Observation of fault-free coherent layer during Ruddlesden–Popper faults generation in LaNiO 3 thin films

Two-dimensional lattice defects, including surface and grain boundaries, in various perovskite oxides have garnered considerable attention, as the overall physical properties of the oxides can significantly vary depending on the atomic and electronic structure of the defects. In particular, because Ruddlesden–Popper (RP) planar faults have a unique lattice structure with [AO] monolayers being inserted in an ABO3 perovskite framework, they have attracted considerable attention, resulting in various relevant studies being conducted. This study focuses on the effect of lattice strain on formation of RP faults and on critical thickness in (001) epitaxial LaNiO3 thin-film fabrication through a sol–gel process. Atomic-scale direct observations identifies that a fault-free coherent buffer layer forms during the RP fault generation and its thickness varies depending on the strain exerted from the substrate. When DyScO3, the lattice mismatch of which is the largest, is used as a substrate, the critical thickness of the fault-free buffer layer remarkably reduces to a few unit cells. This work highlights the significance of direct observation to understand the defect formation in perovskite oxides.