Effect of crystal orientation on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO₃ thin films

The influence of crystal orientations on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO₃ thin films has been investigated using an expanded nonlinear thermodynamic theory. The calculations reveal that crystal orientation has significant influence on the phase stability and phase transitions in the misfit strain-temperature phase diagrams. In particular, the (110) orientation leads to a lower symmetry and more complicated phase transition than the (111) orientation in BaTiO₃ films. The increase of compressive strain will dramatically enhance the Curie temperature T_C of (110)-oriented BaTiO₃ films, which matches well with previous experimental data. The polarization components experience a great change across the boundaries of different phases at room temperature in both (110)- and (111)-oriented films, which leads to the huge dielectric and piezoelectric responses. A good agreement is found between the present thermodynamics calculation and previous first-principles calculations. Our work provides an insight into how to use crystal orientation, epitaxial strain and temperature to tune the structure and properties of ferroelectrics