STUDIES OF STRUCTURAL DISORDER IN REBA2CU3O7-X THIN-FILMS (RE = RARE-EARTH) AS A FUNCTION OF RARE-EARTH IONIC RADIUS AND FILM DEPOSITION CONDITIONS

Structural disorder in expanded ''c'' parameter ReBa2CuO7-x (Re = rare earth) thin films grown by radio-frequency magnetron sputtering has b een quantified by undertaking Rietveld refinements of X-ray data. In o rder to understand why the disorder occurs, we have focused on determi ning the factors which influence the amount of disorder, such as growt h conditions, rare-earth ion size, and overall composition. To underst and more about the disordering mechanism, some additional comparative growth experiments with bulk samples have been carried out. For YBa2Cu 3O7-x films, low oxygen growth pressures and temperatures give rise to disordered films with enlarged ''c'' parameters and degraded supercon ducting properties. There is a correlation between (00l) to (006) peak intensity ratios and the ''c'' parameter with the amount of disorder in the films. In the most disordered YBa2Cu3O7-x films, as much as 14% Y and Ba cation exchange, and a large fraction of apical oxygen vacan cies have been found. The effect of smaller rare-earth ions is to decr ease the amount of cation disorder. However, as was shown for YbBa2Cu3 O7-x films, there are structural vacancies in the rare-earth site and T(c) is reduced. A model for the disordering mechanism is proposed bas ed on the kinetic and thermodynamic driving forces for the growth of Y Ba2Cu3O7-x from the disordered, precursor phase, Re0.33Ba0.66CuOy.