cs __ __ i!!i ELSEVIER April 1996 Materials Letters 26 (I 996) 305-3 I 1 The effect of Fe diffusion on structural and superconducting properties of YBaCuO T.D. Dzhafarov ‘, M. Altunbag * , A. Varilci, U. Cevik, A.i. Kopya zyxwvutsrqponmlk Depurtment zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Physics, Faculty of Science and Arts, Karo&niz Technicd University, 61080 Trahzon, Turkey Received 11 July 199.5; revised 1 I October 1995; accepted 13 October 1995 zyxwvutsrqponmlkjihgfedcbaZYX Abstract The Fe diffusion in superconducting YBa,Cu,O, (YBaCuO) ceramic has been studied over the temperature range 615-880°C by the energy dispersive X-ray fluorescence (EDXRF) technique. The temperature dependences of iron diffusion coefficient in grains (D I] and over the grain boundaries ( D2) are described by the relations D, = 1.5 X lo- 4 exp( - 1.25/H) and D, = 1.8 X 10m5 exp( -0.90/U). Effects of iron diffusion and cooling rate on the crystal structure and superconduct- ing properties of YBaCuO have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEMI and resistivity measurements. It is found that iron diffusion is accompanied by the formation of iron oxides in YBaCuO. The fast cooling of iron-doped sa.mples stimulates the formation of predominantly orthorhombic phase, while the undoped sample, employing the same cooling rate, remained tetragonal. Keywords: Fe diffusion; Superconducting properties; YBaCuO 1. Introduction The partial substitution in the parent YBa,Cu,O, (YBaCuO) structure has been investigated exten- sively in attempt to clarify or improve the supercon- ducting behaviour of this type of superconductors. Substitutions of ions for Y and Ba in YBaCuO have very little effect on superconductivity, while substitu- tions for Cu have a destructive effect. This supports the belief that the superconducting properties are associated with the Cu-0 layers. Substitutions for Cu by magnetic ions of 3d transition metals (Fe, Co, or Ni) present an essential interest for the under- ’ Corresponding author. ’ Permanent address: ‘The Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan. standing of the high-T, superconductivity mecha- nism. The Fe atoms substitute preferentially at Cu(1) sites in chains of YBaCuO and induce an orthorhom- bit-tetragonal phase transition (near 3 at% of Fe) [l-4]. The Fe substitution leads to decrease of q with the increasing dopant concentration. The super- conducting transition temperature is depressed to zero at a concentration of Fe of about 17 at% [3,5,6]. Microscopic studies revealed that the observed “te- tragonal” phase results from the twin structure of microdomains with orthorhombic symmetry [7,8]. There are also indications that iron substitutes for copper at both sites and that the structure is a mixture of the orthorhombic and tetragonal phase in YBaCuO-Fe composition [4,9]. It should be noticed that all studies of the YBaCuO-Fe compound were carried out on the 00167-577X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDf 00 167-577X(95)00240-5