International Journal of Inorganic Materials 1 (1999) 259–268 Sol–gel synthesis and properties of YBa (Cu M)O (M5Co, Ni) 2 1 2x x 4 y q and effects of additional replacement of yttrium by calcium a, b a a a c * M.K. Van Bael , A. Kareiva , R. Nouwen , I. Schildermans , G. Vanhoyland , J. D’Haen , c a a a a M. D’Olieslaeger , D. Franco , J. Mullens , J. Yperman , L.C. Van Poucke a Laboratory of Inorganic and Physical Chemistry, IMO, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium b Department of General and Inorganic Chemistry, Vilnius University, LT-2006 Vilnius, Lithuania c Materials Physics Department, IMO, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium Abstract In this work the influence of replacing Cu by Ni or Co on the structural and superconducting properties of YBa Cu O is studied by 2 4 8 means of XRD, TGA, SEM, EDX and resistivity measurements. The samples are prepared by the acetate–tartrate sol–gel method, which has proved to be very appropriate for the synthesis of Y-124 at 1 atm with even very small amounts of dopants. It is demonstrated that the transition temperature drastically decreases upon Co or Ni substitution, and that superconductivity is lost at 3% Ni and 6% Co doping. XRD measurements show that Ni and Co produce different structural effects to the orthorhombic YBa Cu O phase, possibly because 2 4 8 they are introduced in different copper positions. Interesting results are obtained by additional substitution of Y by Ca in the Co- and Ni-doped samples: It is shown that the superconducting properties are enhanced in the samples with less than 3% Ni or 6% Co, and that superconductivity is even recovered in samples with more than 3% Ni or 6% Co.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Superconductors; B. Sol–gel chemistry; C. electron microscopy; C. thermogravimetric analysis; C. X-ray diffraction 1. Introduction as charge reservoirs with respect to the CuO planes. It can 2 be expected, therefore, that substitution of metals in one of the copper positions (Cu(1) in the CuO chains or Cu(2) in The YBa Cu O superconductor is the n52 member of x 2 4 8 the CuO planes) will cause important consequences for the Y Ba Cu O family, and its structure consists of 2 2 4 61n 141n the superconducting properties. A systematic study of layers of Y, BaO, CuO planes and double layers of CuO 2 x substitutions in both copper positions on the properties of chains perpendicular to the c-axis. The fact that double YBaCuO superconductors could provide information about layers of CuO chains are present (unlike in the n51 x the nature of high temperature superconductivity. member YBa Cu O (Y-123) that has single layers of 2 3 72d A lot of studies have already been reported about CuO chains) results in a better orbital overlap between the x 31 substitution in the copper positions of Y-123 by Fe Cu(3d) and the O(2p) orbitals leading to a stable oxygen 31 21 [2–13], Co [2,3,5–7,9–11,13–17], Ni content and no orthorhombic to tetragonal phase transition 21 31 [2,3,9,10,13,18], Zn [2,3,6,8–10,12,13,19–21], Ga [1]. The superconducting transition temperature of Y-124 31 [2,21] and Al [3,9,10,15]. Fewer studies have been is 80 K. published concerning the influence of doping in the copper To date, no mechanism explaining superconductivity 31 21 positions of Y-124, e.g., by Fe [8,9,11,13,22–28], Zn and the herewith-related properties in high temperature 31 21 [8,10,13,29,30], Co [9,11,13,25,27,31], Ni [9– cuprate superconductors is known. However, it is generally 31 accepted that superconductivity resides in the copper oxide 11,13,25,27,31] and Ga [13,32]. plane layers of the cuprate structures, but it is largely From the published results can be concluded that each affected by the CuO chain layers, that are supposed to act substitution in either copper position causes a drastic x decrease of the superconducting transition temperature and that the effect is much more intense than for substitution in q Paper presented at the First International Conference on Inorganic the Y or Ba positions. The critical temperature of Y-124 Materials, Versailles, France 16–19 September, 1998. changes faster [13] than that of Y-123 upon substitution in *Corresponding author. Tel.: 132-11-268-393; fax: 132-11-268-301. the copper positions and substitution in the CuO planes E-mail address: marlies.vanbael@luc.ac.be (M.K. Van Bael) 2 1466-6049 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S1466-6049(99)00039-2