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 ReBa2Cu3O7−x (Re=rare earth) thin films grown by radio-frequency magnetron sputtering has been quantified by undertaking Rietveld refinements of X-ray data. In order to understand why the disorder occurs, we have focused on determining the factors which influence the amount of disorder, such as growth conditions, rare-earth ion size, and overall composition. To understand more about the disordering mechanism, some additional comparative growth experiments with bulk samples have been carried out. For YBa2Cu3O7−x films, low oxygen growth pressures and temperatures give rise to disordered films with enlarged “c” parameters and degraded superconducting 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 vacancies have been found. The effect of smaller rare-earth ions is to decrease the amount of cation disorder. However, as was shown for YbBa2Cu3O7−x films, there are structural vacancies in the rare-earth site and Tc is reduced. A model for the disordering mechanism is proposed based on the kinetic and thermodynamic driving forces for the growth of YBa2Cu3O7−x from the disordered, precursor phase, Re0.33Ba0.66CuOy.