Gibbs energy and growth habits of YBCO

A simple model of nucleation of YBCO thin films deposited on different substrates by vapour deposition is reported. The role of the surface energy of the smallest crystallites formed at the very early stage of film growth is discussed. Information on the surface Gibbs energy of YBCO is obtained from experimental results reported in the literature, and in particular from the analysis of single crystal growth. etch figures and indentation measurements. An estimate from ab initio calculations of the Gibbs energy of(100), (010) and (001) surfaces is presented. The obtained data allow us to estimate the surface energy of the nuclei and to achieve a qualitative description of the transition from a-axis to c-axis YBCO growth at decreasing supersaturation.