THE EFFECTS OF PARTIAL SUBSTITUTIONS OF ZN AND FE ON THE CU SITES IN LABA2CU3O7+/-X, AND A COMPARISON WITH SIMILAR SUBSTITUTIONS IN YBA2CU3O7-X

The structural effects of Zn and Fe substitutions in LaBa2Cu3O7 +/- x have been examined using time-of-flight neutron powder diffraction, and compared with previous data relating to similar studies of YBa2Cu3O7-x. Although the Fe and Zn ions display a similar site preference (for Cu(1) and Cu(2), respectively) in both systems, significant structural differences have been found. In particular, higher concentrations of dopant are possible in LaBa2Cu3O7 +/- x (up to 25% Cu replacement by Zn and 66% replacement by Fe), and superconductivity is rapidly destroyed due to the incorporation of additional structural defects. For Zn substitutions, a significant concentration of vacancies on the O(4) sites is introduced, whereas Fe substitutions result in a gradual increase in structural disorder of a more general type. Evidence for this disorder is found on both the oxygen sublattice, e.g. partial filling of sites in the La plane at z = 1/2 and the cation sublattice, with some intermixing of the La and Ba ions.