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

摘要

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 BaTiO_3 (BT) and PbTiO_3 (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≠0,P_2=P_3=0) and a_2 (P_2≠0,P_1=P_3=0), in both BT and PTthin films, which do not exist if the misfit strains are equally biaxial.