Stress induced monoclinic phase in epitaxial BaTiO3 on MgO

We present a detailed strain analysis of epitaxial ferroelectric films taking into account multiple sources of strain, including the lattice mismatch between the film and the substrate, thermal strains due to differences in the thermal expansion coefficients of the film and the substrate, and relaxation by the formation of interfacial dislocations. The lattice parameters of the film are calculated using a thermodynamic formalism coupled with the strain analysis. The theoretical model shows that epitaxial (001) BaTiO3 films on (001) MgO are expected to display successive phase transformations with decreasing temperatures that are different than the bulk. This is verified experimentally for 50 nm thick (001) BaTiO3 films grown on (001) MgO substrates at 720 degrees C using pulsed laser deposition. A synchrotron x-ray diffraction study displays two slope changes in the temperature dependence of the out-of-plane lattice constant. This indicates that two phase transformations exist in the film as a function of temperature with transition temperatures and lattice constants that are consistent with the theoretically predicted phases. Theoretical results show that the first transformation at around 270 degrees C corresponds to a paraelectric-ferroelectric transition. The ferroelectric phase has an orthogonal symmetry (Amm2) with in-plane polarization components. The transformation at around -20 degrees C is an Amm2 <-> Pm transition between two ferroelectric phases. The ferroelectric Pm phase is monoclinic compared to the rhombohedral R3m phase in bulk. (c) 2006 American Institute of Physics.