Vacancy filled nickel-cobalt-titanate thin films

Single phase nickel-cobalt-titanate thin films with a formula A_1+2xTi_1−xO₃, where A is Ni²⁺,Co²⁺, and −0.25 < x < 1, were grown by pulsed laser deposition on sapphire substrates. There is a large window in which Ni/Co ratio and x can be chosen independently, allowing a control of properties by the synthesis. In the prototype ilmenite and corundum structures one third of the octahedra are vacant. The reported structure is obtained by filling vacant octahedra (x > 0) or emptying filled octahedra (x < 0). Films with x = 1 have all octahedra filled. X-ray photoelectron spectroscopy, X-ray diffraction measurements and Raman scattering techniques were applied to address the structure as a function of x. Data collected on samples with x ≈ 1 are interpreted in terms of hexagonal P6₃/mmc space group. The hexagonal structure was obtained when films contained both Ni and Co: pure cobalt oxide thin films possessed the spinel structure. Two factors controlling the magnetism and crystal distortion are identified: a direct overlap between the adjacent cation d-orbitals resulting in a bond formation and magnetic interactions, and x controlling the cation shift toward the vacant octahedron. The displacement decreases and the symmetry increases with increasing x. When x ≈ 1 the cations prefer octahedron center positions.