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

Abstract

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 Δqz ≈ 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.

Valence-Ordered Thin-Film Nickelate with Tri-component Nickel Coordination Prepared by Topochemical Reduction

“…The square planar environment occurs at the intersection of orthogonal families and harbors a Ni 1+ indicating a possible route to IL-structure. It should be noted that such (303) pc phases have also been reported as structural defects, constrained in a quasi-2D nanosheet on the top of IL-NNO2 . In fact, such a (303) pc defect might be present as nanodomains, presumably along a single orientation, or overlap along different orientations and order at longer range as we showed here, enabling phase engineering in the ABO 2 –ABO 3 system.…”

“…The tricomponents are distributed in the lattice with a periodicity determined by the Vo ordering, contributing to a commensurate charge order for this system with a periodicity ( 1 3 , 0, 1 3 ) rlu. This periodicity is strikingly similar to that of the CO reported in IL-nickelate thin films. − This possibly indicates the existence of this phase in IL-nickelate thin films as a defect, and this is further strengthened from recent studies reporting apical oxygen ordering in IL-nickelate thin films that exhibited similar CO. , It questions how intrinsic is the ( 1 3 , 0) rlu CO in IL-NdNiO 2 .…”

“…It should be noted that such (303) pc phases have also been reported as structural defects, constrained in a quasi-2D nanosheet on the top of IL-NNO2. 24 In fact, such a (303) pc defect might be present as nanodomains, presumably along a single orientation, or overlap along different orientations and order at longer range as we showed here, enabling phase engineering in the ABO 2 −ABO 3 system. In such case, this A 9 B 9 O 22 phase could be a member of the family of nickel oxides that forms by {n0n} pc ordered Vo.…”

“…This periodicity is strikingly similar to that of the CO reported in IL-nickelate thin films. 20−23 This possibly indicates the existence of this phase in IL-nickelate thin films as a defect, and this is further strengthened from recent studies reporting apical oxygen ordering in IL-nickelate thin films that exhibited similar CO. 24,25 It questions how intrinsic is the ( ), making the phase diagram of this system more complex.…”

“…This often results in a mixture of secondary phases with partial removal of apical oxygen. , This is due to the partial instability attributed to the infinite-layer structure that does not host Ni 2+ (3d 8 ) but a Ni 1+ in a 3d 9 configuration . Along with superconductivity, the observed charge ordering (CO) − in these systems now appears to be connected to a structural reordering due to oxygen intercalation in the apical sites. , Nevertheless, the structure of the charge-ordered systems cannot be directly related to known phases, such as brownmillerite or octahedra and square planar bearing R 3 Ni 3 O 7 phase where Ni 2+ is absent . The complex transport properties and notably the unusual dramatic resistivity increase upon oxygen deintercalation of the perovskite nickelates RNiO 3 is also intriguing …”