Phase-Pure Epitaxial b-Axis-Oriented Bronze TiO2 Films

We demonstrate the heteroepitaxial growth of phase-pure bronze-phase TiO2 films using pulsed laser deposition on MgAl2O4 single-crystal substrates. While the growth on cubic substrates with smaller lattice parameters favors the stabilization of an out-of-plane-oriented anatase phase, and the use of substrates with larger lattice parameters leads to formation of the rutile phase, MgAl2O4 lies in a narrow intermediate range where the bronze phase is stabilized. X-ray diffraction shows that the b-axis is oriented out-of-plane, while the a-c lattice plane lies within the film plane. The bronze films show twinned domains due to their monoclinic structure that are aligned along all four in-plane directions of the MgAl2O4 lattice. In a subsequent step, TiO2 films are grown on top of MgAl2O4-buffered MgO single crystals in order to demonstrate a route to stabilize the bronze phase on a larger variety of substrates. The growth of bronze-type TiO2 films with the unique, open, one-dimensional framework aligned along the film normally may allow for the investigation of its basic functional properties related to ion diffusion that cannot otherwise be studied easily in other crystal forms.