Yoshichika Bando

Phase relation in the oxygen nonstoichiometric system, SrFeOx (2.5 ≤ x ≤ 3.0)

Abstract The SrFeOx system with 2.5 ≤ x ≤ 3.0 was prepared and examined by differential thermal analysis, thermogravimetry, powder X-ray diffraction, and the 57Fe Mossbauer effect. Four single phases with different structures exist in this system, the typical compositions of which are SrFeO3, SrFeO2.86, SrFeO2.73, and SrFeO2.50. SrFeO2.86 and SrFeO2.73 have newly found tetragonal and orthorhombic structures, respectively, and their unit cells are related to the cubic perovskite cell for SrFeO3 in the manner that a t ≈ 2 × 2 1 2 a c , c t ≈ 2a c and a o ≈ 2c o ≈ 2 × 2 1 2 a c , b o ≈ 2a c , where the suffixes c, t, and o stand for the cubic, tetragonal, and orthorhombic structures, respectively. These single-phase compositions suggest the ideal series of SrFeO 3− 1 n , where n = ∞, 8, 4, and 2 give x = 3, 2.875, 2.75, and 2.50, correspondingly. The vacancy ordered phase of SrFeO2.86, SrFeO2.73, and SrFeO2.50 show first-order transitions to their cubic, disordered structures at 250, 320, and 830°C, correspondingly. Formation of the cubic, disordered structure above a temperature depending on the composition is suggested for the whole composition range.

Single-Crystal YBa2Cu3O7-x Thin Films by Activated Reactive Evaporation

By means of the activated reactive evaporation, YBa2Cu3O7-x single-crystal thin films with the c axis perpendicular to the substrate plane have been directly and epitaxially grown on the (100) surface of SrTiO3. The substrate temperature was kept below 600°C and the oxidation treatment was performed at below 500°C. The ac-resistive measurement for a film with 2000 A thickness gave a sharp superconducting transition, where the endpoint was 90.2 K and ΔTc(10–90%)=1.7 K. From the complex susceptibility measurement we confirmed the Meissner effect as well as the structural uniformity of the specimen.

A new homologous series Srn−1Cun+1O2n found in the SrOCuO system treated under high pressure

Abstract A new homologous series of high-pressure phases, Srn−1Cun+1O2n (n = 3, 5, …), were found in the SrOCuO system treated at 6 GPa and 1373–1573 K. These crystallize in orthorhombic structures of space group Cmmm with unit-cell parameters of a ∼ 3.9 A, b ∼ n × a, and c ∼ 3.5 A. Along the c-axis Cu 1+ 1 n O 2 sheets alternate with Sr 1− 1 n sheets without oxygen. Within the CuO sheets, CuO double chains made of corner- and edge-sharing CuO4 squares are intergrown with a corner-sharing CuO4 network; the distance between the double chains is b 2 . The serial chemical composition and structure are interpreted as resulting from a shear operation upon the high-pressure form of SrCuO2 (n = ∞).

Preparation of ferroelectric BaTiO3 thin films by activated reactive evaporation

Ferroelectric BaTiO3 thin films were directly and epitaxially grown on SrTiO3 single crystal and epitaxial Pt film substrates by activated reactive evaporation. The substrate temperature was around 600 °C. For (100) oriented as‐grown films, a typical ferroelectric hysteresis loop and a maximum of dielectric constant at about 115 °C were observed. The resistivity was as high as 109 Ω cm and the breakdown voltage was 2.7 MV/cm for as‐grown BaTiO3 films.

Atomic layer growth of oxide thin films with perovskite‐type structure by reactive evaporation

Epitaxial growth of BaTiO3 and SrTiO3 films by the reactive evaporation method was investigated using reflection high‐energy electron diffraction (RHEED). The investigations were carried out using two growth methods: coevaporation and alternate evaporation of the metal elements in an oxygen atmosphere. Atomic layer growth was achieved by the alternate supply of Ba or Sr and Ti on the growing surface. In the case of coevaporation, epitaxial growth occurred in a two‐dimensional unit‐cell‐by‐unit‐cell mode. The surface of each unit cell is terminated by a (TiO2) layer. Artificial superlattices of BaTiO3/SrTiO3 were fabricated by monitoring the film thickness with the RHEED oscillations.

SUPERCONDUCTIVITY IN THE BA-SR-CU-O-SYSTEM

Abstract Superconductivity with T c s at 90 K and 60 K has been found in the Ba-Sr-Cu-O system treated at 6 GPa and 1473 K. It should be noticed that the system is free from any trivalent or quadrivalent counter cation in contrast with the known superconducting cupric oxides. It is known that use of high pressure for synthesis tends to stabilize a tetragonal structure of the Ca 0.84 Sr 0.16 CuO 2 type for a wide alkaline earth composition range including Sr 1- x Ba x CuO 2 ( x ⪅0.3), while transmission electron microscopic examinations of the present samples revealed three types of modified structures. We suggest that among these, an incommensurate superlattice of a × a × nc with n ≈9 superconducts, though this phase has not been isolated yet.

Epitaxial growth and dielectric properties of BaTiO3 films on Pt electrodes by reactive evaporation

Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high‐energy electron diffraction and cross‐sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two‐dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant er. Good agreement between the experimental results and the theoretical calculations leads to ...

Crystal chemistry and physical properties of La2−xSrxNiO4 (0 ≤ x ≤ 1.6)

Abstract Structural, transport, and magnetic data for the system La 2−x Sr x NiO 4 , 0 ≤ x ≤ 1.6, are reported. The oxygen content of the system was controlled by annealing under various oxygen partial pressures below 150 atm and was determined by iodometric titration. A Rietveld analysis of the room-temperature powder X-ray diffraction data showed that the tetragonal distortion of the NiO 6 octahedra decreases monotonically with increasing x for 0 ≤ x ≤ 1.4, but the La(Sr)O(2) bond length increases anomalously in the interval 0 ≤ x ≤ 0.6. A semiconductor-metal transition decreases monotonically with increasing x from about 675 K(x=0) to 20 K(x=1.2). Samples with x > 1.0 exhibited a weak ferromagnetic moment at 5 K and the onset of a positive component in the Hall effect. Comparison of the La 2−x Sr x CuO 4 and La 2−x Sr x NiO 4 systems shows that the superconductive copper oxides have the equilibrium Cu 3+ + O 2− = Cu 2+ + O − shifted to the right and, for x = 0, antiferromagnetic correlation in a half-filled σ x 2 − y 2 band whereas the weakly ferromagnetic nickel oxides have the equilibrium Ni 4+ + O 2− = Ni 3+ + O − shifted to the right, but ferromagnetic correlations in a quarter-filled σ∗ band.

Dependence of the structure and electronic state of SrFeOx (2.5 ≤ x ≤ 3) on composition and temperature

Abstract The system SrFeOx, 2.5 ≤ x ≤ 3, forms a continuous solid solution at temperatures T ≥ Tt(x), but a series of discrete ordered-vacancy phases SrFeO 3−( 1 n ) (n = ∞, 8, 4, 2) below temperatures Tt(n = 8) = 523 K, Tt(n = 4) = 598 K, and Tt(n = 2) = 1103 K. The most probable vacancy-ordering schemes for Sr8Fe8O23 (n = 8) and Sr4Fe4O11 (n = 4) are proposed. Formation of fivefold-coordinated iron sites on either side of an oxygen vacancy is characteristic of both phases, which contrasts with Sr2Fe2O5 (n = 2) where four-coordinated sites coexist with six-coordinated sites. The high-spin Fe 4+ (t 3 2 σ∗ 1 ) configuration of SrFeO3 (n = ∞) evolves as follows: (a) for x ≈ 3, random vacancies trap Fe3+ ions at five-coordinated sites; (b) for n = 8, Fe4+ ions remain high spin, a localized-electron 5Eg configuration giving rise to a cooperative Jahn-Teller distortion with c a ≤ 1 and a [220]n8 unique axis imposed by structural symmetry; fast Fe3+ + Fe4+ = Fe4+ + Fe3+ electron transfer occurs parallel to this axis at room temperature, but Fe3+ ions are ordered at five-coordinated sites at 4 K; (c) for n = 4, the octahedral-site Fe4+ ions have a low-spin t42 configuration with four near-neighbor Fe3+ ions in five-coordinated sites not making an Fe3+OFe4+OFe3+ linear chain as in n = 8. Oxygen-vacancy hopping times τh ≥ 10−8 sec persist for 200 K above Tt in n = 2, and short-range ordering in this temperature interval is inferred. For n = 8 and n = 4, motional narrowing to a single Mossbauer peak occurs within tens of degrees above Tt, and this narrowing is assumed to reflect rapid electron hopping in a mixed-valence state; this electronic motion masks any line narrowing due to oxygen-vacancy mobility in these phases.

Epitaxial growth of YBa2Cu3O7−x thin films on (110) SrTiO3 single crystals by activated reactive evaporation

The orientation of YBa2Cu3O7−x thin films grown on (110) planes of SrTiO3 by activated reactive evaporation was investigated by means of reflection high‐energy electron diffraction. The orientation of the films depended on the substrate temperature. The films with (110) planes parallel to the substrate surface grew in a narrow range of substrate temperatures around 530 °C, while the films with (103) planes parallel to the surface grew at temperatures above 600 °C. The change of the epitaxial orientation with the substrate temperature is discussed in terms of the temperature dependence of the lattice mismatch between YBa2Cu3O7−x and SrTiO3.