Monoclinic MA domains in anisotropically strained ferroelectric K0.75Na0.25NbO3 films on (110) TbScO3 grown by MOCVD

Abstract

A highly regular one-dimensional domain pattern is formed in ferroelectric K0.75Na0.25NbO3 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 K0.75Na0.25NbO3 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 MA 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 ±[{\overline 1}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 MA 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.