Abstract
Single crystals of Bi 2 Sr 2 CaCu 2 O y with transition temperature T c =80 K have been reduced in hydrogen at 400°C. After hydrogen reduction, there is no observable change in crystal cell parameters and the modulation wave vector, q, but apparently both the Meissner fraction and the superconducting transition temperature decrease. The copper K-edge X-ray-absorption near-edge spectra (XANES) in the oxidized crystals with higher T c and the reduced crystals with lower T c show that the difference in local structure around copper could be a main factor governing the observed changes in superconducting properties. © 1992.