Dorothee Braun: Monoclinic domains in KxNa1-xNbO3 on rare earth scandate substrates

Recent years in ferroelectric material research are characterized by exploring different alternative lead-free materials. These have to exhibit excellent functional properties (e.g. high piezoresponses) for an appropriate exchange of lead-based oxide material. One pathway is offered in monoclinic phases due to the possibility to rotate the polarization vector almost continuously within the monoclinic mirror plane yielding enhanced piezoelectric properties. In lead-containing materials such as PbZrxTi1-xO3, monoclinic bridging phases exist at the morphotropic phase boundary (MPB). In the absence of such a MPB, monoclinic symmetry can be artificially induced in lead-free materials e.g. by the application of anisotropic lattice strain.

One promising material in this context is (K,Na)NbO3. Since the 70ies the material is known in bulk form with outstanding piezo-/ferroelectric properties like high Curie temperature and coupling coefficients. However due to the high volatility of the alkaline components, the challenge is the thin film deposition with low defect densities and controlled stoichiometric composition. As a result, the formation of domains, especially under anisotropic epitaxial lattice strain, is rarely investigated, yet.

In this talk KxNa1-xNbO3 thin films grown on several rare earth scandate substrates by metal-organic chemical vapor deposition are presented. Due to the orthorhombic symmetry of film and substrate material, the incorporated lattice is anisotropic favoring the formation of monoclinic domains. In order to identify appropriate film-substrate combinations, we have performed calculations of the elastic strain energy density based on linear elasticity theory for the system KxNa1-xNbO3 with x = 0.5 – 1 on several rare-earth substrates. On the basis of two examples, the huge variety of different monoclinic domains in (K,Na)NbO3 should be illustrated: (i) K0.75Na0.25NbO3/TbScO3, where films exhibit (001)pc film orientation under almost uniaxial, compressive strain, and (ii) K0.9Na0.1NbO3/NdScO3, where the elastic strain energy density for (001)pc and (100)pc film orientations are equal and provide the coexistence of domains with differently oriented polarization vectors.