  Citation: Yang, C.;Wang, Y.; Putzky, D.; Sigle, W.; Wang, H.; Ortiz, R.A.; Logvenov, G.; Benckiser, E.; Keimer, B.; van Aken, P.A. Ruddlesden– Popper Faults in NdNiO 3 Thin Films. Symmetry 2022, 14, 464. https:// doi.org/10.3390/sym14030464 Academic Editors: Partha Pratim Das, Arturo Ponce-Pedraza, Enrico Mugnaioli and Stavros Nicolopoulos Received: 8 December 2021 Accepted: 20 February 2022 Published: 25 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). symmetry S S Article Ruddlesden–Popper Faults in NdNiO 3 Thin Films Chao Yang 1, *, Yi Wang 1,2, *, Daniel Putzky 1 , Wilfried Sigle 1 , Hongguang Wang 1 , Roberto A. Ortiz 1 , Gennady Logvenov 1 , Eva Benckiser 1 , Bernhard Keimer 1 and Peter A. van Aken 1 1 Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany; d.putzky@fkf.mpg.de (D.P.); w.sigle@fkf.mpg.de (W.S.); hgwang@fkf.mpg.de (H.W.); r.ortiz@fkf.mpg.de (R.A.O.); g.logvenov@fkf.mpg.de (G.L.); benckise@fkf.mpg.de (E.B.); b.keimer@fkf.mpg.de (B.K.); p.vanaken@fkf.mpg.de (P.A.v.A.) 2 Center for Microscopy and Analysis, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China * Correspondence: c.yang@fkf.mpg.de (C.Y.); wang.yi@nuaa.edu.cn(Y.W.) Abstract: The NdNiO 3 (NNO) system has attracted a considerable amount of attention owing to the discovery of superconductivity in Nd 0.8 Sr 0.2 NiO 2 . In rare-earth nickelates, Ruddlesden– Popper (RP) faults play a signiﬁcant role in functional properties, motivating our exploration of its microstructural characteristics and the electronic structure. Here, we employed aberration-corrected scanning transmission electron microscopy and spectroscopy to study a NdNiO 3 ﬁlm grown by layer-by-layer molecular beam epitaxy (MBE). We found RP faults with multiple conﬁgurations in high-angle annular dark-ﬁeld images. Elemental intermixing occurs at the SrTiO 3 –NdNiO 3 interface and in the RP fault regions. Quantitative analysis of the variation in lattice constants indicates that large strains exist around the substrate–ﬁlm interface. We demonstrate that the Ni valence change around RP faults is related to a strain and structure variation. This work provides insights into the microstructure and electronic-structure modiﬁcations around RP faults in nickelates. Keywords: Ruddlesden–Popper faults; NNO thin ﬁlms; EELS; HAADF image 1. Introduction The recent discovery of nickel-based superconductors has ﬁlled the gap in nickel-based oxide materials in superconducting systems [1–5]. The inﬁnite-layer phase NdNiO 2 can only be synthesized from the precursor ABO 3 perovskite structure by removing apical oxygen atoms from NiO 6 octahedra through soft-chemistry topotactical reduction [6]. For stabilization of the superconducting phase, chemical doping with divalent Sr replacing trivalent Nd is crucial. Optimal doping with the highest superconducting transition temper- ature (around 15 K) has been found in Nd 0.8 Sr 0.2 NiO 2 thin ﬁlms [1]. During pulsed-laser deposition growth of Sr-doped thin NdNiO 3 ﬁlms, a strong tendency to form Ruddlesden– Popper (RP) faults has been reported [2]. In addition, the epitaxial strain from the substrate can induce the formation of RP faults [7,8]. These RP faults display an atomic structure, where the inclusion of an additional AO layer breaks the long-range order of the ABO 3 perovskite phase [3,7–17]. As the microstructure of the entire ﬁlm determines its electric properties, investigation of the defect structure and associated variations in the local elec- tronic structure in nickelates is indispensable. Moreover, RP structures are related to a variety of physical and chemical properties, e.g., electro-catalytic activities [11], microwave dielectric performance [18], magnetic properties [19], and ferroelectric properties [20]. Epitaxial strain and/or a cation non-stoichiometry, e.g., an excess of A or B in the ABO 3 structure, can induce the generation of different RP faults that consist of an in- tergrowth of rock-salt-type and perovskite-type building blocks [11,12,21,22]. A single rock-salt layer can be described as an a/2 <111> or a/2 <110> stacking fault displaying a zigzag arrangement of cations [17]. The (AO)(ABO 3 ) n RP structure forms when a rock-salt Symmetry 2022, 14, 464. https://doi.org/10.3390/sym14030464 https://www.mdpi.com/journal/symmetry