Phase transition and ferroelectric properties of epitaxially strained KNbO₃/NaNbO₃ superlattice

The influence of epitaxial strain on the crystal structure and polarization of KNbO₃/NaNbO₃ (KNO/NNO) superlattice has been quantified using density functional theory based on pseudopotential and plane-wave basis. A sequence of structural phase transitions with changing the in-plane misfit strain has been identified. If the compressive strain is more than -0.8%, tetragonal phase with polarization along [001] is stable. For the misfit strain between -0.8% and 0.36%, the stable phase is monoclinic while the stable phase becomes orthorhombic with polarization along [110] when the tensile strain is more than 0.36%. The spontaneous polarization in the orthorhombic phase reaches more than 70 μC/ cm².