Dielectric and pyroelectric properties of barium strontium titanate films on orthorhombic substrates with (110)//(100) epitaxy

The role of anisotropic misfit strains on the spontaneous polarization, dielectric properties, and pyroelectric response of (110) oriented Ba0.6Sr0.4TiO3 (BST 60/40) thin films on (100) orthorhombic substrates is analyzed theoretically. The anisotropic in-plane strain state and the rotation of the elastic and the electrostrictive constants of the BST 60/40 films result in strongly directional and unique properties, different from BST 60/40 films on cubic substrates with (100)(BST)//(100)(substrate) epitaxy. The thermodynamic formalism also incorporates the thickness dependence of the internal stress state due to the anisotropic relaxation of epitaxial stresses through the formation of misfit dislocations along the two in-plane directions. In particular, the model is applied to (110) BST 60/40 ferroelectric films on (100) NdGaO3 orthorhombic substrates. A more generalized analysis treating the in-plane misfit strains as parameters shows that ferroelectric phases that cannot be observed in single-crystal perovskite ferroelectrics can be stabilized due to the reduction in the symmetry induced by the anisotropic strain state.