PnCA ELSEVIER Physica C 341-348 (2000) 613-614 www elsevier.nl/Iocate/physc Substitution for Ba by light rare-earth in Eu 123 solid solutions* Youwen Xu a, M. J. Kramerb, K. W. Dennis b, H. Wu b, A. O'Connor b and R. W. McCalhim b, S.K. Malik c, W.B.Yelon d aMinnesota State University, Mankato, Mankato, MN56001 UAmes Laboratory, U. S. Dept. of Energy, Iowa State University, Ames, IA 50011, U.S.A. CTata Institute of Fundamental Research, Bombay 400005, India aUniversity of Missouri Research Reactor, Columbia, MO 65211, U.S.A. A detailed study on a series samples of Eu(Bal-xRx)2Cu307+~ (R = La, Pr and Eu) was performed using X-ray diffraction, neutron diffraction, magnetic measurements and thermopower measurements. The effect of Pr in this compound was compared to two other rare earths, and we believe that the fast suppression ofT c by Pr substitution in this structure is electronic in nature. 1. INTRODUCTION It is well known that Pr plays a unique role in the superconductivity of rare-earth 123 (R123) compounds. Pr substitute on R site in these com- pounds are well studiedk Since Pr can substitute on both the R site and the Ba site, it is unclear what role Pr plays on Ba site. The question has become more critical as superconductivity in Pr123 was reported2. To differentiate the effect of Pr on Ba site to R site, we chose a solid solution based on Eul23. We studied a series samples of Eu(Bal.xRx)2 Cu307.~ where R = La, Pr and Eu. Since the ionic radius of Eu (0.950 A) is considerably smaller than that of other light rare-earth (1.061A for La, and 1.013/~ for Pr), we hope that Pr would substitute Ba exclusively. If the valence state is the sole factor in hole localization, all of the R 3+ substitutes should have the same effect on superconductivity. 2. EXPERIMENTAL RESULTS X-Ray diffraction was used to determine the lattice parameters of the Eu03aL.xRx)2Cu3OT.~ samples. As R substituted for Ba, the cell volume decreases since the ionic radii of the R 3+are smaller than that of Ba, and the crystal structure gradually changed from orthorhombic to tetragonal due to disorder of the oxygen on the chain and anti-chain sites. A least-squares-fit to the orthorhombicity (defined as [2*(b-a)/(b+a)l) as a function of substitution level x showed that the phase transition occurred at x = 0.11, 0.13 and 0.15 for Eu, Pr, and La substituted samples respectively. Larger R 3÷ allows for a higher anti-chain site occupancy before the phase transition occurs, mostly due to less strain on the bonds. Neutron diffraction confirmed that all of the substitutes went to the Ba site, and the oxygen content slightly increased as x increased. The buckling angle of the Cu-O2 plane decreased as x increased, resulting in flatter Cu-O2 plane. Therc was no distinguished difference in the changes of buckling angle for different R substitutes. As the substitution level increased, the Cu-O apical bond length decreased, 1.4% in Pr samples, and 2.7% in La samples. While distance between Cu(1) and 0(4) stayed the same, the distance between the Cu- O2 planes slightly decreased. This decrease was the most prominent in Eu samples. * 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. Depa.,tment of Energy grant No. DE-FE02-90ER45427 through the Midwest Superconductivity Consortium. 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.M All rights reserved. Pll S0921-4534(00)00614-6