Complex transition metal oxides have enough structural flexibility to produce a wide range of functional properties for electronics and energy storage applications. To control these properties, researchers grow thin films of these materials, seeking to control their crystal structure. Controlling crystal structure opens the door for tuning electronic structure and possibly achieving novel properties. Electronic structure in oxides is largely controlled by the charges on the cations. Ni4+ ions are energetically unstable and rarely found in chemical compounds. We have stabilized Ni4+ in a carefully designed superlattice of SrNiO3 and LaFeO3. The cubic perovskite SrNiO3, which is not observed in bulk form, was shown to form in these superlattices, but only at the thickness of one crystalline unit, or unit cell.
We deposited alternating layers of SrNiO3 and LaFeO3 with varying thicknesses by means of molecular beam epitaxy and found that well-ordered, cubic SrNiO3 nucleated only when a single unit cell of SrNiO3 was included. By controlling the thickness of the adjacent LaFeO3 layers, they could control the charges on the Fe and Ni cations.
If a single unit cell of LaFeO3 was used, resulting in a (SrNiO3)1/(LaFeO3)1 superlattice, the Ni was trivalent (Ni3+) and the Fe was tetravalent (Fe4+). However, as the LaFeO3 layer thickness increased, the Ni became tetravalent (Ni4+) and the Fe trivalent (Fe4+). It was found that additional layers of LaFeO3 induced larger rotations of the FeO6 octahedral cages in the LaFeO3 layers. The result was that the structure of the LaFeO3 layers approached that of bulk LaFeO3. This structural change modified the densities of electronic states such that holes could localize in the SrNiO3 layer, producing Ni4+ in the (SrNiO3)1/(LaFeO3)5 superlattice.