Unravelling atomic disorder and anti-site distortions in non-stoichiometric NiCo2O4 nanoparticles: a pathway to functional electronic and magnetic behaviour

Abstract

We report the impact of atomic disorder and Jahn–Teller (J–T) distortions in inverse spinel NiCo₂O₄ (NCO) nanoparticles synthesized via sol–gel combustion at elevated temperatures. Structural, optical, and electronic characterizations reveal cooperative J–T distortions driven by orbital degeneracy in Ni³⁺ cations, giving rise to local lattice distortions and electronic localization. UV-visible absorption spectroscopy confirms the semiconducting nature of the phase-stable NCO at room temperature. Site-specific cation distributions and charge states were probed through resonant photoemission spectroscopy (RPES) at the Ni and Co 3p–3d transitions, supported by core-level X-ray photoemission spectroscopy (XPS). The presence of oxygen vacancies implies that non-stoichiometric growth contributes to intra- and inter-site electron-hopping mechanisms. RPES analysis reveals the partial localization of Ni 3d states and a more pronounced localization of Co 3d states, indicative of strong correlation effects. X-ray absorption near-edge structure (XANES) measurements at the Ni and Co K-edges further elucidate the influence of anti-site disorder on the local atomic environment, revealing temperature-induced modifications in Ni–Co interatomic distances. These structural changes significantly affect magnetic interactions, particularly the antiferromagnetic exchange pathways mediated via the octahedral site, i.e., B-site cation disorder. These findings show the interplay between atomic disorder and electronic structure as well as magnetism in NCO, with implications for its functional applications in spintronics and photocatalysis.