolution limits in spectroscopic imaging due to inelastic delocalization effects are discussed.

Valence State of Mn-Doped BiFeO3–BaTiO3 Ceramics Probed by Soft X-ray Absorption Spectroscopy

The valence state of Mn-doped BiFeO3–BaTiO3 ceramics has been probed by soft X-ray absorption spectroscopy (XAS). Mn-doped BiFeO3–BaTiO3 has valence states of Fe3+ and Ti4+, although BiFeO3 and Mn-doped BiFeO3 have mixed valence states of Fe2+ and Fe3+. The Mn 2p-XAS peak of Mn-doped BiFeO3–BaTiO3 locates at a lower energy side than that of Mn-doped BiFeO3 that corresponds to the Mn3+ valence state. These findings may indicate that the Fe3+ valence state of Mn-doped BiFeO3–BaTiO3 is stabilized by charge transfer from the Mn 3d state to the Fe 3d state through the Ti 3d state in BaTiO3.

Protonic Conduction in the Single Crystals of SrZr0.95M0.05O3 (M=Y,Sc,Yb,Er).

The protonic conductivities of SrZr0.95M0.05O3 doped with four acceptor ions (M3+=Y3+, Sc3+, Yb3+, Er3+) have been studied in the single crystal form. The protonic conductivity is found in four acceptor ions, indicating that protons migrate by hopping from site to site. The Yb-doped crystal has the lowest activation energy and the highest conductivity amongst the four acceptor ions. This is considered to be due to the difference in strength of the O–H bond with different acceptor doping.

LaVo4: Eu Phosphor Films with Enhanced Eu Solubility

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO{sub 4}:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd-Ofelt theory. However, the low solubility limit (< 3 mol %) of Eu in LaVO{sub 4} prevents its efficient use as a phosphor. We present optical evidence of enhanced Eu solubility as high as 10 mol % in LaVO{sub 4}:Eu thin films grown by pulsed laser deposition and postannealing. The photoluminescent intensity exceeded that of YVO{sub 4}:Eu thin films when excited below the host bandgap, indicating stronger direct emission of Eu in LaVO{sub 4}.

In Situ Tuning of Magnetization and Magnetoresistance in Fe3O4 Thin Film Achieved with All-Solid-State Redox Device

An all-solid-state redox device composed of Fe3O4 thin film and Li(+) ion conducting solid electrolyte was fabricated for use in tuning magnetization and magnetoresistance (MR), which are key factors in the creation of high-density magnetic storage devices. Electrical conductivity, magnetization, and MR were reversibly tuned by Li(+) insertion and removal. Tuning of the various Fe3O4 thin film properties was achieved by donation of an electron to the Fe(3+) ions. This technique should lead to the development of spintronics devices based on the reversible switching of magnetization and spin polarization (P). It should also improve the performance of conventional magnetic random access memory (MRAM) devices in which the ON/OFF ratio has been limited to a small value due to a decrease in P near the tunnel barrier.

Modulation doping of a Mott quantum well by a proximate polar discontinuity

We present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells near a polar surface of LaAlO3 (001). As the surface is brought in proximity to the LaVO3 layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient is observed below a characteristic coupling length of 10-15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the AlO2-terminated LaAlO3 surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole-doping in oxide thin film heterostructures.

Protonic Conduction in the Single Crystal of Sc-Doped SrZrO3

The protonic conduction of Sc-doped SrZrO3 (SrZr1-xScxO3) in the single crystal form is investigated. The SrZr1-xScxO3 crystals exhibit significantly higher conductivity than the pure ones, and the crystal with x=0.05 exhibits the highest conductivity of those masured. The activation energy of the SrZr1-xScxO3 crystal decreases rapidly with increasing Sc3+ concentration when x≤0.05, and increases when x>0.05. The activation energy of SrZr0.95Sc0.05O3 agrees with the energy separation between hole states at the top of the valence band and the Fermi level.

High resolution soft X-ray absorption spectroscopy for the chemical state analysis of Mn

Abstract Mn L 2,3 X-ray absorption spectra of various manganese compounds were measured using a synchrotron radiation facility. Very high-resolution spectra were obtained. Sharp lines, which were not found before, were observed, and reproducibility was checked using a different beam line in the synchrotron radiation facility. Difference in spectral line shape was observed as the difference of quantum yield method. An unknown sample was measured and the specimen was concluded to be a mixture of two different oxidation states of Mn. The effects on the change of spectral profiles are discussed.

Effect of Mn Substitution for Multiferroic BiFeO3 Probed by High-Resolution Soft-X-ray Spectroscopy

The electronic structures of BiFeO3 (BF) and Mn-doped BiFeO3 [BF(Mn)] have been studied by X-ray absorption spectroscopy (XAS) and soft-X-ray emission spectroscopy (SXES). BF and BF(Mn) have a mixed valence state of Fe2+ and Fe3+. Their valence band is mainly composed of the O 2p state hybridized with the majority-spin t2g and eg orbitals of Fe 3d state. The conduction band is composed of the minority-spin t2g and eg orbitals of Fe 3d. The band gaps of BF and BF(Mn) are estimated to be 1.3 eV and 2.7 eV, respectively. The increase in band gap with Mn substitution contributes to a change in the bandwidth of the valence band.