Synthesis and Valence Control of Unconventional Ruddlesden-Popper Nickelates by Integrated Electrochemistry Approaches

Nickelates, a class of high-temperature superconductors, present unique challenges to new materials exploration due to their multi-valence nature and complex materials chemistry. In this work, we focus on stabilizing and tuning nickel valences through lithium-ion incorporation into La₂NiO₄ via solid-state reactions and electrochemical processes. Our study reveals that solid-state reactions with LiOH introduce new phases and lead to a higher Ni oxidation state, while electrochemical lithiation primarily promotes oxygen deintercalation, transforming La₂NiO₄ from the Fmmm to a Bmab phase and effectively reducing the Ni valence. This method further applies on thin-film geometries, enabling precise control over phase transitions by lattice epitaxy. Using advanced techniques like in situ X-rays diffraction (XRD) and neutron powder diffraction (NPD), we establish an innovative platform for the synthesis of nickelates with tunable valence states, highlighting the role of oxygen modulation in achieving desired electronic properties. This work paves new pathways for exploring superconductivity in these compounds.