4PO7-10 Effect of the Doping Level of Sc₂O₃ on the Properties of the YBa₂Cu₃O_7-d - Sc₂O₃ System

Frantisek Hanic 1, Alexander Cigan 1, Pavol Banacky 2, Stefan Buchta 1, and Jan Manka 1. 1 Institute of Measurement Science, Slovak Academy of Sciences, 84219 Bratislava, Slovakia. 2 S-Tech a.s., Ursulinska 3, 811 01 Bratislava, Slovakia.

Presenting Author: P. Banacky

Formation of superconducting and paramagnetic phases in the system YBa₂Cu₃O_7-d - was investigated. The starting compositions were: (Y_xSc_1-x)Ba₂Cu₃O_6.5+ (0 < x £ 1) (A); Y(Ba_2-xSc_x)Cu₃O_6.5+x/2+ (0 < x £ 0.4) (B); YBa₂(Cu_3-xSc_x)O_6.5+x/2+ (0 < x £ 1.1) (C). The porosity p gradually increased with increasing x (for x=1, p=32%).

The products include in the system (C) are: a superconducting (123)-phase Y_1-uBa_2-2u(Cu_3-2u-xSc_x-u)O_6.5+x/2-7u, paramagnetic insulators Y₂BaCuO₅, BaCuO₅, BaCuO₂, and spinel-like cubic phase BaSc_zCu_2-zO_3+z/2 (z=0.5, 1 or 2). The paramagnetism of the BaScCuO_3.5 could be verified well resolved also at temperatures T < T_c. The reaction pass from the basic composition to the final products was accomplished.

Detailed phase equilibria, EPR, magnetization, crystallographic and superconducting studies in the YBa₂Cu₃O_7-d - Sc₂O₃ system were performed.

4PO7-11 Ni Doping Effect on Internal Friction Peaks in YBa₂Cu₃O_7-d

Qingming Zhang, Xuenong Ying, Ang Li, Wuming Chen, Xiaoshan Xu, Baiqing Li, and Yening Wang, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

Presenting Author: Y.N. Wang

Internal friction measurements at ~1.6kHz were performed from 100 to 300K in seven Ni-doping samples YBa₂(Cu_1-xNi_x)₃O_7-d . Three peaks are observed in the temperature dependence of internal friction. One is located at about 220K(P₁), the other two appears near 98K(P₃) and 110K(P₂), respectively. The peak P₁ is suppressed and moves to lower temperatures with increasing Ni content. It is speculated that the peak may be related to a magnetic transition, which is affected by the magnetic impurities Ni. The latter double peaks show no obvious shift with varying Ni content. The height of the peak P₂ has little change as Ni content increases. It is suggested that P₂ may be related to the jumps of O atoms in the Cu-O chains, similar to the idea from Cannelli et al., and P₃ may be associated with the strong interaction between phonons and spin fluctuations.

*4PO7-12 Synthesis, Structure, and Improved Properties of YBa_2-xSr_xCu_3-yMo_yO_7+d Compounds

Bogdan Dabrowski 1, Krzysztof Rogacki 2, Omar Chmaissem 1, and Jim D. Jorgensen 3. 1 Physics Department, Northern Illinois University, DeKalb, IL 60115. 2 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50950 Wroclaw, Poland. 3 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439.

Presenting Author: B. Dabrowski

YBa₂Cu₃O₇ (Y123) has been the subject of vigorous research for power and magnet applica-tions because the parameters of practical importance, the critical current, J_c, and the irreversibility field, B_irr, remain quite large in intense magnetic fields at 77 K. These parameters can be further enhanced by the complex melt-texturing processing that generates finely dispersed extrinsic flux pinning centers. By using the knowledge accumulated during last decade about the relationship between the structural features and the superconducting properties we have attempted to design an improved Y123 material that would display an increased superconducting transition temperature, T_c, and enhanced intrinsic J_c, and B_irr.

Chemical substitutions on various crystallographic sites were made to reduce the buckling of the CuO₂-planes. Substitutions of Sr for Ba and transition elements for copper were made to shorten the distance between the double-CuO₂-planes while preserving the "metallicity" of the blocking layer. It was found that during synthesis in air several transition elements preferentially substitute for Cu in the blocking layer (on the Cu-chain site). In particular, it was found that Mo shows a wide range of solubility and that the Mo-substituted materials are easy to synthesize and can be annealed at elevated oxygen pressure to increase T_c. For the x = 1 compound (T_c = 82 K for y = 0), that are orthorhombic for y < 0.1 and tetragonal for y > 0.1, the highest transitions, T_c = 87 K, were obtained for y = 0.05 – 0.10. The scaled irreversibility fields are comparable to pure Y123 for the sample with y = 0.05. This is the first example of an improved superconducting 123 compound by substitution of a transition element for Cu. The structural studies indicate that the enhanced flux pinning properties may arise from a shorter width of the intermediate region and from the randomly distributed defects, dimers of corner shared Mo-O₆ octahedra in the blocking layer. Similar effects have been observed for the compounds with x = 0 and 0.1 for which the optimized synthesis conditions have produced a wide range of compositions, 0 * y * 0.15, where T_c = 93 K.

This work was supported by the NSF Science and Technology Center for Superconductivity, grant #DMR 91-20000, the U.S. Department of Energy, BES - Materials Sciences, contract No. W-31-109-ENG-38 (JDJ), and the TMR Network, Contract #ERBFMRX-CT98-0189 (KR).

4PO7-13 Influence of Ag substitution on superconductivity of YBa₂Cu₃O_7-x

V.V. Derevysnko, A.A. Blinkin, V.N. Golovin, and V.A. Finkel, National Science Centre Kharkov Institute of Physics & Technology, 310108 Kharkov, Ukraine

Presenting Author: V.V. Derevyanko

Practically constant T_c, decreased Hc1 increased resistance of high- T_c superconductors in the normal state, and decreased critical current with increasing y found for the YBa₂Cu_3-yAg_yO_7-x system examined (0 £ y £ 0.5) can be explained involving our idea of the dominant substitution of the Cu(2) positions in then YBa₂Cu_3-yAg_yO_7-x lattice and hence, the crystal lattice stability. Studies have shown a good evidence in support of the supposition that mainly Cu³⁺ ions contribute to the charge transfer in high- T_c superconductors of 1-2-3 stricture along the chains...-O-Cu-O-Cu-O-...

4PO7-14 Ferroelectric Anomaly Induced by the Ordering of Cu-O Chains

Guangping Zheng and Mo Li, Department of Materials Science & Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA

Presenting Author: G. Zheng

One of the possible mechanism for the opening of spin-gap in the normal state of YBCO cuprate is the ordering of Cu(1)O chains. It is important to study the anomalies of the Cu(1)O chain near 200K. We substitute tin atoms into the Cu(1) positions. Chain ordering is studied by dielectric and acoustic internal friction measurements at different tin doping levels. We find a ferroelectric transition near 200K and it is affected by the tin contents. The relationship between this phenomena and the tin-content dependent superconducting temperature is discussed. We also propose a Ising type Hamiltonian with long-range interaction for the doped dipole-chain. Analytic and Monte Carlo simulation results are compared with the experiment data of structural anomaly near 200K.

4PO7-15 Carbon incorporated YBa₂Cu₃O_7-d crystal; growth, phase separation and enhancement of pinning

Y. Yamada 1, T. Miura 3, Y. Koike 1, I. Hirabayashi 1, H. Ikuta 2, and U. Muzutani 3. 1 Supercond. Res. Lab., ISTEC, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan. 2 CIRSE, Nagoya Univ., Nagoya 456-8601, Japan. 3 Dept. cryst. Mater. Sci., Nagoya Univ. Nagoya 456-8603, Japan.

Presenting Author: Y. Yamada

YBa₂Cu₃O_7-d (YBCO) crystalline films incorporating carbon atoms have been grown from a high temperature solution of BaO-CuO including barium carbonate in air. Since the air contains about 0.035% of carbon dioxide, carbon content in the solution rapidly decreases to a finite value as the time goes by after the BaCO₃ addition, implying that carbon in the solution is released into and equilibrates with the air. Carbon content incorporated in the crystal grown from the solution is proportional to that in the solution and the effective distribution coefficient is about 0.5.

The crystalline film shows two slightly different c-axis lattice constants, suggesting possibility of phase separation. One is almost constant (11.68 Å) and another is slightly larger and dependent on the carbon content. Superconducting properties of YBCO crystal containing carbon have been measured. Transition temperature significantly decreases from about 90K both in transport and magnetic measurements when the carbon content exceeds 0.06wt%. A maximum hysteresis peak in M-H curve have been observed for the sample containing carbon of about 0.04wt%.

4PO7-16 The Role of Pr at Ba-site in YBa₂Cu₃O_y Superconductor

W.H. Tang and J. Gao, Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong

Presenting Author: W.H. Tang

Samples with nominal compositions of YBa_2-xM_xCu₃O_y (M=Pr and Nd, x<=0.25) were prepared by solid state reactions. The influences of Nd and Pr at Ba-sites on the superconducitvity of YBa₂Cu₃O_y were comparatively studied by x-ray diffraction, magnetic and electrical measurements. Our results indicate that Nd at Ba-sites has little influence on the superconductivity of YBa₂Cu₃O_y, but Pr at Ba-sites depresses the superconductivity seriously. The difference roles of Nd and Pr at Ba-sites in YBa₂Cu₃O_y will be discussed. It is also found that Pr at Y- and Ba-sites exhibit different T_c-depression behaviors. The depression in superconductivity of Pr at Y-sites is due to both hole filling and depairing effects. However, Pr at Ba-sites seems to be due to hole filling only.

4PO7-17 Sr substitutions for Ba in different HTS systems

Francesca Licci, Andrea Gauzzi, Edmondo Gilioli, and Massimo Marezio, Istituto MASPEC-CNR, Parco Area delle Scienze 37/A, 43010 Fontanini-Parma, Italy

Presenting Author: F.G. Licci

We are performing systematic studies on different Sr-substituted HTS systems, aimed to accurately investigate the microstructural aspects and their relationships with T_c. The tailoring of the cell dimensions and the interatomic distances by substitutiing Sr for Ba in HTSs has been widely employed since it was thought that it could simulate the application of external pressure, which generally results in T_c increases. Instead the effect of Sr substitution on T_c, with the exception in the La_2-x(Ba_1-ySr_y)_xCuO₄ system, always results in T_c decreases. This discrepancy between the application of the mechanical and chemical pressure has to be attributed to an uncontrolled perturbation of the local structure and of the related electronic properties induced by the size of the substituent.

We report and discuss our recent results on Y(Ba_1-xSr_x)₂Cu₃O_w and Hg(Ba_1-xSr_x)₂CuO_4+d (x * 0.5) systems. In the first case it was found that the strain of the Ba layer scales well with T_c. Since this strain decreases with Sr substitution, it induces a decrease of T_c. The same effect is observed in the Hg-system, but in this case the control of the carrier concentration seems to play a role on T_c as well.

4PO7-18 Effects of Sr Substitution on T_c of Y_0.85Ca_0.15Ba_2-xSr_xCu₃O_y

D.H. Ha 1, Y.I. Kim 1, Y.N. Choi 2, C.H. Lee 2, Jin-Tae Kim 1, and Y.K. Park 1. 1 Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-600, Korea. 2 Neutron Beam Applications, Hanaro Center, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon, Korea.

Presenting Author: D.H. Ha

We have studied the Ar annealing effect on T_c of over-doped Y_0.85Ca_0.15Ba_2-xSr_xCu₃O_y (x = 0.0, 0.5 and 1.0) samples. The annealing was carried out at 480° C for 15 min ~ 10 h. As oxygen content is decreased by the Ar annealing, the hole concentration on CuO₂ plane is gradually moved to the optimum region so that T_c of x = 0.0 and 0.5 samples increases and remains at a maximum. However, x = 1.0 sample shows a different behavior. T_c increases from 68 K to 75 K after Ar annealing for 15 ~ 30 min, but it rapidly decreases to 40~50 K after further Ar annealing. It suggests that x = 1.0 sample has a suppressed T_c in the optimum hole concentration region for YBCO, but a relatively sharp maximum T_c at a slightly over-doped state.