Oxygen isotope exchange kinetics and site-selective oxygen substitution in YBa₂Cu₃¹⁸O_7-χ

Studies of the oxygen isotope exchange kinetics in 18O substituted YBa2Cu3O7 (123) have been performed. As a weight decrease is observed during the 18O→16O exchange, high resolution thermogravimetric measurements (both isothermal and non-isothermal) were used to investigate the kinetics. The exchange process takes place at relatively low temperatures, 225-430°C, and was found to be non-isokinetic in this temperature range. Two ranges with different activation energies have been found: (1) 1.88 eV both in the ranges 433-398°C (total exchange of 18O in the planes+apex+chains) and 353-307°C (exchange mainly in apex+chains) but with different pre-exponential factors and (2) 0.71 eV at t<290°C (exchange mainly in the chains). Based on these investigations, it was possible to determine the thermodynamic and kinetic conditions for the site-selective (O(1) or O(1), O(4)) oxygen exchange. © 1993.