SCIENCE ADVANCES | RESEARCH ARTICLE PHYSICS Copyright © 2021 The Authors, some Spontaneous phase segregation of Sr2NiO3 and SrNi2O3 rights reserved; exclusive licensee during SrNiO3 heteroepitaxy American Association for the Advancement of Science. No claim to Le Wang1*, Zhenzhong Yang1*†, Xinmao Yin2*, Sandra D. Taylor1, Xu He3, Chi Sin Tang4, original U.S. Government Mark E. Bowden5, Jiali Zhao6, Jiaou Wang6, Jishan Liu1,7, Daniel E. Perea8, Linda Wangoh1, Works. Distributed Andrew T. S. Wee2, Hua Zhou9, Scott A. Chambers1‡, Yingge Du1‡ under a Creative Commons Attribution Recent discovery of superconductivity in Nd0.8Sr0.2NiO2 motivates the synthesis of other nickel­ates for pro- NonCommercial viding insights into the origin of high-temperature superconductivity. However, the synthesis of stoichiometric License 4.0 (CC BY-NC). R1−xSrxNiO3 thin films over a range of x has proven challenging. Moreover, little is known about the structures and properties of the end member SrNiO3. Here, we show that spontaneous phase segregation occurs while depositing SrNiO3 thin films on perovskite oxide substrates by molecular beam epitaxy. Two coexisting oxygen-deficient Ruddlesden-Popper phases, Sr2NiO3 and SrNi2O3, are formed to balance the stoichiometry and stabilize the ener- getically preferred Ni2+ cation. Our study sheds light on an unusual oxide thin-film nucleation process driven by the instability in perovskite structured SrNiO3 and the tendency of transition metal cations to form their most stable valence (i.e., Ni2+ in this case). The resulting metastable reduced Ruddlesden-Popper structures offer a testbed for further studying emerging phenomena in nickel-based oxides. INTRODUCTION samples (12, 13), suggesting that superconductivity in this system Rare-earth nickelates (RNiO3, where R denotes the lanthanide) ex- may arise from interface or strain-related effects. Hence, the synthe- hibit a rich array of structural and physical properties (1–3). Although sis of high-quality stoichiometric Nd1−xSrxNiO3 thin films with bulk LaNiO3 (LNO) is a rhombohedral structured paramagnetic controlled x becomes a prerequisite for further harnessing super- metal at all temperatures, other RNiO3 compounds display metal-­ conductivity in this system. However, the synthesis of stoichiomet- to-insulator transitions (MITs) from a high-temperature orthor- ric R1−xSrxNiO3 thin films over a wide range of x has proven to be hombic metallic phase to a low-temperature monoclinic insulating difficult (10, 11, 14) because of the instability of Ni oxidation states phase. The orthorhombic structure (Fig. 1A) evolves from a distor- greater than +3. It is thus of considerable interest to explore how the tion of the cubic structure due to a small Goldschmidt tolerance structure and properties of nickelate thin films evolve at higher x, why factor (t). The drop in t with decreasing R cation radius tends to structural decay occurs as x increases, and whether the end member drive the distortion and reduce the Ni─O─Ni bond angle, resulting SrNiO3 can be stabilized as a (pseudo)cubic perovskite by epitaxial in a reduction in Ni 3d–O 2p orbital overlap. Consequently, the growth. In addition, a Ruddlesden-Popper (RP)–type secondary bandwidth drops and the transition temperature goes up. phase is often observed during the growth of nickelate thin films Doping and strain engineering are two common ways to modify (14). However, it is not clear why the RP secondary phase forms so the electronic states of RNiO3 for exploring emergent physical prop- readily, and further exploration is needed. erties and potential applications (4–10). The recent discovery of In light of the fact that the t for SrNiO3 is ~1.08, bulk SrNiO3 is superconductivity in infinite-layered Nd0.8Sr0.2NiO2 on SrTiO3 sub- expected to form a hexagonal structure (see Fig. 1A) with face-shared strates motivates the synthesis of other related nickelates in search- NiO6 octahedra (15). This structure is not symmetry-matched with ing for high Tc materials and exploring the origin of the observed traditional perovskite substrates such as SrTiO3 or LaAlO3. First- superconductivity (11). However, insulating behavior without the principles calculations for hexagonal SrNiO3 predict this material to presence of superconductivity was observed in bulk Nd0.8Sr0.2NiO2 be a half-metallic ferromagnet with promising applications in spin- tronics and magneto-optical devices (16). In addition, Ni4+ has been 1 Physical and Computational Sciences Directorate, Pacific Northwest National Lab- proposed to be responsible for the high OER performance found in oratory, Richland, WA 99354, USA. 2Department of Physics, Faculty of Science, Na- modified nickelate electrocatalysts (17, 18). SrNiO3 with Ni4+ is tional University of Singapore, Singapore 117542, Singapore. 3Catalan Institute of unstable due to its high Gibbs formation energy (19), and as a result, Nanoscience and Nanotechnology-ICN2, CSIC and BIST, Campus UAB, 08193 Bellaterra, Spain. 4NUS Graduate School for Integrative Sciences and Engineering, National there has been limited success in the synthesis of bulk, hexagonal- University of Singapore, Singapore 117456, Singapore. 5Environmental Molecular structured SrNiO3 (15, 20). In addition, successful epitaxial growth Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, of SrNiO3− (where 0 ≤  < 1) thin films has not yet been reported. USA. 6Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China. 7State Key Laboratory of Functional Here, we present a study of the synthesis of SrNiO3− thin films Materials for Informatics, Shanghai Institute of Microsystem and Information Tech- by oxygen plasma–assisted molecular beam epitaxy (OPA-MBE). nology, Chinese Academy of Sciences (CAS) , Shanghai 200050, China. 8Earth and By keeping the Sr/Ni ratio at 1:1 during the deposition, we find that Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, instead of forming phase pure SrNiO3, spontaneous phase segrega- WA 99354, USA. 9X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA. tion occurs. Our combined diffraction, spectroscopy, and micros- *These authors contributed equally to this work. copy dataset reveals that two complementary oxygen-deficient RP †Present address: Key Laboratory of Polar Materials and Devices (MOE) and Depart- phases Sr2NiO3 and SrNi2O3 evolve together, and that these effec- ment of Electronics, East China Normal University, Shanghai 200062, China ‡Corresponding author. Email: sa.chambers@pnnl.gov (S.A.C.); yingge.du@pnnl. tively balance the overall film stoichiometry, stabilize the preferred gov (Y.D.) Ni2+ oxidation state, and satisfy the epitaxial relationships for both Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 1 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE clearly not 1:1. Scanning transmission electron microscopy (STEM) images in Fig. 2 reveal that the film consists of layered RP phases. The body-centered symmetry of these compounds results in the ab- sence of diffraction peaks for an odd value of h + k + l. Consequent- ly, the XRD indicates an epitaxial RP phase with a c lattice parameter of 12.6 Å aligned perpendicular to the LSAT substrate surface. Phase segregation in epitaxial SNO films The high-angle annular dark field (HAADF) STEM image taken along [110] (Fig. 2A) reveals an abrupt interface between the first unit cell (u.c.) and the substrate. Phase segregation is apparent in the thin films consisting of more than 1 u.c. and two different phases can be clearly observed in the STEM image (see the dashed-circle regions). At the onset of SNO film growth, the cubic perovskite substrate can provide a lattice template for nucleation of a NiO2 monolayer fol- lowed by a SrO monolayer. This nucleation sequence does not pre- clude the formation of isolated point defects. As a result, interface engineering and limiting SNO deposition to 1 u.c. in the superlattices could generate novel ground states (such as Ni4+) unobtainable in bulk crystals (22). However, once the SNO film thickness exceeds 1 u.c., the large ionic radius ratio R(Sr2+)/R(Ni4+) and the instability of Ni4+ could promote phase segregation. Figure 2B shows magni- Fig. 1. Epitaxial synthesis of SrNiO3− (SNO) films with a 1:1 Sr/Ni ratio. (A) Schematic fied images of these two different phases that are marked by green crystal structures for bulk NdNi3+O3 and SrNi4+O3. Tolerance factor t​ = ​ _ ​r​ ​​  + ​r​O​​ _A ​​, and blue rectangles in Fig. 2A. Such spatially distributed, well-­ ​√2 ​(​r​B​​  + ​r​O​​) where rA, rB, and rO are the ionic radius of A and B cations and oxygen, respectively, separated phases are similar to nanocomposite films reported in a set of ABO3 crystal structures. Sr doping in NdNiO3 (Nd1−xSrxNiO3) would in- earlier (23–27). These phases are tentatively assigned to Sr2NiO3 and crease t due to the larger ionic radius of Sr2+ ion. (B) XRD -2 scan of SNO films. The SrNi2O3 based on the HAADF STEM contrast, the 1:1 Sr/Ni ratio left inset shows the RHEED pattern for an as-grown SNO film on LSAT(001) viewed during deposition, and the presence of Ni2+ as determined by x-ray along the [100] zone axis. The right inset shows the magnified 004 peak with clear photoemission spectroscopy (XPS) and x-ray absorption spectroscopy thickness fringes. # indicates that the peak originated from our XRD setup. a.u., ar- (XAS) measurements (see Fig. 3). As seen in Fig. 2A, Sr2NiO3 and bitrary units. SrNi2O3 show atomically matched, epitaxial interfaces along the film growth direction. Moreover, both phases show the same c-axis in-plane (IP) film/substrate and out-of-plane (OOP) Sr2NiO3/SrNi2O3 lattice constant (~12.6 Å), consistent with the XRD analysis. The grain interfaces. In addition, a combination of x-ray linear dichro- HAADF-STEM images and atomically resolved electron energy ism (XLD) measurements and density functional theory (DFT) cal- loss spectroscopy (EELS) mappings (Fig. 2, C and D) for the two culations reveals the presence of a planar structure in the Sr2NiO3 domains viewed along [100] further confirm the phase assignment. phase with a large IP orbital occupancy, which may open new ave- Within Sr2NiO3 regions, repeating SrO bilayers and NiO monolayers nues for investigation of emergent phenomena in this system. are clearly resolved along the vertical direction (c axis), as shown in Fig. 2C. Within SrNi2O3 regions, SrO single layers and NiO bilayers are connected along the c axis (Fig. 2D), where a weak signal/inter- RESULTS AND DISCUSSION ference of Sr in the second Ni layer is also observed, indicating that Epitaxial growth of strontium nickel oxide thin films the phase purity of SrNi2O3 is not as high as that of the Sr2NiO3, or Twenty-five-nanometer-thick SrNiO3− (SNO) thin films with a fixed the layer may be more accurately defined as Sr1+xNi2−xO3. In addition, 1:1 Sr/Ni flux ratio (as calibrated by a quartz crystal microbalance at localized NiO clusters are observed in the Sr1+xNi2−xO3 regions by the growth position before growth) were deposited on (001)-oriented EELS mappings at lower magnification (fig. S2), which may account for (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates by OPA-MBE. A typical the overall stoichiometry of SrNi2O3. reflection high-energy electron diffraction (RHEED) pattern taken Although similar structured SrCu2O3 and CaCu2O3 have been along the [100] zone axis (Fig. 1B) reveals bright, sharp streaks, in- reported in previous studies (28–30), the structure of SrNi2O3 has dicative of a planar surface and an epitaxial orientation with the not been predicted or characterized. SrCu2O3 is the first member substrate. The high-resolution x-ray diffraction (XRD)  to 2 scan (m = 3) of the homologous series of compounds with a stoichiome- in Fig. 1B shows clear thickness fringes (right inset in Fig. 1B) and try of Srm−1Cum+1O2m (m = 3, 5, ...). The Cu-O sheet in SrCu2O3 is strong Bragg peaks, indicating a high extent of crystallographic or- made of only Cu-O double chains without any bridging network dering. A wide-range reciprocal space map (RSM) (fig. S1) confirms (29). The CaCu2O3 structure is similar to that of SrCu2O3, but the the epitaxial relationship. All SNO film peaks in Fig. 1B can be as- Cu-O sheets in CaCu2O3 are strongly puckered. On the contrary, signed to (00l) reflections with an OOP (c) lattice parameter that is Sr2NiO3 films have previously been grown on Sr2TiO4-buffered a multiple of 6.3 Å, a value much larger than that of either bulk LaSrAlO4 (100) by pulsed laser deposition (21), indicating that hexagonal SrNiO3 (a = 5.355 Å and c = 4.86 Å) or the expected cubic Sr2NiO3 phase is more stable and easier to form than SrNi2O3 phase. perovskite counterpart (a = 3.81 Å) (15, 21). This expanded lattice Therefore, when the SNO film thickness exceeds 1 u.c., the large ionic parameter matches that of Sr2NiO3 (21), for which the Sr/Ni ratio is radius ratio R(Sr2+)/R(Ni4+) and highly unstable Ni4+ could promote Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 2 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE Fig. 2. Phase segregation occurs in SNO films. (A) Cross-sectional STEM image of SNO/LSAT along the [110] LSAT direction. Dashed circles clearly show phase segrega- tion. (B) Magnified images of the Sr2NiO3 phase and the SrNi2O3 phase marked by green and blue rectangles in (A). (C and D) Cross-sectional STEM images of two different phases in SNO/LSAT along the [100] LSAT direction. EELS maps (right) from the area marked with a yellow rectangle in the STEM images (left). Scale bar, 2 nm. (E) APT tip reconstruction for SNO/LSAT. (F) A 3D volume rendering showing regions with greater than 13.5% Ni2+ (upper, gray scale). 2D Ni2+ (middle) and Sr2+ (lower) concentration maps highlighting where spatially localized Ni and Sr enrichment occurs. the formation of Sr2NiO3 phase first. At the same time, a 1:1 Sr/Ni fluxes to the desired values (either 2:1 or 1:2). By normalizing the Ni flux ratio could induce the formation of impure SrNi2O3, leading to 3p XPS spectra in the survey scans, the resulting Sr 3d XPS spectra the observation of Sr1+xNi2−xO3 phase and NiO clusters. Sr2NiO3 show distinctly different intensities (fig. S3A), quantitatively con- structure can be considered to be a derivative of the layered RP firming the different Sr/Ni ratios in these three samples. Moreover, phases (An+1BnO3n+1, where n is the number of the layers of octahe- as shown in fig. S3B, the Sr2NiO3 film displays a RHEED pattern dra in the perovskite-like stack) and can be constructed by remov- similar to that of the SNO film with a 1:1 Sr/Ni flux ratio. However, ing alternate oxygen ions in the two-dimensional (2D) planes along the Sr2NiO3 film becomes amorphous once exposed to air. By cap- the IP direction of the A2BO4 (n = 1) structure. For simplicity, we ping the sample with a 10-nm LaFeO3, the XRD OOP scan of the continue to refer to the mixture of Sr1+xNi2−xO3 and NiO clusters as Sr2NiO3 film shows that the diffraction peak is weak and no thick- SrNi2O3 and refer to both Sr2NiO3 and SrNi2O3 as reduced RP phases ness fringes are observed. In contrast, the RHEED pattern of the because they have the same c-axis lattice parameter (~12.6 Å). The SrNi2O3 film indicates a much rougher surface and a mix of phases. difference between the epitaxial relationships shown in the two The diffraction peak for the SrNi2O3 film is also weak, but the peak white dotted circles in Fig. 2A may be induced by interfacial defects. position is the same as those of the Sr2NiO3 film and the SrNiO3− In the left circle, Sr planes align well to form a coherent interface film, indicating that all three films have the same c-axis lattice between the two different phases. In contrast, the grain boundary parameter (~12.6 Å). Therefore, the epitaxial interfaces between defects that are commonly found in metal oxide nanocomposites Sr2NiO3 and SrNi2O3 columns appear to be responsible for stabilizing (31) result in poor alignment of Sr planes, as seen in the right circle. these metastable phases to form the vertical “nanocomposite” struc- To verify the apparent stabilities of Sr2NiO3 and SrNi2O3, we tures when a 1:1 Sr/Ni flux ratio is used. also deposited and characterized nominally stoichiometric Sr2NiO3 The phase-separated reduced RP film was further analyzed by and SrNi2O3 films on LSAT, respectively, by setting the Sr and Ni 3D atom probe tomography (3D APT) (Fig. 2E). The 3D elemental Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 3 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE Fig. 3. Spectroscopy results of SNO films. Ni 2p XPS (A) and Ni L edge XAS (B) of SNO films. The reference spectra for Ni2+ and Ni3+ were measured using a vacuum-annealed NdNiO3− film (VA-NNO) and a plasma-annealed NdNiO3 film (PA-NNO), respectively, both grown on SrTiO3(001). The spectrum for a NiO film grown on MgO (001) is included for comparison. The blue dashed lines in (A) denote the Ni 2p1/2 and Ni 2p3/2 peak positions of SNO films. The purple and blue dashed lines in (B) are guidelines for peak positions. reconstruction was accomplished through assignment of ionic spe- located at 853.3 and 854.3 eV, respectively, at the Ni L3 edge in the cies from the mass spectra to best reproduce the SNO stoichiometry PA-NNO spectrum and a single feature at ~871.6 eV at the Ni L2 (fig. S4). In general, the major Ni and Sr species were Ni2+ at 29 to edge. In contrast, SNO shows one asymmetric feature located at 32 Da and Sr2+ at 42 to 44 Da. Here, 2+ is not the valence of Ni in ~853 eV at the Ni L3 edge and the Ni L2 edge is clearly shifted to the solid, but rather the mass-to-charge ratio of the ion detected in lower photon energy at ~870.6 eV. Previous studies show that in- APT. The minor isotope 64Ni interferes with 16O21+ at 32 Da and is creasing oxygen nonstoichiometry results in a gradual change in excluded from analysis of the Ni2+ species. Additional polyatomic/ valence from Ni3+ to Ni2+ and the Ni L3 edge is marked by an in- isobaric interferences for the Ni2+ and Sr2+ species were not ob- crease (decrease) in the intensity of peak  () as well as a shift to served. In turn, the Ni2+ and Sr2+ ionic species were used in evaluat- lower (higher) photon energy (35–37). This change is clearly seen at ing the Ni and Sr distribution for simplicity. Potential elemental the Ni L3 edge in Fig. 3B. Together, the Ni XPS and XAS spectra partitioning within a small subvolume of the SNO layer was visual- support the presence of divalent Ni in the SNO films. However, it ized in 3D using iso-concentration surfaces of the major ionic should be noted that the spectra of VA-NNO and SNO are different species Ni2+ and Sr2+ species. Ni-rich regions are visualized using from that of the Ni2+-containing standard, a NiO film grown on iso-concentration surfaces exceeding the average Ni2+ concentra- MgO(001). This difference is most likely due to the significantly tion (e.g., 13.5 ionic % Ni2+) in turn enabling observation of more different local atomic (RP versus rock salt) structures and bonding Ni-rich phases (Fig. 2F). Ni-rich regions within the SNO matrix are configurations. generally circuitous and randomly distributed in space. 2D concentra- As with the Cu L edge spectra in cuprates (38), peak  in the Ni tion maps (middle panel and lower panel of Fig. 2F) further highlight L edge (Fig. 3B) is indicative of ligand holes formed as a result of Ni Ni-rich regions correlated to Sr-poor regions, strongly supporting our 3d–O 2p hybridization. The diminution of peak  in SNO and VA- proposed Sr2NiO3 and SrNi2O3 model. NNO implies that the Ni 3d–O 2p hybridization strength decreases relative to PA-NNO. Owing to the strong hybridization between Ni In situ x-ray photoemission and ex situ absorption 3d and O 2p bands at the Fermi level, O K edge XAS becomes an- spectroscopy other useful way to probe the valence of Ni with adjacent ligand Figure 3A shows Ni 2p XPS spectra of a representative SNO film holes. Figure 4A shows the O K edge XAS for the SNO film and and three reference samples. Surface charging was consistently ob- reference samples. The spectra are normalized to zero and uni- served, and all spectra were shifted so that the O 1s peaks fall at 530 eV. ty at the O K pre-edge and post-edge, respectively, and the spec- Oxygen plasma annealing has been shown to be effective at healing tra are shifted vertically for clarity. The spectra for PA-NNO and oxygen vacancies in oxide thin films (32). Accordingly, a plasma-­ VA-NNO agree well with those reported in the literature (5, 37, 39, 40). nnealed NdNiO3 film (PA-NNO) was used as a Ni3+ standard and a For PA-NNO, a sharp pre-edge peak (marked by a dashed purple vacuum-annealed NdNiO3− film (VA-NNO) was used as a Ni2+ line) centered at ~528 eV, a broad feature near 535 eV, and a dou- reference (33). The Ni 2p XPS spectrum for PA-NNO consists of blet at 540 to 545 eV reveal hybridization of the O 2p band with Ni two dominant spin-orbit split features at ~856.3 and 873.9 eV, re- 3d, Nd 4f, and Ni 4sp bands, respectively. The sharp pre-edge peak spectively. These peaks shift to lower binding energy in VA-NNO, at ~528 eV in PA-NNO originates from the 3d8L̲ → c̲ 3d8 (Ni3+) tran- consistent with the previous reports (34). These two peaks are sig- sition, where L̲ denotes a ligand hole and c̲ denotes an O 1s core hole nificantly narrower in SNO than in PA-NNO, and the binding en- (41). The intensity of the pre-edge peak is directly related to the li- ergies are nearly the same as those for VA-NNO, suggesting the gand hole density, which in turn strongly affects transport behavior same Ni2+ valence in the two films. In addition, the Ni L edge XAS in nickelates films (8, 36, 42, 43). As seen in Fig. 4B and fig. S5, PA- spectrum for SNO (Fig. 3B) shows the same shape and peak posi- NNO shows metallic bulk-like -T behavior with low resistivity tions as the spectrum for VA-NNO. However, these differ signifi- of ~3.2 × 10−4 ohm·cm at 300 K. In sharp contrast, the pre-edge peak cantly from those of PA-NNO. There are two distinct peaks  and  is strongly suppressed in SNO, revealing the insulating behavior at Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 4 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE To better understand the XLD results in light of the coexistence of Sr2NiO3 and SrNi2O3 in SNO/LSAT, we conducted first-principles calculations (see Materials and Methods for details). We calculated the density of states (DOS) of four different structures: Sr2NiO4 (SNO_214), pyramidal Sr 2NiO 3 (SNO_213 pyramid), planar Sr 2NiO3 (SNO_213 planar), and SrNi2O3 (SNO_123). The calculat- ed minimum-energy structures are shown in Fig. 5B. The calculated c-axis lattice parameters of these four different structures are 12.24 Å for SNO_214, 12.86 Å for SNO_213 pyramid, 12.57 Å for SNO_213 planar, and 10.39 Å for SNO_123, respectively. However, experimental results (XRD and STEM) show that both phases have the same c-axis lattice constant (~12.6 Å). The Sr2NiO3 phase has a calculated lattice parameter closest to our measured value, whereas the SrNi2O3 phase has a much-reduced lattice parameter compared Fig. 4. O K edge XAS and electrical transport of SNO. (A) O K edge XAS for to the experimental value. Partial Sr intermixing within the Ni PA-NNO, SNO, and VA-NNO films measured at room temperature. (B) Temperature- planes (Sr1+xNi2−xO3) can be invoked to maintain a 12.6-Å c lattice dependent resistivity of these three samples. The purple arrow denotes the metal- parameter and an epitaxial relationship between the two phases. insulator transition point (TMI). Figure 5C shows the partial DOS (PDOS) results from which the orbital polarization in each structure can be obtained. For SNO_214 300 K. Moreover, the O K edge spectrum for VA-NNO exhibits no- (Ni4+), there is no bandgap and the IP orbital has a larger band- table changes compared with PA-NNO, especially in the Ni 3d band width and a slightly lower average energy than the OOP orbital. For region. For VA-NNO, the pre-edge peak located at ~528 eV disap- SNO_213 pyramid (Ni2+), the IP orbital has a higher energy than pears and two small features appear at ~528.8 and ~ 530.1 eV the OOP orbital and a large ∆eg (~0.7 eV) is observed. For SNO_213 (marked by two yellow arrows in Fig. 4A), which match well with planar (Ni2+), ∆eg = 0, and it is almost fully polarized with the OOP those observed in LaNi2+O2.5 (44, 45). Even though Ni ions in both orbital unoccupied, in qualitative agreement with our XLD mea- SNO with its mixed phases and VA-NNO are both Ni2+, their O K surements. SNO_123 (Ni 2+) shows a clear orbital polarization edge XAS spectra are quite distinct due to differences in band struc- and the IP orbital is mainly unoccupied, which is the opposite of ture that also greatly affect conductivity (see Fig. 4B). Thus, the local SNO_213 planar. Because of the phase segregation that occurs coordination environment and associated electronic structure of Ni in our SNO films, the orbital polarization in SNO_123 will par- atoms in SNO with its mix of Sr2NiO3 and SrNi2O3 are different tially cancel out that in SNO_213. In addition, the phase purity of from that found in VA-NNO and LaNiO2.5, in which Ni atoms are SrNi2O3 is not as high as that of the Sr2NiO3. Although it will not in a square-planar structure with chains of NiO6 octahedra (Oh) and affect the net orbital polarization of our SNO films, which is domi- NiO4 square-planar (D4h) configurations (44, 46, 47). nated by the contribution from the SNO_213 phase, the orbital polarization P for our SNO film (P ≈ 5.7%) is smaller than that Orbital polarization in SNO films found in the square-planar phase R4Ni3O8 (R = La, Pr) obtained by We also investigated the orbital polarization in the SNO films us- hydrogen reduction (51), in which ~70% of the holes occupy the dx2 − y2 ing XLD. The dx2 − y2 and the d3z2 − r2 orbitals can be probed inde- IP orbital. This value is the highest published in the literature for pendently by using IP and OOP x-ray polarization, respectively. Ni-based complex oxides (48–50, 52). Although a large IP orbital The absorption spectra for these two polarizations are shown in occupation has been observed in our SNO film, it should be men- Fig. 5A. The XLD (defined here as IEIP− IEOOP) indicates the pre- tioned here that the infinite NiO2 plane in the SNO_213 planar dominant occupation of the dx2 − y2 IP orbital, suggesting the atom- structure is along the OOP direction, different from the IP infinite ic orbital scheme shown in the inset in Fig. 5A. To quantify the NiO2 planes shown in superconducting Nd0.8Sr0.2NiO2 films (11). orbital polarization (P), we first calculate the ratio of unoccupied Moreover, spectroscopy data demonstrate the 3d8 configuration in states, r (48) our insulating SNO films, while a predicted 3d9 configuration should 3 ​I​ ​E​ OOP​​​​ exist in the superconducting Nd0.8Sr0.2NiO2 films (11). ​h​ 3​z​​  2​−​r​​  2​​​ ​ r = ​ ─  ​  = ​ ─  ​​ (1) In summary, our study demonstrates that the end member of ​h​ ​x​​  2​−​y​​  2​​​ 4 ​I​ ​E​ IP​​​​  − ​I​ ​E​ OOP​​​​ phase pure SrNi4+O3 thin films in Nd1−xSrxNiO3 where x = 1 cannot Here, hi is the number of holes in orbital i and Ij is the integrated be stabilized as a pseudo-cubic structure by epitaxial growth. How- L edge spectral intensity with polarization j. We obtain r = 1.12 by ever, suitable perovskite substrates can be effective templates for the integrating the spectra over the range 850 to 875 eV. This hole ratio nucleation and growth of structurally compatible oxygen-deficient value is comparable to previously reported values of r = 1.19 for RP phases, Sr2NiO3 and SrNi2O3. Their coexistence effectively satis- nickel oxide superlattices in tension and r = 1.05 resulting from spa- fies an overall Sr/Ni ratio of 1:1, leading to an epitaxial c-axis–oriented tial confinement in LaAlO3/LNO superlattices (48, 49). The r value nanocomposite. The dynamic phase segregation process that occurs can be used to evaluate the orbital polarization, P (50) during MBE growth of this material requires high cation diffu- sion rates and extensive vertical redistribution. Although the exact ​​P = ​(​​─ ​ ​ ​e​ g ​ ​​​​ − 1​)​​ ​ ─ ​  n4 1 − r ​​​ (2) growth mechanism remains to be elucidated, our observations indi- 1+r cate that this system presents a fascinating testbed for an unusual where neg is the total number of eg electrons. We obtain a value of kind of film nucleation that appears to be driven by the high stabil- P = 5.7% for our SNO films if neg = 2, as expected for Ni2+ (3d8). ity of a particular cation valence, Ni2+. We envision that this kind of Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 5 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE Fig. 5. Orbital polarization in SNO films. (A) Polarization-dependent XAS across the Ni L edge for SNO/LSAT(001). The inset shows a sketch of the engineered orbital and spin states, which forms the electronic configuration (Ni2+, 3d8) and orbital polarization (ndx2− y2> nd3z2 − r2). (B) Schematic of the crystalline structures of Sr2NiO4 (SNO_214), pyramid Sr2NiO3 (SNO_213 pyramid), planar Sr2NiO3 (SNO_213 planar), and SrNi2O3 (SNO_123). (C) Partial density of states (PDOS) of these four different systems. The Fermi level (the black dashed line) is placed at 0 eV. The black arrows denote the spin state. spontaneous phase segregation and RP faults may occur during the XPS and XAS measurements synthesis of Sr-doped RNiO3 where the Sr doping level is high. The In situ XPS measurements were carried out on the as-grown sam- presence of randomly distributed, mixed RP or reduced RP phases ples. A low-energy electron flood gun was used to neutralize the adds additional synthesis challenges in studying superconductivity charging effect. The TEY XAS measurements of the Ni L edge and but may open other avenues to explore the emergent phenomena in O K edge were performed at the Singapore Synchrotron Light nickelates. Source and the photoemission station at Beijing Synchrotron Radi- ation Facility of Institute of High Energy Physics, Chinese Academy of Sciences. All XPS and XAS were measured with the samples at MATERIALS AND METHODS 300 K. The x-ray incident angles 90° and 30° correspond to IP (Exy) Sample growth and OOP (Ez) polarizations, respectively. SNO thin films (~25 nm) were grown on (001)-oriented LSAT sub- strates by OPA-MBE. Sr and Ni were evaporated from the effusion Sample characterization cells, and the evaporation rates were calibrated using a quartz crys- A high-resolution x-ray diffractometer (Rigaku SmartLab) was used tal oscillator. The Sr (Ni) fluxes were calibrated by growing SrTiO3 to measure the lattice structure. HAADF images were obtained by (LaNiO3) films on (001)-oriented SrTiO3 substrate, respectively. using a probe-corrected ARM200F (JEOL, Tokyo, Japan) STEM op- Once we obtain stable RHEED oscillations, the exact flux was ex- erated at 200 kV using a ~1-Å probe with a 27.5-mrad convergence tracted from the period (deposition time per unit cell) and used for semiangle and an 82-mrad collection inner angle. EELS mapping SNO film growth. The substrate temperature was set to 700°C, was performed using a probe-corrected GrandARM-300F (JEOL, and the activated oxygen partial pressure was kept at ~3 × 10−6 torr Tokyo, Japan) STEM operating at 300 kV, using a ~1-Å probe with a during growth. In situ RHEED was used to monitor the overall 29-mrad convergence semiangle and a ~113-mrad spectrometer col- growth rate, and surface crystallography and structure. After growth, lection inner angle. the sample was cooled down to room temperature at 30°C/min un- Conventional focused-ion beam milling techniques were applied der activated oxygen. Two 12-nm-thick NdNiO3 films grown on to fabricate APT tips using a dual-beam scanning electron micro- (001)-oriented Nb-doped SrTiO3 substrate were annealed in oxy- scope (Thermo Fisher Scientific Helios Nanolab 600i) (53). A de- gen plasma and vacuum, respectively, to use as reference samples scription of the preparation techniques, including protective coatings for comparing the Ni XPS and XAS spectra with SNO. One 20-u.c.- used, is provided in the Supplementary Materials. APT was performed thick NiO film grown on MgO (001) substrate was also used as ref- using a CAMECA local electrode atom probe (LEAP) 4000 X-HR at erence sample. a base temperature and pressure of 40 K and < 3 × 10−11 torr, respectively. Wang et al., Sci. Adv. 2021; 7 : eabe2866 5 March 2021 6 of 8 SCIENCE ADVANCES | RESEARCH ARTICLE The detection efficiency is nominally 36%. Field evaporation of ions Y. Du, Tuning bifunctional oxygen electrocatalysts by changing the A-site rare-earth from the specimen was induced by a 355-nm-wavelength picosecond element in perovskite nickelates. Adv. Funct. Mater. 28, 1803712 (2018). 9. L. Chang, L. Wang, L. You, Z. Yang, A. Abdelsamie, Q. Zhang, Y. Zhou, L. 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