Properties of Pb(Zr,Ti)O$_3$ ultrathin films under stress-free and   open-circuit electrical boundary conditions
  A first-principles-based scheme is developed to simulate properties of (001) PbO-terminated Pb(Zr$_{1-x}$Ti$_{x}$)O$_3$ thin films that are under stress-free and open-circuit boundary conditions. Their low-temperature spontaneous polarization never vanishes down to the minimal thickness, and continuously rotates between the in-plane <010> and <110> directions when varying the Ti composition around x=0.50. Such rotation dramatically enhances piezoelectricity and dielectricity. Furthermore, the order of some phase transitions changes when going from bulk to thin films. 
Emad Almahmoud, Yulia Navtsenya, Igor Kornev, Huaxiang Fu and L.   Bellaiche