Strain-Driven Oxygen Vacancy Ordering in LaNiO3 Thin Films: Impact of Ruddlesden-Popper Faults

The study of rare-earth nickelates, such as LaNiO ₃ (LNO), is significant due to their complex electronic properties. Ordered oxygen vacancies (OOV) in LaNiO _3 − x decrease conductivity, converting it from metallic to insulating state as 'x' approaches 0.5, and semiconducting behavior near x = 0.75. These OOV also influence magnetic properties, causing LNO to exhibit anti-ferromagnetic and ferromagnetic behavior instead of its usual paramagnetic state. Interfacial strain in thin-film heterostructures is utilized to regulate the creation of oxygen vacancies and Ruddlesden-Popper (RP) faults, leading to notable impacts on materials' structural and electronic phases. The effect of strain on the formation of RP faults and the critical thickness of a fault-free layer in LNO has been studied, but atomic-scale insights into the relationship between strain, OOV, and RP faults are still limited. In this paper, we systematically investigated the effect of strain and RP faults on the formation of OOV in LNO thin films grown on SrTiO ₃ (STO) substrates. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM imaging, we conducted atomic-scale structural and compositional analyses of OOV. Geometric phase analysis (GPA) was employed to measure the strain in fault-free and RP fault regions, while density functional theory (DFT) calculations explored different OOV arrangements in the LNO phase. Simulated iDPC-STEM imaging of energy-stabilized structures was performed to correlate with experimental results. Our findings reveal superstructure modulation in the chemical composition and atomic-scale lattice structure in LNO, primarily due to the formation of the OOV in Ni-O layer of LaNiO _2.5 phase. The out-of-plane compressive strain of about 2% stabilizes this phase, reducing the strain, diminishing OOV, and transforming them into LNO.