  We present an ab initio study of the interface energies of cubic
yttria-stabilized zirconia (YSZ) epitaxial layers on a (0001)_{alpha-Al_2O_3}
substrate. The interfaces are modelled using a supercell geometry and the
calculations are carried out in the framework of density-functional theory
(DFT) and the local-density approximation (LDA) using the
projector-augmented-wave (PAW) pseudopotential approach. Our calculations
clearly demonstrate that the (111)_{YSZ} || (0001)_{alpha-Al_2O_3} interface
energy is lower than that of (100)_{YSZ} || (0001)_{alpha-Al_2O_3}. This result
is central to understanding the behaviour of YSZ thin solid film islanding on
(0001)_{alpha-Al_2O_3} substrates, either flat or in presence of defects.
