Scanning tunnelling microscopy and spectroscopy, with atomic-scale resolution, were performed on single crystals in order to understand the marked differences in the behaviours of Pr- and Y-containing phases; both of which had the identical crystal structure. With increasing Pr content, the twin-boundary density was systematically reduced. The results confirmed the unusual electronic structure of CuO2 planes and Cu-O chains in the Pr-compound. They indicated that one of the main ways in which the Pr doping differed from other rare-earth substitutions was that it led to substantial O disorder on CuO2 planes, although the overall crystal structure - including the net O content of the unit cell - was invariant. It was noted that O vacancies on CuO2 planes increased monotonically with increasing Pr content. The Pr-compound could contain almost 20 times more O vacancies than did the Y-compound; giving some O6.95. It was suggested that the disorder which was generated by the presence of planar O vacancies made any band which involved planar O 2p-orbitals vulnerable to localization and led directly to a loss of superconductivity in Y(Pr)-systems. It was concluded that the avoidance of aliovalent substitutions, such as Pr3+ at Ba2+ sites and Pr4+ at Y3+ sites, which led indirectly to O vacancies in CuO2 networks, could be crucial in the preparation of superconducting Pr-compound samples.

Nanolevel Studies of Lattice Defects and Electronic Structure of YBa2Cu3O7 Single Crystals Doped with Praseodymium. A.V.Narlikar, A.Gupta, S.B.Samanta, C.Chen, Y.Hu, F.Wondre, B.M.Wanklyn, J.W.Hodby: Philosophical Magazine B, 1999, 79[5], 717-28