Oxides with perovskite-like structure represent a fascinating class of advanced materials which have been extensively studied in the past decades since they exhibit a variety of functionalities, e.g., ferromagnetism, ferroelectricity or ferroelasticity. In ferroelectric materials phase symmetry and structural distortions are strongly coupled to piezoelectric properties. Periodic domain structures are of particular interest from both fundamental and technological point of view. Such periodic polarization modulations on a nanometer scale can be engineered by the use of substrates with suitable (anisotropic) misfit strains. However, understanding and controlling of ferroelectric phases and dimensions of the domain pattern is still challenging.
We focus on (K,Na)NbO3 thin films grown on various rare earth scandate substrates using metal-organic chemical vapor deposition (MOCVD). Highly regular one dimensional ferroelectric domains pattern are formed with a lateral periodicity of typically 50 nm. The monoclinic symmetry of the domains is controlled by the anisotropic epitaxial lattice strain. Using piezoresponse force microscopy and conventional x-ray diffraction the monoclinic Mc phase can identified, which is associated with both a strong vertical and lateral electrical polarization component.
Interestingly, depending on the lattice strain induced by the underlying substrate and the MOCVD growth conditions four structural variants of superdomains are found, which differ in the three-dimensional arrangement of the domain walls and the corresponding electrical polarization vectors. These superdomains with typical lateral sizes of a few micrometers were individually characterized by a focused 100 nm x-ray beam as provided by highly brilliant synchrotron radiation sources (ESRF, PETRA III). Distinct differences (e.g. strain state, monoclinic shearing angle, domain wall angles) between the superdomain variants were observed and will be discussed in more detail.