Temperature dependence of three-dimensional domain wall arrangement in ferroelectric K_0.9Na_0.1NbO₃epitaxial thin films

The three-dimensional arrangement and orientation of domain walls in ferroelectric K0.9Na0.1NbO3/(110)NdScO3 epitaxial thin films were investigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffraction and theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain pattern appears in which there is a periodic arrangement of a1a2/MC monoclinic phases. Four different types of domain walls are observed, which can be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, a periodic a1/a2 stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed, both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimental results are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreement between the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In this temperature range, a complex interplay of coexisting monoclinic a1a2/MC and orthorhombic a1/a2 phases takes place.