Effects of nonequally biaxial misfit strains on the phase diagram and dielectric properties of epitaxial ferroelectric thin films

The effects of nonequally biaxial in-plane misfit strains on the equilibrium polarization states and the dielectric properties of single-domain epitaxial ferroelectric thin films are investigated by a nonlinear thermodynamic theory. The "misfit strain-misfit strain" and "temperature- misfit strain" phase diagrams for single-domain BaTiO3 (BT) and PbTiO3 (PT) thin films grown on tetragonal substrates are developed by minimizing the Helmholtz free energy. The nonequally biaxial misfit strains cause the presence of two in-plane tetragonal ferreoelectric phases, a(1) (P-1 not equal 0, P-2= P-3= 0) and a(2) (P-2 not equal 0, P-1= P-3= 0), in both BT and PT thin films, which do not exist if the misfit strains are equally biaxial. (c) 2005 American Institute of Physics