Microscopic Studies on Y₂Ba₄Cu₇O_15-δby Use of TEM and NQR Techniques

Y₂Ba₄Cu₇O_15-δcompounds were characterized by microscopic investigation using TEM and NQR techniques. We synthesized Y247 compounds by two kinds of preparation methods: the conventional solid-state reaction (sample A) and the polymerized-complex method (sample B). The value ofT_cfor sample A was found to be 65 K and that for sample B was 93 K by ac-χmeasurements. As a result of TEM experiments, stacking faults along thecaxis were observed more frequently in sample B than in sample A. These stacking faults resulted in microdomains containing pure Y123 or Y124 thin blocks of several unit cells. NQR experiments revealed that the microscopic environment of Cu(2) sites in sample A differed from either of those in the Y123 and Y124 compounds. NQR frequency values (ν_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 coincident with the result of TEM experiments. We concluded that the superconductivity of Y247 with higherT_coriginates from the thin block of Y123.