Takahiro Wada

High transition temperature superconductor LaBa2Cu3O7−y with zero resistance at 92 K

Samples of LaBa2 Cu3 O7−y have been synthesized, which show a superconducting transition above 90 K. One of the samples shows a superconducting transition with an onset at 93 K and a zero resistivity at 92 K. These temperatures are the highest among those reported so far on this system and even higher than those in YBa2 Cu3 O7−y. The magnetic susceptibility measurement also shows that it has a superconducting transition with onset at around 93 K. To synthesize such good LaBa2 Cu3 O7−y requires two important procedures. First is sintering above 950u2009°C in N2 gas atmosphere and the second is annealing at 300u2009°C in dried O2 gas atmosphere.

New families of layered cuprates containing double-MO2-unit fluorite blocks: (Ho, Ce)3Sr2Cu3O11 and (Ho, Ce)3Sr2Cu2 (Cu, M)O11 (M = Pb, Fe, Al)

Abstract Two new families of layered cuprates, (Ho, Ce)3Sr2Cu2(Cu, Pb)O11 and (Ho, Ce)3Sr2Cu2(Cu, M)O11 (M = Cu, Fe, Al), have been synthesized. These compounds have tetragonal unit cells with the lattice constants, all being approximately equal to a = 3.8A and c = 17 A . A Rietveld analysis using X-ray powder diffraction data shows that the crystal structure of (Ho 1 3 Ce 2 3 )3Sr2Cu2 (Cu 1 2 Pb 1 2 )O11 consists of a new double-MO2-unit fluorite block, [(Ho, Ce)O2]2, and a Pb-based “1212” block, (Cu, Pb)Sr2(Ho, Ce)Cu2O7. The unit cell for (Ho, Ce)3Sr2Cu3O11 or (Ho, Ce)3Sr2Cu2(Cu, M)O11 (M = FeorAl) is considered to consist of the double-MO2-unit fluorite block and an oxygen-deficient-triperovskite block, (Ho, Ce)Sr2(Cu, M)3O7 (M = Cu, AlorFe).

Substitution Effect of Sr for Ba of High-Tc Superconducting YBa2Cu3O7-y Ceramics

Single phase Y(Ba1-xSrx)2Cu3O7-y [x=0, 0.1, 0.2, 0.3, 0.4] ceramics were fabricated. In the fracture surface of the ceramics, many prism crystals were observed. Superconducting transition temperature measured by the magnetic susceptibility was linearly lowered from 94 to 84 K with increasing x from 0 to 0.4 and all the lattice constants of orthorhombic unit cell became short.

Phase stability and decomposition of superconductive YBa2Cu4O8

The stability of the YBa2Cu4O8 phase has been studied with respect to oxygen partial pressure P(O2 ) and temperature by x‐ray diffraction, thermogravimetry, and differential thermal analysis. The critical temperature Td for the decomposition of YBa2Cu4O8 phase was determined from thermal analyses. It was found that Td was strongly dependent on P(O2 ) and the following empirical relation was obtained: log[P(O2)(atm)]=12.41−(1.449×104)/Td(K). It was found that, when heated, YBa2Cu4O8 first decomposed into YBa2 Cu3.5 O7.5−z plus CuO for 0.02 atm<P(O2 )<9 atm, while for P(O2 )≤0.02 atm, it decomposed into YBa2Cu3O7−z plus CuO without yielding YBa2Cu3.5O7.5−z .

Study on the preparation of 90‐K superconductor LaBa2Cu3Oy

The high‐Tc superconductor LaBa2Cu3Oy has been prepared under various conditions, and characterized by x‐ray and neutron powder diffraction techniques, dc and ac magnetic susceptibility measurements, and dc resistivity measurement. It has been found that samples with excellent superconducting characteristics and also with good structural properties are obtained in a reproducible way if the following three procedures are performed: preheat treatment of La2O3 powders, sintering in N2, and low‐temperature annealing in dried O2. Long‐time annealing in undried O2 was sometimes found to degrade the grain boundaries and to lead to a lower zero‐resistance temperature.

Preparation of High-Tc (110 K) Superconducting Phase by the Annealing of Low-Tc (80 K) Superconductor Bi2(Sr0.5Ca0.5)3-xCu2Oy

The high-Tc (110 K) superconducting phase of Bi-Sr-Ca-Cu-O system is obtained by the transformation of the low-Tc (80 K) phase, Bi2(Sr0.5Ca0.5)3-xCu2Oy [x=0, 0.1, 0.2, 0.4] through long time sintering. One sample with x=0.4 whose sintering temperature is 868°C shows zero resistivity at 106 K and a sharp onset of the diamagnetic susceptibility. The Meissner signal reaches about 30% of the ideal value at around 100 K. The relation between starting composition and the process of the formation of the 110 K phase is briefly discussed, based on the results of ac and dc magnetic susceptibility measurements.

Phase stability and decomposition of superconductive (Y1-xCax)Ba2Cu4O8 (0 x 0.1)

The stability of (Y1-xCax)Ba2Cu4O8 has been studied as a function of Ca content, temperature and oxygen partial pressure, P(O2), by means of thermogravimetry and differential thermal analysis. When heated at P(O2)=1 atm, YBa2Cu4O8 decomposed into YBa2Cu3.5O7.5-Y (1-2-3.5) and CuO above 899°C and the resultant 1-2-3.5 phase decomposed into YBa2Cu3O7-z (1-2-3) and CuO above 926°C. As the Ca content increased, the decomposition temperatures of both 1-2-4 and 1-2-3.5 phases were decreased and they tended to merge. Actually, when the Ca content was as high as 10% of the Y content, the 1-2-4 phase decomposed directly into a 1-2-3 phase and CuO at 884°C.

Yb (Ba, Sr)2Cu4O8: a “124” phase superconductor with a reduced unit cell volume

Abstract Yb(Ba 1− x Sr x ) 2 Cu 4 O 8 (0.1≦ x ≦0.3) superconductors of the YBa 2 Cu 4 O 8 (“124”) structure were successfully synthesized using an O 2 -hot-isostaticc-pressing (HIP) technique. The samples were characterized with respect to the crystal structure and superconducting properties. The lattice parameters of the samples decreased as the substitution of Sr for Ba proceeded. The superconducting transition temperatures of all the Yb-“124” samples were more or less the same, being around 80 K.

(Tl, Pb, Bi)Sr2Ca2Cu3Oz superconductors with zero resistance at 120 K

We have synthesized new superconductors of nearly single phase with zero‐resistance temperatures around 120 K employing a novel method for sintering. The samples included a nominal composition of (Tl0.64Pb0.2Bi0.16)Sr2Ca3Cu4Oz. This sample was prepared from the mixture of presynthesized (Tl0.64Pb0.2Bi0.16)Sr2CaCu2Oz (1212 phase), CaO, and CuO powders. The mixed powder was sintered at 920u2009°C in O2 gas and post‐annealed at 400–600u2009°C in O2 gas. The powder x‐ray diffraction revealed that the sample was isostructural with the Tl‐based (Tl, Pb, Bi)Sr2Ca2Cu3Oz (1223 phase). The sample exhibited a sharp superconducting transition at Tonsetc=125 K and TR=0c=120 K and the superconducting diamagnetic signal at 10 K was about 20% of a full Meissner effect.

Structural changes and redistribution of holes by Sr-doping in YBa2Cu4O8

Abstract The crystal structure of Y(Ba 0.7 Sr 0.3 ) 2 Cu 4 O 8 at room temperature and 18 K has been refined by Rietveld analysis of the time-of-flight neutron powder diffraction data. The lattice parameters and the interatomic distances in the Sr-doped compound were compared with those of YBa 2 Cu 4 O 8 under high pressure. The crystal lattice contracts considerably by substitution of Sr 2+ ions for Ba 2+ ions, as if pressure is applied to it. Nevertheless, T c of the compound is nearly independent of Sr-doping, which is in contrast with YBa 2 Cu 4 O 8 showing a marked pressure-induced rise in T c due to the hole transfer from the double chain to the CuO 2 plane. We calculated Madelung energies on the basis of the refined structural parameters, distributing holes on two Cu sites: Cu(1) on the double chain and Cu(2) on the CuO 2 plane. The number of holes per Cu(2) at the lowest Madelung energy is 0.24 at room temperature and 0.26 at 18 K. The number of holes at 18 K is essentially the same as that of YBa 2 Cu 4 O 8 at 0 GPa, which is a reasonable explanation for the invariance of T c by Sr-doping.