THE EXCHANGE OF OXYGEN ISOTOPES IN HIGH-TEMPERATURE SUPERCONDUCTORS

The following paper presents a new method of characterizing the exchan ge of oxygen in oxygen-containing solids. High-temperature superconduc tors of Y-Ba-Cu-O systems display a unique exchange process which is d etermined to a great extent by stoichiometric compositions and structu re. A preliminary characterization of the exchange may be stated as fo llows: the greater the oxygen affinity, the smaller the tendency towar ds isotope exchange. As the oxygen affinity decreases, the initial tem perature required for diffusion and exchange processes to the site inc reases, the initial temperature required for diffusion and exchange pr ocesses to the site increases, and the temperature at which emission a nd absorption of oxygen are equal decreases. The extent of the relativ e accumulation of O-18 in the gas phase bears a correlation to the tra nsition temperature in the superconducting Y-Ba-Cu-O phase. A very sim ilar mechanism for the diffusion of oxygen in solid phases can be infe rred from the alteration of the O-18 content in the O-34(2) in the gas phase. Qualitative conclusions regarding isotope exchange processes, surface conditions, and the movement of oxygen may be drawn from the t ime- and temperature-dependent changes in various formulated coefficie nts. In addition, the method presented allows opposing processes withi n a solid to be distinguished and recorded separately.