Oxygen diffusion in the superconductors of the YBaCuO family: isotope exchange measurements and models

The thermodynamic and kinetic aspects of the oxygen exchange in the solid oxide-oxygen phase system are discussed for the YBaCuO compounds. For both YBa2Cu3O7−x (Y123) and Y2Ba4Cu7O15−x (Y247) a vacancy model can successfully be used to describe the phase equilibrium. The dependence of some properties (lattice constants, Raman scattering, magnetic properties, and local structure EXAFS) of the nearly-equilibrium Y123 samples on the oxygen-stoichiometry is briefly presented. Studies of the carbon dioxide content in the YBaCuO family compounds show that apart of YBa2Cu4O8 (Y124), a complete removal of the carbon without decomposition of the compounds is not possible. In the case of Y247 the influence of the CO2 content on TC was found to be large. ygen-isotope exchange kinetics (tracer diffusion) for all three YBaCuO compounds is discussed in detail. In the structure of Y123 and Y247 the CuO chains are the ‘fastest paths’ for the 18O diffusion, from which further exchange with the apex and planar sites takes place. A three-dimensional diffusion model for the oxygen exchange is presented. The model and the experimentally determined values of the diffusion coefficients allow the calculation of the isotope distribution for all three oxygen sites as a function of temperature, time and grain size. The kinetics of the low temperature (<325°C) oxidation of the oxygen depleted Y123 are discussed. An oxidation model assuming the existence of a boundary between the fully oxidized and the reduced material is proposed. The model allows the calculation of the conditions necessary to oxidize Y123 depending on crystalline size and temperature. The total and the site-selective oxygen-isotope effect (OIE) are discussed in the case of YBa2Cu3O7−x and Y0.7Pr0.3Ba2Cu3O6.98. The total OIE in underdoped oxygen depleted Y123 increases with decreasing TC. It has been shown that the OIE in site-selectively substituted samples is mainly due to the oxygen in CuO2 sites.