Koukichi Tomimoto

Superconductivity in the new compound (Y1-xCax)0.95Sr2.05Cu2.4(CO3)0.6Oy

Abstract A new Cu-oxide superconductor (Y 1− x Ca x ) 0.95 Sr 2.05 Cu 2.4 (CO 3 ) 0.6 O y has been found by resistivity and magnetization measurements. The transition temperature is about 63 K. The crystal structure consists of the periodic replacements of the Cu(I) site with a Co 3 group in the basic YBa 2 Cu 3 O y structure.

Crystal Substructure of the Bi-Sr-Cu-O System

A substructure with a pseudotetragonal unit cell is proposed for a low-Tc superconducting oxide Bi-Sr-Cu-O system on the basis of Rietveld analysis of powder X-ray diffraction data. It is a layered structure having the composition Bi2Sr2CuOy. The relationship between the crystal structure of the Bi-Sr-Ca-Cu-O system and that of Bi-Sr-Cu-O system was discussed.

(C0.35Cu0.65)Sr2(Y0.73Ce0.27)2Cu2Ox: A new superconductor containing CO3

A new “1222”-type compound containing carbonates, (C0.35Cu0.65)Sr2(Y0.73Ce0.27)2Cu2Ox, was synthesized at 1303 K under an atmosphere of O2 (80%)-CO2 (20%) (partial pressure of CO2: PCO2=0.020 MPa). The sample annealed at 1023 K under the high oxygen pressure of 90 MPa has a superconducting transition temperature, Tc, of 18 K and a superconducting volume fraction of about 10% at 5 K. The structure of the superconductor was analyzed using the Rietveld method for the X-ray powder diffraction pattern. The refined lattice parameters were a=3.8272(1), b=3.8313(1) and c=27.7077(6) A.

Long Period Structures in the Bi-Sr-Cu-O System

The structure of the superconducting Bi-Sr-Cu oxide system was studied by means of electron diffraction. Three kinds of long period structures were found in this system. The relation between these long period structures and the nonstoichiometry in this system is discussed.

51V knight shift and quadrupole interaction in the low-temperature phase of LiVO2

The 51 V Knight shift and the quadrupole interaction in a triangular lattice compound LiVO 2 have been studied by means of the pulsed-NMR technique below the transition temperature T t of about 450 K. From the analysis of the NMR spectrum, we determined the principal values of the electric field gradient (EFG) tensor and the Knight shift components along the principal axes of the EFG tensor. The Knight shift was found to be temperature independent, and its anisotropy could be attributed to the anisotropy of the Van Vleck orbital susceptibility. Spin contribution to the susceptibility is largely reduced below T t , indicating a nonmagnetic spin-singlet state probably due to the clustering of V 3+ ions.

Superconductivity in Sr(Ln)NbO system (Ln:La,Nd,Pr,Ce,Gd and Ho)

Abstract A new non-Cu oxide superconductor Sr(Ln)NbO system (Ln:La,Nd,Pr,Ce,Gd and Ho) has been found by the resistivity and magnetization measurements. The highest onset transition temperature is about 12K. Preliminary electron and X-ray diffraction experiments indicate that main phase (∼90%) in this system is (Sr 1− x Ln x )Nb 2 O 6−gd and another impurity phase (∼10%) is (Sr 1− x Ln x )Nb 2 O 7−δ , suggesting that the basic crystal structure is probably an oxygen deficient CaTa 2 O 6 type structure.

Phase transition in LiVO2

Abstract The magnetic susceptibility χ and specific heat C p were measured for the two-dimensional triangular antiferromagnet LiVO 2 . The phase transition was observed between T down t = 450 K and T up t = 485 K in the χ-result. The χ showed a reproducible thermal hysteresis between T down t and T up t . The entropy change around T up t was estimated as about 11 J/K mol from the C p -result. By the cation-exchange chromatography, it was indicated that the chemical formula of the measured sample was Li 0.84 VO 2 .

High-Tc Superconductivity of Y-Ba-Cu-O System

A high-Tc superconductivity has been observed in the Y-Ba-Cu-O system by both resistivity and magnetic susceptibility measurements. A superconducting transition with an onset temperature of 120 K has been achieved. About 8% Meissner effect has been confirmed indicating that superconductivity in this material is of the bulk nature. Preliminary X-ray diffraction experiments show that the crystal structure is a non-primitive perovskite structure.