Study of the effect of Zn substitution for Cu on the properties of Y-Ba-Cu-O system

Abstract

Since the discovery of high-Tc cuprates the system have been. the subjected of extensive substitutional studies to investigate how their superconductivity responds to substitutional disorder resulting from incorporation of metallic dopants at different cationic sites of their crystallographic unit cell. In the present investigation the YBa2Cu~7_6 (Y-123), YBa~u,f)g (Y-124) and Zn doped Y-124 samples have been prepared at difftrent pressure and difftrent sintering temperature by solid state reaction method using Y/J3' BaC03, CU20, and ZnO. The samples are annealed at 60ifC for 24 hours in oxygen atmosphere. The main objectives is to investigate the effict of the substitution of Zn for Cu-site in Y-124 system having the composition YBa~u.4-,Zn,Oo where x =0.0 to 0.5 in steps ofO.1. DC resistance studies have been carried out using the four point probe method for all samples down to liquid nitrogen temperature. Tcfor pure Y-123 & Y-124 are found to above liquid nitrogen temperature. Tcfor Zn doped Y-124 samples are found to be below liquid nitrogen temperature. The Tc for these samples are determined by susceptibility measurements using liquid helium. Structural analysis are done taking Xray diffraction (XRD) pattern of the samples with CuKa radiation using Ni filter. The oxygen content have been estimated by iodometric technique. DC resistance measurement for Y-123 sample showed a sharp superconducting transition having onset temperature (Tc,) of92 K and a mid point transition (Tc"Jof 91 K. XRD pattern shows that the sample is of orthorhombic single phase and the lattice constants are a = 3.818A, b = 3.887 A and c = 11.685 A. These values are in close agreement with JCPDS Grant in Aid report 1987 (a = 3.8185 A, b = 3.8856 A and c = 11.6804 A). The oxygen content in this sample is observed to be 6.8 i.e. the Y-123 compound is ofYBa~u306.8 state. Tcofor Y-124 (x = 0.0) sample is found to vary from 84 to 89 K and Tem80 to 86 K for different batches of samples prepared at difftrent sintering temperature. The Teofor Zn doped specimens YBa2CU4-,Zn,Ogdrops quite drastically and falls below liquid nitrogen temperature. The magnetic susceptibility measurement performed on the. same batches of samples gives Tco of 87 to 89 K and Temis 84 to 86 K for x = 0.0, consistent with the values of resistance measurement. Tco drastically drops to around 60 Kfor x = O.land about 50 Kfor x = 0.5. The superconducting Tem was about 50 K and 40 K respectively. An analysis of the peak position and intensities of the XRD pattern of the samples show that the parent sample x = 0.0 has the orthorhombic structure. The Zn doped samples preserve the basic orthorhombic structure though some modifications of the diffraction are observed. There is no significant change of a, b, and c parameters with Zn incorporation. The intensity of the XRD pattern for the maximum intense peak is however found to gradually decrease with increasing Zn content. From the iodometry test, the oxygen content for x = 0.0 is found to vary 7.79 to 7.81 and the oxygen content slightly decrease with increasing Zn dopant. It can be concluded that oxygen content or oxygen vacancy order are not disturbed significantly by Zn substitution and these results suggest that divalent Zn substitutes preferentially in the CuD2 planes in Cu(2) sites. However the slight variation in oxygen content with increased Zn-content is found to play a significant role in determining the superconducting properties of this system. Ratan Lal et al. from their measurements on YBa2Cu4-,Zn,Dofor maximum dopant of5% (X = 0.2) reported the suppression effects o/Zn in Tc to be 18Klat.%. In the present measurement conducted up to 12.5% Zn substitution we observed suppression effect of8 to 12 Klat.% be/11Jeen0 and 2.5%. The suppression is nearly 3 Klat.% in bet11Jeen2.5 and 5% and 1 Klat.% in bet11Jeen5 and 12.5%. Thus with higher dopant concentration drop in Tc gets slower. This is probably that at higher concentration of Zn substitutional atom in the system not only gains atomic character but also occupies some of the chainer site Cu(l) besides occupying the planar Cu(2) site: It thus appears that Zn in the chain site [cu(l)] has little effect either on the stability a/the double chain or in the oxygen occupation in the chains.