Influence of the growth direction on properties of ferroelectric ultrathin films

A first-principles-based approach is developed and used to investigate Pb(Zr0.4Ti0.6)O-3 ultrathin films having different growth directions and subject to different boundary conditions. A wide variety of dipole patterns is found, including ferroelectric phases absent in the bulk and different periodic stripe nanodomains. Moreover, a large enhancement of dielectricity is found in ultrathin films exhibiting a growth direction that differs from a possible direction of the polarization in the corresponding bulk. A set of two general and simple rules is provided to analyze and understand all these features.