Phase and Orientation Control of NiTiO₃ Thin Films

Subtle changes in the atomic arrangement of NiTiO₃ in the ilmenite structure affects its symmetry and properties. At high temperatures, the cations are randomly distributed throughout the structure, resulting in the corundum structure with R-3c symmetry. Upon cooling, the cations order in alternating layers along the crystallographic c axis, resulting in the ilmenite structure with R-3 symmetry. Related to this is the R3c symmetry, where the cations alternate both perpendicularly and along the c axis. NiTiO₃ with the latter structure is highly interesting as it exhibits ferroelectric properties. The close relationship between structure and properties for ilmenite-related structures emphasizes the importance of being able to control the symmetry during synthesis. We show that the orientation and symmetry of thin films of NiTiO₃ formed by atomic layer deposition (ALD) can be controlled by choice of substrate. The disordered phase (R-3c), previously only observed at elevated temperatures, have been deposited at 250 °C on α-Al₂O₃ substrates, while post-deposition annealing at moderate temperatures (650 °C) induces ordering (R-3). We have in addition explored the symmetry and epitaxial orientation obtained when deposited on substrates of LaAlO₃(100), SrTiO₃(100) and MgO(100). The presented work demonstrates the possibilities of ALD to form metastable phases through choice of substrates.