Masatsune Ohta

Studies on the Thermal Behavior of Ba2YCu3O7-x by X-Ray Powder Diffraction Method

Thermal behavior of Ba2YCu3O7-x was investigated mainly by X-ray powder diffraction method between 70 K and 1173 K. Oxygen-rich(x~0.5) specimen is orthorhombic in symmetry in the temperature range between 70 K and 943 K. It undergoes a phase transition to tetragonal form at 943 K or 783 K in air or N2 gas, respectively. On the other hand, oxygen-poor specimen(x~1.0) semiconductive down to 4.2 k is tetragonal up to 1173 K in N2 gas. Thermal expansion coefficient of a cell parameter is about 1.1 × 10-5 K-1 and that of c cell parameter is 2.5 × 10,,5 K-1, in air.

Preparation and Physical Properties of PrBa2Cu3Oy

A new compound, PrBa2Cu3Oy, which is isomorphic with YBa2Cu3Oy, was prepared as single phase. With specimens quenched from 920°C, the oxygen content y was 6.8 ± 0.1 and the ratio of Pr3+ to Pr4+ was 0.4:0.6. With furnace-cooled specimens, the value of y was 7.1 ± 0.1 and the ratio became 0.1:0.9. The average valence of Cu ions was 2.0 ± 0.1 for the former and 2.1 ± 0.1 for the latter. Specimens handled both ways were nonmetallic and nonsuperconducting down to 4.2 K.

Preparation of new strontium niobium sulfide and strontium tantalum sulfide

New strontium ternary sulfides, SrM[sub 2]S[sub 5] (M=NB, Ta), have been prepared by the reaction of CS[sub 2] with a mixture of SrCO[sub 3] and Nb[sub 2]O[sub 5], or SrCO[sub 3] and Ta[sub 2]O[sub 5]. The powder X-ray diffraction patterns were indexable on the basis of hexagonal cells with lattice parameters of a=3.3592 (7) [angstrom], c=23.995 (6) [angstrom] for SrNb[sub 2]S[sub 5], and a=3.3063 (5) [angstrom], c=24.276 (4) [angstrom] for SrTa[sub 2]S[sub 5]. The electron diffraction measurements, however, revealed the existence of the supercells of [radical]28a x [radical]28a for SrTa[sub 2]S[sub 5]. Superconductivities with Tcs around 3K were observed in SrTa[sub 2]S[sub 5] and SrNb[sub 2]S[sub 5].

Crystal Structure and Tc of YBa2Cu3Oy (y>7)

At a high oxygen pressure of 6 GPa, YBa2Cu3Oy was heat-treated. With an increasing supply of oxygen, the crystal structure of the processed specimens gradually changed from orthorhombic (y=7) to tetragonal (y=7.7), whereas the onset of superconducting transition temperature consistently remained at 92 K for all of the specimens. The distribution of charges among the copper-oxygen chains and the CuO2 sheets in YBa2Cu3Oy (y>7) are discussed.

Crystallographic Study in the Nd2CuO4-Sr2CuO3 System

A crystallographic study was done in the Nd2CuO4-Sr2CuO3 system. Five phases were found in this system; A: Nd2CuO4, B: Nd2SrCu2O6, C: (Nd0.45Sr0.55)3Cu2O6-y, D: (Nd1-xSrx)2CuO4-y, and E: Sr2CuO3. Phases B and C possess Sr3Ti2O7-type structures, and phase D has the same structure as La2CuO4. No superconductivity was observed in any of the above-mentioned phases down to 4.2 K.

Enhancement of Jc in Y1-xCa1.1xBa2-0.1xCu3Oy

With specimens of Y1-xCa1.1xBa2-0.1xCu3Oy prepared under the same conditions, Y0.95Ca0.055Ba1.995Cu3Oy possessed a larger superconducting critical current density Jc than the nondoped YBa2Cu3Oy. The Jc of the former composition was 275 A/cm2 at 77 K, whereas it was 145 A/cm2 with the latter stoichiometric composition.

Effect of Pressure on Superconducting Transition Temperatures in the Bi–Sr–Ca–Cu–O System

Superconducting transition temperatures were measured under pressure for the Bi-Sr-Ca-Cu-O system. The transition temperature near 80 K (Tc1) slightly increases with pressure: dTc1/dp=0.06 K/GPa (onset), while Tc near 110 K (Tc2) decreases with pressure: dTc2/dp=-0.2 K/GPa (onset) and -0.3 K/GPa (complete). The negative pressure-dependence was discussed with regard to the resonating-valence-bond theory.

Preparation of Superconductive YBa2Cu3O7-x Thick Films by the Rapid-Quenching Technique

Thick films of YBa2Cu3O7-x (e.g. 50 µm thick × 13 mm wide × 45 mm long) with maximum density were prepared by rapid quenching. For the film heat-treated at 950°C in air, superconductivity was realized with an onset temperature of 98 K and zero resistivity of 87 K.

Preparation and physical properties of (Bi1−xLnx)2.2Sr1.8CaCu2Oy (Ln=La, Pr and Nd, x≤0.25)

Solid solutions (Bi1-xLnx)2.2Sr1.8CaCu2Oy(Ln=La, Pr and Nd, x0.25) were prepared. Of these three Ln ions replacing Bi3+, Pr was not trivalent, the average valence of Pr being shown to be 3.5. With increase of x, the onset of superconducting transition temperature and hole concentration initially increase, then decrease steadily with increase of x equally for all the three series, irrespective of difference in lanthanide elements substituted.

Preparation and High Tc Superconductivity of Ba-Y-Cu Oxides

High Tc superconductors of monophase, Ba2YCu3O8-x, and mixed phase samples were prepared from mixtures with Ba/Y/Cu ratios of 2/1/3 and 2/1/3.5, respectively. They were investigated by dc electrical measurements and X-ray diffraction. All samples have basically an orthogonal structure. The zero-resistivity temperatures for the monophase and mixed phase samples were 86 K and 77 K, respectively. The reason for the different superconducting characteristics is also discussed.