Charge Distribution across Capped and Uncapped Infinite-Layer Neodymium Nickelate Thin Films

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, the structural and charge distribution in NdNiO₂ thin films prepared with and without capping layers, and characterized by the absence and presence of CO are elucidated. The microstructural and spectroscopic analysis is done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard X-ray photoemission spectroscopy (HAXPES). Capped samples show Ni¹⁺ , with an out-of-plane (o-o-p) lattice parameter of around 3.30 Å indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite features indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 Å on the thin film top with a valence toward Ni²⁺ in this region. Here, 4D-STEM demonstrates (303)-oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with Δq_z ≈ 0.24 reciprocal lattice units (r.l.u.) extension located at Q = ( $$ ) and ( $$ ) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggest a possible link to the previously reported CO in infinite-layer nickelate thin films.