MICROSCOPIC STUDIES ON Y2BA4CU7O15-DELTA BY USE OF TEM AND NQR TECHNIQUES

Y2Ba4Cu7O15-delta compounds were characterized by microscopic investig ation using TEM and NQR techniques. We synthesized Y247 compounds by t wo kinds of preparation methods: the conventional solid-state reaction (sample A) and the polymerized-complex method (sample B), The value o f T-c for sample A was found to be 65 It and that for sample B was 93 It by ac-chi measurements. As a result of TEM experiments, stacking fa ults along the c axis were observed more frequently in sample B than i n sample a. These stacking faults resulted in microdomains containing pure Y123 or Y124 thin blocks of several unit cells. NQR experiments r evealed that the microscopic environment of Co(2) sites in sample A di ffered from either of those in the Y123 and Y124 compounds, NQR freque ncy values (nu(Q)) of Cu(2) sites agreed well with those calculated by the point charge model applied for the Y247 structure, The spectra of Cu(2) sites in sample B could be regarded, however, as a combination of those of pure Y123, Y124, and also Y247 compounds. This fact was co incident with the result off TEM experiments. We concluded that the su perconductivity of Y247 with higher T-c originates from the thin block of Y123. (C) 1998 Academic Press.