INFLUENCE OF STACKING-FAULTS AND TEMPERATURE ON THE STRUCTURE OF Y2BA4CU7O15, INVESTIGATED BY HIGH-RESOLUTION NEUTRON-DIFFRACTION AND ELECTRON-MICROSCOPY

The temperature dependence of the structure of the high-T-c supercondu ctor Y2Ba4Cu7O15-delta (Y-247) has been studied by high-resolution neu tron powder diffraction. This structure can be described as alternatin g single CuO chain (123) and double CuO chain (124) blocks. Two fully oxidized polycrystalline Y-247 samples with T-c,T- onset=94.8 and 89.3 K, prepared by a sol-gel technique at 1 atm oxygen pressure, were inv estigated in the temperature range from 5 to 100 K, and the structural changes were determined by Rietveld analysis. The oxygen content was found to be equal in both samples but the unit cell parameters and int eratomic bond distances differed significantly. The structural changes with temperature in the compound with T(c, onset)approximate to 95 K showed that the displacements of the apical oxygen in the 123 and 124 blocks are different. In agreement with the decreased coherence length along the c axis detected by neutron diffraction, electron diffractio n studies showed considerable streaking in the sample with T-c=89 K wh ereas the sample with T-c=95 K was more ordered. High-resolution elect ron microscopy studies indicated that the reason for streaking are sta cking faults in the -123-124-123-124-sequence that build up the struct ure. (C) 1996 Academic Press, Inc.