Structure of the High T_c Superconductor, Ba₂LaCu_3-xO_7-y –Studies on La-Ba Ordering and Oxygen Defects

Abstract

Crystal structures of semiconductor Ba 2 LaCu 3- x O 7- y sample and 56.5 K superconductor sample, which was obtained by annealing the former in O 2 , were determined by X-ray Rietveld analysis. The ordering of Ba and La has been clarified by the use of the anomalous dispersion effects. An increase in the site occupancy factor of square planer oxygen (O(2):at z=0) around Cu(1) and shortening of Ba-O(2) distance were observed in the high T c sample compared with the semiconductor. This oxygen behavior, i.e. formation of fully occupied square planer oxygen group (O(1)×2+O(2)×2) around Cu(1), determines the appearance of the superconductivity.