Shin-ichi Koriyama

New superconducting cuprates (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz

Abstract New superconducting Pb-based layered copper oxides, (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz, have been synthesized. This new superconducting phase or the “1222” phase characteristically contains a (Pb, Cu)-O monolayer and a “fluorite-type” block layer which is sandwiched between the basal planes of double Cu-O5 pyramids. Crystallographycally, this structure agrees with that recently proposed for nonsuperconducting PbPr2.4Sr1.6Cu2.6Oz by Adachi et al. A superconductivity transition with the onset temperature of 25 K is measured for the nearly single-phase sample of the nominal composition of ( Pb 1 2 Cu 1 2 ) ( Eu 3 4 Ce 1 4 )2( Sr 7 8 Eu 1 8 )2Cu2Oz. This is the first superconductor of the “1222” structure.

Superconductivity in Pb-based layered copper oxides, (Pb0.5Cu0.5)(Sr,Ba)2(Y,Ca)Cu2O7

Abstract The effects of Ba-substitution for Sr in Pb-based superconducting layered copper oxides. (Pb0.5Cu0.5)Sr2(Y,Ca)Cu2Oz (z∼7), are investigated. Ba-free samples are not superconducting at temperatures above 4.2 K, probably because the solubility of Ca is too low to exhibit superconductivity. However, for the samples in which Ba partially substitutes for Sr, superconductivity transitions with an onset temperature of 70 K have been observed. The Tc value of 70 K is as high as that previously reported for Pb2Sr2(Y,Ca)Cu3O8. The superconducting phase is identified to have the crystallographic structure of the “1212” type.

Preparation of “124” superconductors using only alkoxides

Abstract The “124” phase (YBa 2 Cu 4 O 8 ) is synthesized at a low temperature of 700°C in O 2 gas flow (PO 2 =1 atm) by means of a sol-gel method using only alkoxides. The prepared Cu-butoxide for the present synthesis is soluble up to 36 mMol/l in butanol and hydrolyzable. The sample sintered at 760°C exhibits superconductivity with the zero-resistance temperature (Tc R=0 ) at 78K.

(Pb,Cu)Sr/sub 2/(Y,Ca)Cu/sub 2/O/sub z/, new superconducting layered copper oxides

Single-phase ceramics of Pb-based layered copper oxides, (Pb/sub (1+x)/2/Cu/sub (1-x)/2/)Sr/sub 2/(Y/sub 1-x/Ca/sub x/)Cu/sub 2/O/sub x/ (x=0 approximately 0.35), were successfully synthesized in an oxidizing atmosphere in contrast to the case of Pb/sub 2/Sr/sub 2/(Y,Ca)Cu/sub 3/O/sub 8/. Superconductivity is observed for the samples containing appropriate amounts of Ca such that x>0.25 when annealed in air and then quenched into liquid nitrogen. As the Ca content, x, increases, the superconductivity transition temperature increases. The superconducting onset temperature of 65 K is measured for the multiphase samples with x=0.5.

Preparation of “124” Superconductors Containing Ca from Alkoxides

The samples of “124” superconductors containing Ca, i.e., (Y1−xCax)Ba2Cu4Oy, are synthesized in flowing O2 gas of P(O2)=l atm by means of a sol-gel method using only alkoxides. The sample with the Ca content, x, equal to 0.05 shows sharp x-ray diffraction peaks for the “124” phase and sharp superconducting transitions in both electrical resistivity (Tc(on)=89K, Tc(R=0)=84K) and magnetic susceptibility (Tc mag(on)=90K). The solubility limit, x, for Ca in the “124” phase when it was synthesized in flowing O2 gas is less than 0.1 as previously reported for high-pressure syntheses. The “124” particles in the synthesized samples are flakes with flat c planes.

Superconductivity in Pb-Based Copper Oxides with 1212 Structure

Superconducting Pb-based copper oxides with the 1212 structure have been synthesized in an oxidizing atmosphere in contrast to those with the 2213 structure usually synthesized in a reducing atmosphere. Superconductivity of the Pb-based 1212 copper oxides are strongly dependent on the synthetic conditions as well as the chemical compositions. Although the samples of nearly single- phase of the 1212 phase are obtained in a starting composition of Pb0.5 (Sr1.5Ca1.33Y0.67)Cu2Oz, they are not superconductive at temperatures above 4.2 K. On the other hand, the samples with a starting composition of PbSr2Ca0.5 Cu2Oz are of multi-phase but show a superconductivity transition with an onset temperature of 27 K and a zero resistivity temperature of 16 K.