4PO7-20 Effect of Ca doping in Yb(Ba_0.8Sr_0.2)₂Cu₃O_7-d ; d £ 0.2 and d ³ 0.5

Tushar Anvekar 1, K.R. Priolkar 2, P.R. Sarode 2, V.N. Kamat Dalal 1, N.Y. Vasanthacharya 3, and A.V. Narlikar 4. 1 Department of Chemistry, Goa University, Taleigao Plateau, Goa 403 206 India. 2 Department of Physics, Goa University, Taleigao Plateau, Goa 403 206 India. 3 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560 012 India. 4 National Physical Laboratory, Dr. K. S. Krishnan Road, Pusa, New Delhi 110 012 India.

Presenting Author: K.R. Priolkar

We report the preparation and characterization of Yb(Ba_0.8Sr_0.2)₂Cu₃O_7-d ; d £ 0.2 (oxygen rich compounds) and d ³ 0.5 (oxygen poor compounds) as a function Ca substitution through the measurement of T_c, X-ray Reitveld refinements and iodometric titration's.

In the oxygen rich compounds, the results show that Ca substitutes both Yb and Ba sites and as the Ca content increases the relative fraction on Yb site decreases. Estimation of hole concentration and normal state resistivity indicates the samples to be slightly overdoped.

In the oxygen poor compounds Ca prefers only the rare-earth site, possibly due to oxygen ordering thereby increasing the hole concentration in the Cu-O planes and rendering the Ca doped samples superconducting.

4PO7-21 ^63,65Cu NQR Studies of Y_1-xCa_xBa₂Cu₃O_y

Daniel M. Potrepka 1, Joseph I. Budnick 2, Arnold R. Moodenbaugh 3, and Daniel A. Fischer 4. 1 US Army Research Lab, 2800 Powder Mill Rd, Adelphi, MD 20783. 2 Physics Dept., University of Connecticut, Storrs, CT 06269-3046. 3 Materials Science Division, Brookhaven National Lab., Upton, NY 11973. 4 Materials Science and Engineering Lab., National Institute of Standards and Technology, Gaithersburg, MD 20899.

Presenting Author: D.M. Potrepka

The nuclear quadrupole resonance (NQR) for ^63,65Cu was studied in powder samples of Y_1-xCa_xBa₂Cu₃O_y, where 0 £ x £ 0.20 and (estimated) 6.05 £ y £ 6.8. Oxygen K-edge x-ray absorption spectra for x = 0.12 and 6.12 £ y £ 6.94 were obtained and the relationship between O content, y, and hole doping was analyzed. Samples were characterized by x-ray diffraction and DC magnetometry. The Cu NQR plane spectra, obtained at 60 - 110 MHz and 1.3 K for the stated calcium concentration range, provide insight into the relationship between superconductivity and antiferromagnetism as a function of hole doping in the Cu planes due to Ca content, x. NQR spectra consistent with the presence of a magnetic contribution at the Cu plane sites are observed for (Ca) x £ 0.2. A discussion of the effect of Ca doping on the copper and oxygen will be presented. DMP and JIB supported by Connecticut Critical Technologies Program. ARM supported by US Dept. of Energy.

4PO7-22 Substitution of Ba by light rare-earth in Eu123 solid solutions

Youwen Xu 1, M.J. Kramer 2, K.W. Dennis 2, H. Wu 2, A. O'Connor 2, R.W. McCallum 2, S.K. Malik 3, and W.B. Yelon 4. 1 Department of Physics, Minnesota State University, Mankato, MN 56001, USA. 2 Ames Laboratory, Iowa State University, Ames, IA 50011, USA. 3 Tata Institute of Fundamental Research, Bombay 400005, India. 4 University of Missouri Research Reactor, Columbia, MO 65211, USA.

Presenting Author: R.W. McCallum

A detailed study on a series samples of Eu(Ba_1-xR_x)₂Cu₃O_7+d (R = La, Nd, Pr and Eu) was performed using X-ray diffraction, neutron diffraction, magnetic and thermopower measurements. Neutron diffraction measurements confirmed that the rare earth substitutes went to Ba site as expected. We could study the effects of ionic size and charge transfer separately, and probe the unique characteristic of Pr in the 123 structure. The rate which superconductivity was suppressed by Pr was faster than other rare-earth. The structural distortions caused by rare earth on Ba site include a crystallographic transition from orthorhombic to tetragonal, changes of "buckling angle" of CuO₂ plan layer, and changes in ionic bond lengths. The distortion decreased the density of mobile holes in the structure. The effect of Pr in this compound was compared to that of other two rare earthes, and we believe that the fast suppression of T_c by Pr substitution in this structure is electronic in nature.

This work was performed at Ames Laboratory, and supported by the Director of Energy Research, U.S. Department of Energy, under contract No. W-7405-ENG-82. The work at MURR was supported by the U.S. Department of Energy grant No. DE-FE02-90ER45427 through the Midwest Superconductivity Consortium.

4PO7-23 Restitution of superconductivity by Sr doping in Pr-substituted at Eu-site and Ba-site in EuBa₂Cu₃O_z system.

D.G. Kuberkar 1, R.S. Thampi 1, C.M. Thakker 1, H.H. Ranavayya 1, M.R. Gonal 2, Ramprasad 2, R.G. Kulkarni 1. 1 Department of Physics, Saurashtra University, Rajkot-360005, India. 2 Material Processing Division, BARC, Trombay, Mumbai-400 005, India.

Presenting Author: D.G Kuberkar

We report the effect of Sr-doping in the restitution of superconductivity in Pr-substituted at (Eu_1-xPr_x)Ba₂Cu₃O_z [EuPrBCO] and Eu(Ba_2-xPr_x)Cu₃O_z [EuBPrCO] systems.It is observed that for 40% Pr-substitution at Eu-site, the T_c value <20K while the samples with 15% Pr at Ba-site becomes nonsuperconducting. The Sr doping effect at Ba-site in EuPrBCO and EuBPrCO systems shows that the rate of T_c revival is different for both the systems suggesting altogether different mechanism of hole delocalisation responsible for superconductivity revival. These results were obtained as an outcome of the structural and superconducting property investigations using XRD, SEM susceptibility, iodometry and resistivity measurements.

4PO7-24 A Comparison of the Fehrenbacher-Rice Model and the Liechtenstein-Mazin Model

S.J. Liu 1, J.-Y. Lin 2, J.M. Chen 3, P. Nachimuthu 3, C.W. Luo 1, K.H. Wu 1, J.Y. Juang 1, T.M. Uen 1, and Y.S. Gou 1. 1 Department of Electrophysics, National Chaio Tung University, Hsinchu, Taiwan. 2 Institute of Physics, National Chaio Tung University, Hsinchu, Taiwan. 3 Synchrotron Radiation Research Center, Hsinchu, Taiwan.

Presenting Author: S.J. Liu

To explain the absence of superconductivity in PrBa₂Cu₃O_7-y, Fehrenbacher and Rice (FR) proposed a local Pr 4ƒ_z(x2-y2)-O 2p band in which doped holes are bound to Pr sites. The absence of superconductivity was explained by the existence of the hybridization. Based on the hybridization of Pr 4ƒ_z(x2-y2) with O 2p orbitals, Liechtenstein and Mazin (LM) proposed another model in which the direct hopping between oxygen orbitals is taken into account.

Using polarization-dependent O 1s near-edge x-ray absorption spectroscopy of epitaxial Y_1-xPr_xBa₂Cu₃O_7-y thin films, we can identify the unoccupied electronic states of the samples and study its dependence on Pr concentration. By determining the hole distribution of the Pr-doped thin films, we make a comparison between the FR model and the LM model, and find that our experimental results are more consistent with the FR model.

4PO7-25 The effect of Y1/Nd1(211) additions on the properties of the Y0.5/Nd0.5123) composite.

A.S. Mahmoud 1, and G.J. Russell 1, M.R. Koblischka 2, N. Chikumoto 2, and M. Murakami 2. 1 Advanced Electronic Materials Group, School of Physics, University of New South Wales, NSW 2052 Sydney, Australia. 2 Superconductivity Research Laboratory, International Superconductivity Technology Centre, 1-16-25, Shibaura, Minato-ku, Tokyo 105, Japan.

Presenting Author: A.S. Mahmoud

The effect of Y1/Nd1(211) additions on the microstructure and superconducting properties of the Y0.5/Nd0.5 (123) system was investigated using a melt-processing technique in air. The microstructure investigation revealed a significant preference for Nd to dominate the Re site of the 123 phase with a similar trend for Y to dominate the Re site of the 211 inclusions. Transition temperatures (T_c) exceeding 90K and magnetically induced critical current densities (J_c) ~ 10000A/cm² at zero fields were obtained for many samples. All samples exhibited a peak effect that appeared to flatten or even vanish at temperatures of 77K. Up to the refinement of the 211 particles reached in this study, no significant effect of these particles was noticed on the flux-pinning properties of Y/Nd(123) system. Extensive analysis of pinning for both a Y/Nd (123) single crystal and melt-textured samples are being reported elsewhere.

4PO7-26 Phase diagrams, crystal growth and structural features of RE_1+xBa_2-xCu₃O_z superconductors (RE= La, Nd, Pr, Sm, Eu)

Eugene Goodilin 1, Eugene Trofimenko 2, Ekaterina Pomerantseva 2, Anna Soloshenko 2, Anna Kravchenko 2, Igor Bezverkhy 1, James Hester 3, Valery Petrykin 4, Nikolay N.Oleynikov, and Yury D.Tretyakov 1. 1 Department of Chemistry, Moscow State University, Moscow 119899, Russia. 2 Higher School of Materials Science, Moscow State University. 3 Australian Nuclear Science and Technology Organization, New Illawarra Road, Lucas Heights 2234, Australia. 4 Materials and Structures Lab., Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan.

Presenting Author: Y.D. Tretyakov

The RE_1+xBa_2-xCu₃O_z phases with light Rare-Earths allowed to achieve a great progress in preparation of HTSCs with advanced properties. At the same time, the compounds demonstrate a number of features related to ordering/disordering phenomena at different x and z. Phase relations in RE-Ba-Cu-O systems (RE=La, Pr, Nd, Sm, Eu), including BaCuO₂ – "RE₂Cu₂O₅" quasibinary sections (pO₂=10-3-1 atm.), were studied by XRD, DTA, EPMA, AES ICP techniques, and a special attention was given to structural changes typically occurring when x and z values vary from 0 to 1 as probed by X-Ray, Neutron, electron diffraction, EXAFS, Mossbauer, Raman scattering spectroscopy, susceptibility measurements. Some possible structural transitions at x~0.3 and x~0.6 within the homogeneity field of a model compound Nd_1+xBa_2-xCu₃O_z were discussed in a frame of Nd and Ba atoms ordering and subsequent oxygen sublattice reconstruction. New "anomalous" examples of a disordered low-T_c quasicubic NdBa₂Cu₃O_z phase and a superstructural ordered orthorhombic NdBa₂Cu₃O_z compound, both with no a tetra-ortho-transition, were analyzed on the base of their structure refinement and behavior of Cu-O, Nd-O and Nd-Cu polyhedra during oxygenation. Finally, interrelations of the structural evolution and physical properties were reconsidered to improve a possibility of production of high-quality rare-earth-containing HTSC materials.

4PO7-27 Electrical and Magnetic Properties of Gd(Ba_2-xGd_x)Cu₃O_6+d

Hajime Shimizu 1, Takahiro Tomimiatsu 1, and Kiyoichiro Motoya 2. 1 Department of Electronics and Computer Science, Science University of Tokyo in Yamaguchi, Onoda, Yamaguchi 756-0884, Japan. 2 Department of Physics, Science University of Tokyo, Noda, Chiba 278-8510, Japan.

Presenting Author: H. Shimizu

Influence of gadolinium and oxygen contents on the electrical and magnetic properties of Gd(Ba_2-xGd_x)Cu₃O_6+d has been investigated. Polycrystalline samples of Gd(Ba_2-xGd_x)Cu₃O_6+d (0<x<0.25) were prepared by ordinary solid-state reaction. Electrical resistivity, X-ray diffraction, susceptibility and Gd³⁺-EPR measurements have been carried out as function of x and d .

We have obtained the following experimental results: (I) With increasing x, d increases, orthorhombicity (a-b)/(a+b) decreases, critical temperature T_c decreases and Gd-ordering temperature T_N increases from 2.2K(x=0) to 2.4K(x=0.2). The orthorhombic to tetragonal phase transition occurs around x=0.2. (II) The Gd³⁺-EPR linewidth can be analyzed as the sum of the linewidths due to Gd-Gd and Gd-carrier interactions.

Informations about the carrier number on the CuO₂ planes are obtained from the analysis of the linewidth caused by the Gd-carrier interaction. The influence of x- and d -values on the T_c of these compounds will be discussed on the basis of carrier number and lattice constants.

4PO7-28 Er³⁺-EPR Study of Y_1-xCa_xBa₂Cu₃O_6+d :Er

Hajime Shimizu, Masatoshi Nakamura and Takahiro Tomimatsu, Department of Electronics and Computer Science, Science University of Tokyo in Yamaguchi, Onoda, Yamaguchi 756-0884, Japan

Presenting Author: H. Shimizu

In the Y_1-xCa_xBa₂Cu₃O_6+d superconductor the Y(Ca) layer is sandwiched with CuO₂ plane and located apart from CuO chain. Therefore, by using EPR-signal of the magnetic spin embedded in the Y lattice site we should be able to obtain the informations about electronic and magnetic states of the CuO₂ plane. We employed Er³⁺-ion having L¹ 0 as EPR-probe because we also want to obtain the informations about the lattice systems of this compound. Er³⁺-EPR measurements were carried out for Er-substituted Y_1-xCa_xBa₂Cu₃O_6+d samples with various delta and x values. Polycrystalline samples of Y_0.97-xCa_xEr_0.03Ba₂Cu₃O_6+d were prepared by ordinary solid-state reaction. We have obtained the following experimental results: (I) A clear resonance signal is observed in all the samples all over low temperature region except for just below T_c. (II) The Er³⁺-EPR linewidth can be fitted by D H = D H₀ + bT + cD ³ / [exp(D /T)-1]. The first term is the residual linewidth. The second corresponds to the Korringa relaxation mechanism where the Er³⁺ local moments couple to the host conduction carriers. The third corresponds to the relaxation due to the Orbach process which gives informations about the lattice system.

4PO7-29 Rietveld analysis of Gd(Ba_2-xA_x)Cu₃O_y (A = Ca, Sr) Superconductors

H.-C.I. Kao, C.H. Chin, J.T. Huang, and C.M. Wang, Department of Chemistry, Tamkang University, Tamsui, Taiwan 25137

Presenting Author: H.-C.I. Kao

Two series of orthorhombic triple-perovskite superconducting samples with nominal compositions of Gd(Ba_2-xCa_x)Cu₃O_y Ca series, 0 £ x £ 0.25 and Gd(Ba_2-x'Sr_x')Cu₃O_y Sr series, 0 £ x' £ 1.0 are prepared. Orthorhombicity decreases with increasing the amount of Ca or Sr substitution. Part of Ca ions occupies R-site causes the decrease of all the unit-cell parameters in the R perovskite layer for all the samples in Ca series. However, all of the Sr ions only occupy the Ba-site. T_c is related to orthorhombicity, the vertical distance between two Cu(2) atoms and the angle of O(1)-Ba-O(1) along a-axis.