OXYGEN-ISOTOPE EXCHANGE KINETICS AND SITE-SELECTIVE OXYGEN SUBSTITUTION IN (YBA2CU3O7-X)-O-18

Studies of the oxygen isotope exchange kinetics in O-18 substituted YB a2Cu3O7 (123) have been performed. As a weight is observed during the O-18 --> O-16 exchange, high resolution thermogravimetric measurements (both isothermal and non-isothermal) were used to investigate the kin etics. The exchange process takes place at relatively low temperatures , 225-430-degrees-C, and was found to be non-isokinetic in this temper ature range. Two ranges with different activation energies have been f ound: ( 1 ) 1.88 eV both in the ranges 433-398-degrees-C (total exchan ge of O-18 in the planes + apex + chains) and 353-307-degrees-C (excha nge mainly in apex + chains) but with different pre-exponential factor s and (2) 0.71 eV at t < 290-degrees-C (exchange mainly in the chains) . Based on these investigations, it was possible to determine the ther modynamic and kinetic conditions for the site-selective (O(1) or O ( 1 ), O (4) ) oxygen exchange.