Monoclinic M_A domains in anisotropically strained ferroelectric K_0.75Na_0.25NbO₃ films on (110) TbScO₃ grown by MOCVD

A highly regular one-dimensional domain pattern is formed in ferroelectric K_0.75Na_0.25NbO₃ thin films grown on (110) TbScO₃ substrates using metal–organic chemical vapour deposition (MOCVD). The domain pattern exhibits a lateral periodicity of about 50 nm and extends over several micrometres. The monoclinic symmetry of the domains is controlled by the elastic anisotropy of K_0.75Na_0.25NbO₃ and the anisotropic lattice strain, which is highly compressive in one in-plane direction and weakly tensile in the corresponding orthogonal direction. Using piezoresponse force microscopy and X-ray diffraction, the monoclinic M_A phase is identified, which is associated with both a strong vertical and a lateral electric polarization component. The lateral component of the polarization vector is collinear with the ±[10]_pc shear direction of the pseudocubic unit cell of the film and changes periodically by 180° in adjacent domains. A structural variant of a 90° rotated M_A domain pattern, where the monoclinic distortion of the pseudocubic unit cells occurs along ±[110]_pc, is also observed. However, this variant appears with significantly lower probability, in agreement with energy considerations based on linear elasticity theory. Thus, the incorporation of highly anisotropic lattice strain provides the opportunity to grow one-dimensional nanostructures with high ferroelectric properties.