Ayako Tokiwa

Crystal structure and phase transition of PbBaSrYCu3Oy(y=7∼8.4

Abstract PbBaSrYCu3Oy prepared by slow cooling, oxygen annealing and quenching methods was studied by XPD, EPMA, HREM and TGA. From a high resolution electron microscopic investigation, the structure of PbBaSrYCu3O7 quenched from above 800°C was found to be a derivative of Pb2Sr2YCu3O8, in which one of the PbO layers in the PbO-Cu-PbO block was missing. The slowly cooled specimen with y⋍8.2 lost 1.2 oxygen atoms above 580°C with an abrupt and reversible transition to PbBaSrYCu3O7. All of the slowly cooled, oxygen annealed and quenched specimens showed no superconductivity and behaved like a semiconductor.

Crystal structure and superconductivity controlled by cation substitution and oxygen annealing in y1-xcaxba2cu3oyand yba2-xlaxcu3oy

The effects of Ca2+ substitution for Y3+ and La3+ substitution for Ba2+ in YBa2Cu3O7 on the structural and superconducting properties were studied to control the average copper charge. Single-phase tripled perovskites were obtained for 0x0.2 and 0x1.0 for Ca and La doped series, respectively. A slight decrease of Tc with increasing x was found for small x values for both kinds of substituted specimens prepared by furnace cooling in air. Oxygen annealing experiments were also carried out. These suggested that the superconduction in the Y-Ba-Cu-O system cannot be interpreted only by oxygen content or average copper charge.

Synthesis and crystal structure of Pb2Sr2(Ln,Ce)nCu3O6+2n+δ and Pb(Ba,Sr)2(Ln,Ce)nCu3O5+2n+δ (Ln=Y,n=3,4,… and 0⩽δ⩽2.0), layered structure compounds with multiple fluorite layers

Abstract Pb 2 Sr 2 (Ln,Ce) n Cu 3 O 6+2n+δ (Pb32 n 2) and Pb(Ba,Sr) 2 (Ln,Ce) n Cu 3 O 5+2n+δ (Pb22 n 2) (Ln=Y, n=3, 4, … and 0⩽δ⩽2.0) were synthesized and investigated by XPD, HREM, TGA and SEM-EDX. Almost-single phase of Pb 2 Sr 2 YCe 2 Cu 3 O 12+δ (Pb3232) was prepared using a gel-citric acid method, although it was not obtained through a solid state reaction. The crystal structure of Pb32 n 2 and Pb22 n 2, which was studied by XPD analysis and HREM, was found to be layered constructed by stacking of (Pb, Cu) triple or double layers and (Ln, Ce) multiple fluorite layers. These compounds showed a phase transition accompanied with oxygen absorption and desorption, similar to Pb3212 and Pb3222. SEM observation and EDX analysis of sintered surface indicated that homogeneous dispersion of CeO 2 led the products to intergrowth structure of Pb32 n 2 and Pb22 n 2 ( n = 3, 4, …) with multiple fluorite layers.

Thermal and x-ray analyses of high temperature superconductor YBa2Cu3O6.74

In situ observation of oxygen absorption and desorption during heating and cooling cycle with TG and DTA was done to determine the optimum preparation conditions of high Tc Y-Ba-Cu-O compounds. We suggest the best recipe in air is to fire the raw mixture of 1/2Y2O3, 2BaCO3 and 3CuO at 930°C for more than 10 h, followed by slow cooling in the furnace to take as much oxygen as possible within the specimen. With this method single phase material of an orthorhombic superconducting oxide YBa2Cu3O6.74 with Tc=91.3 K was synthesized, with orthorhombic unit cell dimensions of a=3.8180(6) A, b=3.8888(7) A, and c=11.668(2) A.

Synthesis and superconductivity of PbBaSrY1−xCaxCu3O7

Abstract Superconducting PbBaSrY 1− x Ca x Cu 3 O 7 with a maximum T c onset at 65 K was synthesized. Single-phase materials were obtained for 0 ⩽ x ⩽ 0.25 by sintering in a 1% O 2 -N 2 gas at 750–830°C and quenching from 785°C. Superconductivity appeared for x ⩾ 0.15 and T c increased with increasing Ca content. PbBaSrY 0.7 Ca 0.3 O 7 was confirmed to achieve zero resistance at 37 K with about 45% bulk superconductivity. Superconductivity was considered to occur by hole introduction in CuO 2 sheets by substituting Y 3+ with Ca 2+ , the same mechanism as in Pb 2 Sr 2 Y 1− x Ca x Cu 3 O 8 .

An X-ray diffraction and electron microscopic study of a new high-Tc superconductor based on the Bi-Ca-Sr-Cu-O system

Observed high-resolution images of a new high-Tc superconductor based on the Bi-Ca-Sr-Cu-O system (Tc above 70 K) indicated a structure derived from the Bi4Ti3O12 type in which doubled bismuth oxide layers alternate with double perovskite layers along the c-axis. The orthorhombic unit cell dimensions showed an approximate relation of a=√2ap, b=5√2ap and c=8ap, where ap=3.82 A, the cell parameter of a cubic perovskite. At least a quarter of the oxygen should be deleted in accordance with the average copper valence of less than 3 and two-dimensional Cu-O layers are supposed to be responsible for the high-Tc superconductivity.

Effect of Co Substitution on Tc in YBa2Cu3-xCoxO7-y,(x=0-1)

The crystal structure of YBa2Cu3-xCoxO7-y(x=0~1) was investigated by X-ray and electron diffraction and the crystallographic site of Co atoms was directly determined by electron-channelling-enhanced microanalysis. It was shown that the structure is tetragonal based on the tripled perovskite structure, and that Co atoms occupy the Cu atom sites. Electric resistivity was measured with changing Co atom content and it was shown that Tc did not decrease monotonically with increasing Co atom concentration.

Specific heat of YBa2Cu3O7−δ and BaCuO2+x in magnetic fields up to 6 T

Abstract The specific heat C of YBa 2 Cu 3 O 7−δ ( δ =0, 0.69, 0.91), Y 1.1 Ba 1.9 Cu 3 O 7− δ and BaCuO 2+ x has been investigated in magnetic fields up to 6 T. In addition to the γ ∗ T term and the upturn, characteristic behaviors of C/T consisting of the increase of γ ∗ with magnetic fields, an appearance of a broad hump and a rapid decrease of C/T toward zero at the lowest temperatures are observed in all the YBa 2 Cu 3 O 7−δ samples investigated. BaCuO 2+ x , one of the possible impurity phases included in the Y-Ba-Cu-O system, also shows a similar behavior of C/T as YBa 2 Cu 3 O 7−δ . Particularly, BaCuO 2+ x explains the anomalous phenomenon observed in YBa 2 Cu 3 O 6.31 at 6 T that the C/T at 1.5 K attains to the γ ∗ value at 0 T. The γ ∗ value is attributed to the BaCuO 2+ x included in the matrix.

Synthesis and crystal structure of Pb (Ba,Sr)2(Ln,Ce)2Cu3Oy (Ln: lanthanoid, y=9–10.4), a new member of the Pb system

Abstract Pb(Ba, Sr) 2 (Ln, Ce) 2 Cu 3 O y (Ln: lanthanoid, y =9.0–10.4) with (Pb, Cu) double layers and (Ln, Ce) fluorite layers was designed and obtained as a single phase. The structure of Pb(Ba, Sr) 2 (Ln, Ce) 2 Cu 3 O y , which was derived from Pb(Ba, Sr) 2 YCu 3 O y by replacing oxygen-defect Y layers with (Ln, Ce) 2 O 2 layers, was confirmed by high-resolution electron microscopy. The microstructure of (Pb, Cu) double layers of a quenched specimen was examined by means of high-resolution image with an imaging plate. A slowly cooled specimen with an oxygen content of y =10.2 lost 1.2 oxygen atoms above 580°C with an abrupt and reversible transition. The nature of this phase transition was similar to that of PbBaSrYCu 3 O y described in our previous paper. No superconductivity was found in Pb(Ba, Sr) 2 (Ln, Ce) 2 Cu 3 O y in spite of the CuO 2 doublet sheets, similar to Pb 2 Sr 2 LnCe Cu 3 O y with (Ln, Ce) fluorite layers.

Upper and lower critical fields of REBa2Cu3Oz compounds

Abstract Magnetization and electrical resistance were measured on polycrystalline samples of REBa 2 Cu 3 O z . The lower critical field defined by a point at which the magnetization deviates from the initial slope is a little larger for RE=Dy and Gd than for RE=Y. However, other superconducting parameters are almost independent of RE elements.