ANALISIS ECONOMICO

XRD data have shown that the crystal structure remains orthorhombic for Zn substitution whereas Fe substitution causes changes in the lattice parameters towards becoming a tetragonal structure at about 5% substitution. This is consistent with earlier work(2J,7). For both Fe and Zn substitution a rapid decrease in Tc was observed. This is also in good agreement with earlier work(1-3\

We first discuss the NMR data for Zn substitution. Zn substitution cause very small changes in shift, but a dramatic increase in relaxation rate. This is not consistent with the Korringa relation and thus an indication for the existence o f an additional source for the relaxation sim ilar to that observed for Zn and Ni substituted YBa2Cu307_g. The correlation between the relaxation rate and Tc as presented in section 5.4.2 is evidence that the fluctuating magnetic fields with Zn substitution creating extra relaxation are the probable source for the rapid decrease in Tc. The effects on the shift and relaxation rates with Zn substitution in YBa2Cu4Og are similar to those for Zn substitution in YBa2Cu307_8 strongly suggesting that Zn substitutes onto the Cu(2) site. The suppression of Tc with Zn substitution has been

T (K )

Fig.5.14. K2T ,T as a function of temperature for 1.8% Fe substituted YBa:(Cu,_yMy)40 8 and for pure YBa2Cu40 8l8).

discussed by Miyatake et al(3) who have suggested that the destruction of the local antiferromagnetic correlations is the mechanism responsible for the rapid decrease in Tc. The fluctuating magnetic fields created by Zn doping as suggested by Warren et al(11* and discussed in section 4.9 for Zn doped Y B a ^ t t j O ^ might be the source for the destruction o f the local antiferromagnetic correlations. Therefore the conclusion o f Miyateka et al. agrees with what has been suggested from our 89Y data. The temperature dependence of the 89Y shift for Zn doped YBa2Cu40 8 is similar to that for pure YBa2Cu4Og as was found for Zn doped YBa2Cu30 7_g i.e. consistent with substitution in the plane site and no change in N(EF).

89Y shift and relaxation rate behaviour at room temperature for Fe substituted YBa2Cu4Og is similar to that for chain site substituted YBa2Cu307_5 although there is a slight deviation from the Korringa relation. This could be taken to suggest that Fe mostly occupies the C u ( l) site. On the other hand (remembering that the temperature dependence o f the shift for plane site substituted YBa2Cu30 7_g is similar to that for undoped YBa2Cu30 7_g (chapter 4)) for 1.8% Fe doped YBa2Cu4Og the temperature dependence of shift is similar to that for undoped YBa2Cu4Og consistent with Fe mainly occupying the plane site. From a 57Fe Mossbauer study of 2.5% Fe substituted YBa2Cu4Og, Felner et al(,) have observed that one quadrupole doublet is present at 90K. They have attributed this doublet to iron ions which replace copper in the C u (l) site. However, it was claimed by Boolchand et al(5) that Fe appears to occupy the Cu(2) site. In their study Boolchand et al have analyzed Mossbauer spectra line shapes in terms of two majority (from the plane site) and two minority (from the chain site) quadrupole doublets. These contradictory results might be because o f an unstable structure in Fe substituted

YBajCi^Og as reported elsewhere(7). Also the 89Y MAS widths o f F e substituted YBa2Cu4Og are broader than those of Zn substituted YBa2Cu4O g (table 5.1) suggesting that the structure in Fe substituted YBa2Cu4Og is much m ore disordered than that in Zn doped material. Although our 89Y NM R results suggest that Fe might substitute on both C u (l) and Cu(2), sites because o f these complications discussed above we can not make a conclusion on the site occupancy o f Fe. It also seems likely that some part o f the reduction in Tc is due to the change in N (EF) with Fe substitution.

References.

1. I. Felner, I. Nowik, B. Brosh, D. Heckel and E. R. Baum inger, Phys. Rev. B 43 (1991) 8737.

2. L Felner. B. Brosh. Phys. Rev. B 43 (1991) 10364

3. T. Miyatake. K. Yamaguchi. T. Takata, N. Koshizuka and S. Tanaka, Phys. Rev. B 44 (1991) 10139.

4. Y. Wu. S. Pradhan and P. Boolchand, Phys. Rev. L e tt 6 7 (1991) 3184. 5. P. Boolchand. S. Pradhan and Y. Wu, M. Abdegadir, W H uff, D. Farrell, R.

Coussement and D. McDaniel, Phys. Rev. B 45 (1992) 921.

6. K. Yanagisawa, Y. Matsui, Y. Kodama. Y. Yamada and T. Matsumoto, Physica C 183 (1991) 197.

7. K. Yanagisawa, Y. Matsui, Y. Kodama, Y. Yamada and T. Matsumoto, Physica C 191 (1992) 32.

8. R. Dupree, Z.P. Han, D. McK. Paul, T.G.N. Babu and C. Graves, Physica C 179(1991) 311.

9. Z.P. Han, R. Dupree, D. McK. Paul, A.P. Howes and L.W .J. Caves, Physica C 181 (1991) 355.

10. P. Carretta, M. Cord, A. Rigamonti, R.De Renzi, F. Licci, C. Paris, L. Bonoldi, M. Sparpaglione and L. Zini, Physica C 191 (1992) 97. 11. W.W. W arren Jr., R.E. Walstedt, R.F. Bell, L.F. Schneemeyer, J.V.

Waszczak, R. Dupree and A. Gencten, to be published.