Hole-Trapping-Induced Stabilization of Ni^{4 +} in SrNiO₃/LaFeO₃ Superlattices

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO₃)_m/(LaFeO₃)_n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi⁴⁺O₃ with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni³⁺ and Fe⁴⁺, whereas Ni⁴⁺ and Fe³⁺ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO₆ octahedral rotations, which, in turn, determine the energy balance between Ni³⁺/Fe⁴⁺ and Ni⁴⁺/Fe³⁺, thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.