Complex Epitaxy of Tetragonal Tungsten Bronze K–Ta–Nb–O Nanorods

Tetragonal tungsten bronze (TTB) phases possess numerous important properties (ferroelectricity, multiferroicity, piezoelectricity, optical nonlinearity, electro-optics) that can be achieved by modifying their composition, in addition to their ability to grow as very anisotropic crystals. In this study, K_5.06(Ta_0.57Nb_0.43)_10.99O₃₀ tetragonal tungsten bronze phase thin films were grown by a pulsed laser deposition technique on (001)­SrTiO₃ and R-plane sapphire substrates. The films grew according to two modes with respect to the substrate surface, that is, as vertical nanorods with the [001] direction perpendicular to the substrate surface and as horizontal nanorods with the [001] orientation parallel to the substrate surface and ⟨310⟩ out-of-plane direction. Both vertical and horizontal nanorods present epitaxial relationships with the substrates. Careful study of epitaxial relationships showed a complex growth on both substrates that can be described in the framework of domain matching epitaxy resulting in several antiphase domain formations for both kinds of nanorods. These particular configurations are due to a high degree of coincidence between cations (anions) of the film with those of the substrate. This study shows the ability of ferroelectric TTB phases to grow as one-dimensional objects with the possibility to tailor their polarization direction either normal to or parallel to the substrate surface.