Research Article  www.advelectronicmat.de Competition between Carrier Injection and Structural Distortions in Electron-Doped Perovskite Nickelate Thin Films Marios Hadjimichael,* Bernat Mundet, Claribel Domínguez, Adrien Waelchli, Gabriele De Luca, Jonathan Spring, Simon Jöhr, Siobhan McKeown Walker, Cinthia Piamonteze, Duncan T. L. Alexander, Jean-Marc Triscone, and Marta Gibert 1. Introduction The discovery of superconductivity in doped infinite-layer nickelate thin films has brought increased attention to the behavior of the doped Transition metal oxides are characterized perovskite phase. Despite this interest, the majority of existing studies by their close proximity to a large range of structural and electronic instabilities, pertain to hole-doped perovskite rare-earth nickelate thin films, while most and exhibit complex phase diagrams with electron-doping studies have been performed on bulk materials so far. To a variety of competing phases that can tackle this imbalance, a detailed study that addresses doping of NdNiO3 be spanned using chemical substitution. thin films using A-site substitution is presented, using Pb as a dopant Among them, materials that exhibit metal- and taking advantage of its valence-skipping nature. Through a combina- to-insulator transitions are especially tion of complementary techniques including X-ray diffraction, transport interesting, as this transition is associ- ated with fluctuations and ordering in the measurements, X-ray absorption spectroscopy, electron energy-loss spec- orbital, spin, charge, and lattice degrees troscopy and scanning transmission electron microscopy, the valence of of freedom.[1] Due to this proximity to Pb in the Nd1−xPbxNiO3 structure is confirmed to be 4+, and the behavior various types of ordering, chemical doping of the doped thin films is found to be controlled by a competition between in these materials can lead to many exotic carrier injection and structural distortions, which respectively reduce and phenomena including colossal magnetore- sistance and superconductivity.[2,3] increase the metal-to-insulator transition temperature. This work provides Rare-earth nickelates (RNiO3, where a systematic study of electron doping in NdNiO3, demonstrating that A-site R is a trivalent rare earth cation) are a substitution with Pb is an appropriate method for such doping in perovskite canonical example of a material with a rare-earth nickelate systems. bandwidth-controlled metal-to-insulator transition. In these compounds, the M. Hadjimichael, B. Mundet, C. Domínguez, A. Waelchli, G. De Luca S. McKeown Walker, J.-M. Triscone Catalan Institute of Nanoscience and Nanotechnology (ICN2) Department of Quantum Matter Physics Campus UAB, Bellaterra, Barcelona 08193, Spain University of Geneva J. Spring, S. Jöhr 24 Quai Ernest-Ansermet, Geneva 1211, Switzerland Physik-Institut E-mail: marios.hadjimichael@unige.ch University of Zurich B. Mundet, D. T. L. Alexander Winterthurerstrasse 190, Zurich 8057, Switzerland Electron Spectrometry and Microscopy Laboratory (LSME) S. McKeown Walker Institute of Physics (IPHYS) Laboratory of Advanced Technology (LTA) École Polytechnique Fédérale de Lausanne (EPFL) 24 Quai Ernest-Ansermet, Geneva 1211, Switzerland Lausanne 1015, Switzerland C. Piamonteze Photon Science Division The ORCID identification number(s) for the author(s) of this article Paul Scherrer Institute can be found under https://doi.org/10.1002/aelm.202201182. Villigen 5232, Switzerland © 2023 The Authors. Advanced Electronic Materials published by M. Gibert Wiley-VCH GmbH. This is an open access article under the terms of Solid State Physics Institute the Creative Commons Attribution License, which permits use, distri- TU Wien bution and reproduction in any medium, provided the original work is Wiedner Hauptstr. 8-10/138, Vienna 1040, Austria properly cited. DOI: 10.1002/aelm.202201182 Adv. Electron. Mater. 2023, 9, 2201182 2201182 (1 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de size of the rare-earth cation determines the metal-to-insulator interest in chemically-substituted nickelates after the discovery transition and the paramagnetic-to-antiferromagnetic transi- of superconductivity in Nd1−xSrxNiO2,[28] systematic studies of tion temperatures.[4–6] As the rare-earth cation size is reduced doped perovskite rare-earth nickelate films are still lacking, from La to Lu, the tolerance factor decreases and the RNiO3 with most works focusing on hole-doped films with Ca or structure becomes more distorted, adopting smaller Ni–O–Ni Sr.[29–31,49,50] More importantly, the effects of electron doping bond angles. This reduces the overlap between Ni 3d and O 2p through chemical substitution in RNiO3 thin films have not orbitals (decreasing the bandwidth W) and therefore increases been investigated yet, and are crucial for advancing our under- the temperature of the metal-to-insulator transition.[1] Despite standing of these materials and for the development of elec- their interesting properties, due to the difficulty in making bulk tron-doped infinite-layer nickelate superconductors. materials, the study of rare-earth nickelates and the full exploi- In this work, we electron dope NdNiO3 thin films using tation of their potential were initially limited. The possibility A-site substitution. Given the radioactive nature of thorium and of depositing these systems as high-quality epitaxial thin films the possibility of adopting a valence of 3+ and 4+ in cerium, renewed attention on the nickelates, with studies on the effect our approach instead consists of doping NdNiO3 with Pb. To of strain,[7–12] heterostructuring,[13–19] field effect,[9,20,21] thick- do so, we use off-axis radiofrequency magnetron sputtering to ness,[22–24] and optical excitations.[25–27] fabricate Nd1−xPbxNiO3 thin films (x = 0–0.08) of high struc- The rare-earth nickelate family has recently attracted further tural quality on NdGaO3 substrates. Resistivity measurements interest due to the discovery of superconductivity in infinite- as a function of temperature show that the TMI decreases with layer NdNiO2 thin films hole doped with Sr,[28] a phase achieved increasing Pb content. However, despite doping levels up to by topotactic reduction of the perovskite Nd1−xSrxNiO3 phase. x = 0.08, the metal-to-insulator transition persists and the films After this discovery, films with a range of rare earth cations do not become fully metallic at low temperatures. Using a (R = La, Pr, and Nd) and different dopants (Sr and Ca) were combination of X-ray absorption spectroscopy (XAS) and elec- found to exhibit superconductivity,[29–33] including the function- tron energy-loss spectroscopy (EELS), we show that the films ally-doped Nd6Ni5O12 phase.[34] So far, superconductivity has are electron-doped, with a nominal Ni valence shifting from only been observed in infinite-layer nickelate thin films, but not 3+ toward 2+ with increasing Pb content. Finally, we use aber- in bulk single crystals or powders.[35,36] The study of doped rare- ration-corrected scanning transmission electron microscopy earth nickelates in thin film form is therefore critical to provide (STEM) to map the atomic positions of the A (Nd/Pb) and B deeper understanding into the behavior of both the perovskite (Ni) sites, and extract the magnitude of the structural distor- and the infinite-layer phases. tions in the Nd1−xPbxNiO3 thin films. We find that when we The RNiO3 compounds are negative charge-transfer insula- introduce Pb into the structure the distortions become larger, tors and their behavior is therefore extremely sensitive to elec- signifying a smaller Ni–O–Ni bond angle due to more pro- tronic doping.[1] This doping can be achieved using an electric nounced octahedral rotations. This allows us to conclude that field,[9,21,22,37] by oxygen off-stoichiometry control[38–43] or by Pb has a smaller ionic radius compared to Nd, consistent with chemical substitution of the rare earth cation (with a nominal Pb in a 4+ valence. valence of 3+) with another cation of a different valence.[44–47] Both electron and hole doping have been shown to reduce the metal-to-insulator transition temperature (TMI) in bulk powders 2. Results of NdNiO3 when doped with Ca2+, Sr2+, Th4+, and Ce4+, at dif- ferent rates depending on the dopant used.[44–47] In these doped 2.1. Deposition and Structure of Nd1−xPbxNiO3 Thin Films compounds, the TMI is affected both by charge carrier injection, and by the change in the Ni–O–Ni bond angles that arises due Bulk NdNiO3 is orthorhombic, with a pseudocubic unit cell to the difference in the size of Nd compared to the divalent or with lattice parameter of 3.808 Å and an average Ni–O–Ni bond tetravalent dopants.[47] Carrier injection reduces the TMI for both angle of ≈157°.[51] It displays a metal-to-insulator transition at electron and hole doping, but the effect of A-cation size on the 200 K with a simultaneous appearance of antiferromagnetic TMI is opposite depending on whether the dopant is larger or order.[52–54] Bulk PbNiO3 is hexagonal and its structure can be smaller than Nd. In the case of doping with Ca2+ and Sr2+, both expressed using a rhombohedral subcell, with lattice parameter ions have a larger ionic radius than Nd3+,[48] therefore reducing of 3.886 Å and αr = 87.26°.[55] It is semiconducting, with an anti- the magnitude of the structural distortions and increasing the ferromagnetic transition temperature of ≈205 K.[55] The average Ni–O–Ni bond angle with doping, thus making the octahedral Ni–O–Ni bond angle in PbNiO3 is ≈137°, and in this system Pb rotations less pronounced. The increase in the Ni–O–Ni bond is in a 4+ valence, making it the first observed perovskite struc- angle cooperates with the carrier injection and reduces the TMI ture with Pb in a fully tetravalent state.[56] even further. Conversely, the ionic radii of Th4+ and Ce4+ are The solid solution Nd1−xPbxNiO3 thin films were depos- smaller than Nd3+, increasing the distortions in the structure ited by sequential sub-monolayer deposition from two dif- and reducing the Ni–O–Ni bond angle with doping, making ferent sputtering targets, NdNiO3 and Pb1.1NiO3, as described the octahedral rotations more pronounced. This reduction in in Experimental Section. After a preliminary calibration of the the Ni–O–Ni bond angle increases TMI and therefore competes rates of the two constituent materials, the precise Pb content with the carrier injection, making the suppression of TMI as a of each Nd1−xPbxNiO3 film was determined after deposition function of doping less dramatic. using energy-dispersive X-ray spectroscopy (EDX) performed Despite the comprehensive results on both hole- and elec- on the cross-section of each film (see Experimental Section). To tron-doped NdNiO3 bulk samples,[44–47] as well as the renewed achieve the growth of films with high homogeneity, a capping Adv. Electron. Mater. 2023, 9, 2201182 2201182 (2 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Figure 1. EDX measurements of two Nd1−xPbxNiO3 thin films deposited under identical growth conditions, showing the importance of the SrTiO3 capping layer. a) EDX analysis of a Nd1−xPbxNiO3 film with estimated x = 0.07 (from individual material growth rates), without any capping layer. The analysis shows that most of the Pb has migrated to the surface. A Pb/(Pb+Nd) atomic percentage of 1–2% is found in the bulk of the film, whereas 20% is found on the surface. b) EDX profiles of the film without any capping layer showing the large concentration of Pb on the surface. c) EDX analysis of a Nd1−xPbxNiO3 film with x = 0.07, deposited under identical growth conditions and capped with 5 unit cells of SrTiO3. The images show a uniform Pb distribution across the thickness of the film, with a Pb/(Pb+Nd) atomic percentage of 7%. d) EDX profiles of the film capped with 5 unit cells of SrTiO3, where the uniform Pb distribution is clear. The Nd profiles in (b) and (d) are not shown for clarity, as their behavior is identical for both films. layer has to be deposited immediately after the deposition of the shown in Figure 1d clearly indicates that the Pb profile is con- Pb-doped NdNiO3. Figure 1a is an EDX map of a Nd1−xPbxNiO3 stant throughout the thickness of the film. film deposited on a (110)-oriented NdGaO3 substrate with a free The exact origin of the Pb migration to the surface and the surface. We find that this film (as well as other Nd1−xPbxNiO3 effect of capping is unclear, but it is most likely due to the films with free surfaces) shows Pb migration to the surface, highly volatile nature of Pb and its interaction with oxygen spe- with a Pb/(Pb+Nd) atomic percentage of ≈20% concentrated cies in the chamber during growth.[57,58] Consistent with this in the surface of the sample while a much smaller percentage assumption, in some uncapped films in which we aimed for (≈1–2%) remains in the bulk of the film. The migration of Pb to a large concentration of Pb we have observed weak peaks in the surface of the film is confirmed in Figure 1b, a plot of the the X-ray diffraction scans that could be consistent with PbO EDX line profiles for Ga (present only in the substrate), Pb and or Pb3O4. Based on these results, all Nd1−xPbxNiO3 films that Ni for this uncapped Nd1−xPbxNiO3 film. The Pb profile peaks have been studied in this work have been capped with 5 unit close to the surface of the deposited Nd1−xPbxNiO3 layer, due to cells of SrTiO3 to maintain a uniform Pb content and a high the large concentration of Pb on the surface as compared to the structural quality. rest of the film. Figure 2a is a plot of large-range specular θ –2θ X-ray diffrac- The deposition of the SrTiO3 capping layer (in this case, tion scans for a series of Nd1−xPbxNiO3 thin films (x = 0 up to 5 unit cells of SrTiO3) stops the migration of Pb to the surface, x = 0.08) deposited on (110)-oriented NdGaO3 substrates and rendering the distribution of Pb across the film uniform (see capped with 5 unit cells of SrTiO3. The scans display intense Experimental Section for growth conditions of SrTiO3). An EDX Bragg peaks associated with the perovskite unit cell of the film, map of a capped film (with Nd1−xPbxNiO3 deposited in identical and no parasitic phases. As shown in Figure 2b, the θ –2θ scans conditions as the uncapped case) is shown in Figure 1c, with a are characterized by sharp finite-size oscillations, indicating a uniform Pb concentration of 7% in this case. A plot of the EDX well-defined thickness and high structural quality. Additionally, profiles for Ga, Pb, Ni, and Ti for the capped Nd1−xPbxNiO3 film the quality of the Nd1−xPbxNiO3 thin films is confirmed using Adv. Electron. Mater. 2023, 9, 2201182 2201182 (3 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Figure 2. Structural characterization of Nd1−xPbxNiO3 films deposited on NdGaO3, capped with 5 unit cells of SrTiO3. a) Large-range θ–2θ scans of a series of Nd1−xPbxNiO3 thin films, showing finite-size oscillations and no parasitic phases. The films are ≈22–30 unit cells (8–10 nm) thick and are capped with 5 unit cells of SrTiO3. b) Short-range θ–2θ scans around the orthorhombic 220 peak of NdGaO3. c) Atomic force microscopy image of a film with x = 0.07, characterized by unit-cell-high steps and a flat surface morphology. d) Evolution of the out-of-plane lattice parameter of the Nd1−xPbxNiO3 thin films as a function of Pb content, x. atomic force microscopy (AFM). Figure 2c is an AFM scan for Figure 2d is a plot of the extracted out-of-plane lattice para- a sample with x = 0.07, showing that the film has a flat sur- meter of Nd1−xPbxNiO3 as a function of Pb content, x. The face morphology with a roughness smaller than one unit cell, out-of-plane lattice parameter increases as the Pb content as well as unit-cell-high steps. Similarly, all studied composi- increases, indicating an expansion in the volume of the unit tions exhibit atomically flat surfaces, as seen in Figure S1, Sup- cell, as observed in electron-doped polycrystalline samples[44–47] porting Information. We note that we were unable to deposit as well as bulk samples and thin films doped with F, Li, or any high-quality Nd1−xPbxNiO3 films with x > 0.08, since a oxygen vacancies.[38,60–63] We note that this change in lattice drastic degradation of their structural quality occurs for such parameter is not associated with strain relaxation, since the Pb concentrations. For this reason, only films with Pb content Nd1−xPbxNiO3 films remain strained to the NdGaO3 substrate, up to 8% are studied in this work. as seen in the reciprocal space map around the 103 pseudo- The lattice parameters and the thickness of the Nd1−xPbxNiO3 cubic NdGaO3 peak for a sample with x = 0.05 (Figure S3, Sup- and SrTiO3 layers can be extracted from the profile of the 002 porting Information). peak by simulating the θ –2θ scans using a kinematic scattering model developed in ref. [59], by simulating the scattered inten- sity from a bilayer of NdNiO3 and SrTiO3 on top of a NdGaO3 2.2. Transport Properties of Nd1−xPbxNiO3 Thin Films substrate. Figure S2, Supporting Information shows the simu- lated diffraction patterns and their comparison with the experi- To understand the effect of Pb-doping on the electronic proper- mental measurements, evidencing excellent agreement. No sig- ties of the Nd1−xPbxNiO3 thin films we measure their resistivity nificant differences are found in the simulated diffractograms as a function of temperature. Figure 3a is a plot of the resistivity if a Nd1−xPbxNiO3 solid solution is modeled instead of NdNiO3, versus temperature for a series of Nd1−xPbxNiO3 thin films with most likely due to the small Pb content. x ranging from 0 to 0.08, with the cooling and heating branch Adv. Electron. Mater. 2023, 9, 2201182 2201182 (4 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Figure 3. Transport properties of Nd1−xPbxNiO3 films deposited on NdGaO3. a) Resistivity (in logarithmic scale) as a function of temperature for a series of Nd1−xPbxNiO3 thin films, showing the upturn in temperature due to the metal-to-insulator transition. The solid and dashed lines indicate resistivity values measured upon cooling and heating respectively. b) Metal-to-insulator transition temperatures (TMI) as a function of doping x for the studied films. The dashed line is a linear fit to the TMI data points (TMI = mx + c), with gradient m = (−1350 ± 110) K and intercept c = 158 K, fixed to the TMI of the undoped film. of the resistivity indicated by solid and dashed lines respec-  dTMI ( x , R0 ( x ))   ∂TMI ( x , R0 ( x ))   ∂TMI ( x , R0 ( x ))  dR0 ( x ) tively. All studied films show a characteristic jump due to the   =   +   (1) dx ∂x R0 =R0 [ Nd ] ∂R0 x =0 dx metal-to-insulator transition.[64] This can be seen more clearly in a smaller-scale plot of the resistivity in Figure S4, Supporting Here, dTMI/dx is the observed dependence of TMI on doping Information, where the upturn due to the metal-to-insulator (carrier injection and change in A-cation size), ∂TMI/∂x is transition still occurs even for the highest Pb concentrations. the dependence of TMI on carrier injection only, ∂TMI/∂R0 Additionally, as clearly seen in the behavior of the films with is the dependence of TMI on the A-cation size and dR0/dx is the lowest Pb concentrations, the behavior of the resistivity the change in the ionic radius of the A-cation with respect to is hysteretic due to the first-order nature of the transition in Nd as a function of doping. The authors have found that the NdNiO3.[4,65] The hysteretic behavior of the resistivity persists measured suppression rate of the TMI, dTMI/dx, is drastically as the films are doped with Pb, but the width of the hysteresis different for Sr (−4190 K), Ca (−3180 K), Th (−1029 K), and Ce is reduced with doping. At the moment, it is unclear whether (−220 K). Additionally, they found that the extracted suppres- this effect could be attributed to behavior that is intrinsic to sion rate of the TMI when cation size effects are removed is the material, or associated with a change in sample quality (∂TMI/∂x)h ≈ −3200 K for holes and (∂TMI/∂x)e ≈ −1200 K for with doping,[54] and to clarify the reason for this behavior fur- electrons, reflecting the large electron–hole asymmetry in ther investigation is required. Finally, our measurements show this system. that as the Pb concentration increases, two dominant effects We have fitted the data of Figure 3b using a straight line, are observed in the transport properties of the films: i) the fixing the intercept at x = 0 to be equal to 158 K, the TMI of the TMI decreases from ≈150 K for pure NdNiO3 (x = 0) to 57 K undoped film. We find that the observed gradient, dTMI/dx, is for Nd0.92Pb0.08NiO3 (x = 0.08), and ii) the room-temperature equal to (−1350 ± 110) K, comparable to the values measured resistivity increases. by García-Muñoz and co-workers for electron-doped bulk sam- The effect of Pb-doping on the TMI is summarized in ples.[47] Additionally, the linear fit of Figure 3b shows that the Figure 3b, a plot of the temperature at which the onset of the TMI would be completely suppressed at a Pb content of 12%, metal-to-insulator transition occurs for each sample during much higher than the equivalent value for hole-doped bulk cooling, defined as the minimum of the resistivity on the samples and thin films. As mentioned previously, such a high cooldown branch. The plot shows that despite Pb-doping levels Pb content could not be reached due to a drastic degradation in up to 8%, the metal-to-insulator transition persists, and the sample quality for x > 0.08. samples do not become completely metallic up to these doping For our Nd1−xPbxNiO3 thin films, expressing the change of levels. This is in contrast to hole-doped (A-site substitution the TMI as a function of known quantities similarly to Equa- using Ca and Sr) NdNiO3 thin films and bulk samples, where tion (1) is more complicated than the case of bulk samples, a complete suppression of the metal-to-insulator transition since the magnitude of the epitaxial strain changes as we occurs at around x = 0.05–0.07.[29,44–47,50] dope with Pb. By increasing the Pb content we move from In ref. [47], García-Muñoz and co-workers were able to isolate the smaller pseudocubic unit cell of NdNiO3[51] toward the the effect of the carrier injection on the TMI of NdNiO3 from larger pseudocubic unit cell of PbNiO3,[55] therefore making the change in the cation size with doping using the following the strain less tensile and so reducing the TMI.[9,10,66] Never- equation: theless, to confirm that the observed reduction of the TMI is Adv. Electron. Mater. 2023, 9, 2201182 2201182 (5 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de primarily due to a true electronic doping effect and is not just 2.3. Spectroscopic Evidence for Electron Doping in an effect of the change of strain with Pb doping, we calcu- Nd1−xPbxNiO3 late the expected change of the TMI as a function of the latter. Assuming that Vegard’s law is valid in the Nd1−xPbxNiO3 To shed light on the effect of Pb doping on the valence of Ni, we system, we expect the pseudocubic in-plane lattice parameter use a combination of XAS and STEM-EELS. Figure 4a shows of bulk Nd1−xPbxNiO3 to have the following doping depend- X-ray absorption spectra around the Ni L2,3 edges for the series ence: a = 3.808 + (3.886 − 3.808)x. When bulk Nd1−xPbxNiO3 of capped Nd1−xPbxNiO3 films with x ranging from 0 to 0.07. is strained on NdGaO3, the in-plane percentage strain is equal The top spectrum is a reference spectrum for Ni2+, measured to ε = (3.86 − a)/a × 100. The dependence of the in-plane per- using a NiO powder. In this spectrum, the main peaks at the centage strain on doping, x, can then be calculated for small L2 and L3 edges correspond to transitions from Ni 2p1/2 to Ni values of x: ∂ε/∂x = −2.07. 3d and from Ni 2p3/2 to Ni 3d orbitals, and occur at ≈868 and To calculate the dependence of the TMI on the change of 851 eV respectively. For Ni in a nominal 2+ valence, the pre- strain, we deposited undoped NdNiO3 on LaAlO3 (in-plane dominant transitions are labeled as 2p63d8 → 2p53d9.[67] The lattice parameter equal to 3.79 Å, ε = −0.47% ), NdGaO3 (in- bottom spectrum in Figure 4a is a reference spectrum from a plane lattice parameter equal to 3.86 Å, ε = 1.37% ) and SrTiO3 pure NdNiO3 film with a free surface, deposited on NdGaO3. In (in-plane lattice parameter equal to 3.905 Å, ε = 2.55% ). The this system, the nominal Ni valence is 3+, with features in the TMI of each film, determined by measuring their resistivity spectra predominantly due to a mixture of 2p63d7 → 2p53d8 and versus temperature, is 90, 152, and 200 K respectively. This 2p 6 3d 8 L → 2p 5 3d 9 L transitions, where L denotes an oxygen allows us to calculate the strain-dependence of the TMI, finding 2p hole.[68–71] This configuration is associated with clear spec- ∂TMI/∂ε = −36 K. troscopic features in the Ni L2,3 edges, which can be observed The expected dependence of the TMI on the change of the clearly in this reference Ni3+ spectrum.[70] percentage of strain induced by doping (without accounting for The spectra for the SrTiO3-capped Nd1−xPbxNiO3 thin films any electronic effects) can therefore be calculated as follows as a function of doping in Figure 4a demonstrate that, starting from a nominally Ni3+ valence for x = 0, a redistribution of the dTMI ( x , ε )  ∂TMI  ∂ε Ni L2,3 intensity and a shift of the center of mass of the L2 and = = −74.5 K. (2) dx  ∂ε  ∂x L3 peaks toward lower energies occurs as Pb is added to the films. This shift toward lower energies is associated with the This rate is much smaller than the experimentally observed appearance of a Ni2+ component, as demonstrated in the refer- gradient, dTMI/dx = −1350 K, showing that the reduction of the ence spectrum of NiO. The Ni2+ features become more intense TMI cannot be explained only by a change in strain and that the with increasing Pb content (x), signifying a gradual increase electronic doping also has to be taken into account. in the Ni2+ percentage (as seen more clearly in Figure 4b, where The second observed effect in the plot of the resistivity as the spectra around the Ni L3 edge are plotted). The increase a function of temperature is the increase of the room-tem- in the Ni2+ percentage with increasing Pb content confirms perature resistivity with increasing Pb content. This likely electron doping in the system.[63] occurs due to disorder introduced by the A-site substitution, Figure 4c is a plot of the electron energy-loss spectra around as observed in doped bulk samples[44–47] and thin films.[29,49,50] the Ni L2,3 edges for an uncapped NdNiO3 (x = 0, blue) and a Additionally, defects like oxygen vacancies can also contribute Nd0.93Pb0.07NiO3 film capped with SrTiO3 (x = 0.07, red). The to the increase of the room temperature resistivity,[38,42,43] spectra were acquired from the middle of each film using and these defects can also appear in our Nd1−xPbxNiO3 thin exactly the same acquisition conditions for both samples, films. To decouple the effect of Pb-doping through A-site by preparing both TEM specimens on the same Cu grid (see substitution and oxygen vacancies that could develop during Figure S6, Supporting Information and Experimental Section). growth, we have annealed a Nd0.98Pb0.02NiO3 (x = 0.02) and The substitution of Pb for Nd in the NdNiO3 structure leads a Nd0.93Pb0.07NiO3 (x = 0.07) thin film in flowing oxygen for to a change in the Ni L2,3 spectral shapes that is similar to the various steps as described in Figure S5, Supporting Infor- change observed in the X-ray absorption spectra of Figure 4a. mation. The θ –2θ scans for both films show a shift of the In contrast, the Nd M4,5 spectral shapes do not change with Bragg peak to higher angles, and therefore a decrease in the Pb doping, as shown in Figure 4d, indicating that there is no out-of-plane lattice parameter with annealing (a percentage change in the valence of Nd. The spectroscopy studies summa- decrease of 0.05% and 0.4% for the x = 0.02 and the x = 0.07 rized in Figure 4a–c therefore demonstrate that there is a clear films respectively). This reduction in the volume of the unit shift from a nominal Ni 3+ valence toward 2+ as the samples cell indicates that oxygen vacancies in the as-grown film are are doped with Pb, providing clear evidence for a 4+ Pb valence being filled with each annealing step. Nevertheless, the resis- in this system. tivity of the films as a function of temperature indicates that Having confirmed this change from Ni3+ valence toward a even though the room-temperature resistivity decreases with 2+ Ni character, we now turn to verifying the homogeneity of each annealing step, the TMI does not change significantly, the Ni valence in the Pb-doped film. Figure 4e is a plot of the consistent with the effect of oxygen vacancies studied in bulk EELS measurements around the Ni L2,3 edges for the x = 0.07 NdNiO3 samples[38] and thin films.[42,43] This allows us to film, at different positions through the thickness of the film conclude that the observed decrease of the TMI is predomi- as defined in Figure S6, Supporting Information. We find that nantly due to doping with Pb and not the effect of defects like the peaks do not change in shape or position as we move from oxygen vacancies. the bottom interface with NdGaO3 to the middle of the film Adv. Electron. Mater. 2023, 9, 2201182 2201182 (6 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Figure 4. Spectroscopic evidence of increasing content of Ni2+ with Pb doping. a) X-ray absorption spectra around the Ni L2,3 edges for a series of Nd1−xPbxNiO3 thin films as a function of Pb concentration (shifted vertically for clarity). The films are deposited on NdGaO3 and are capped with 5 unit cells of SrTiO3. The bottom spectrum is a reference spectrum for Ni3+ (uncapped NdNiO3 thin film deposited on NdGaO3), with the vertical line at 852.4 eV indicating the position of the main Ni3+ peak at the L3 edge. The top spectrum is a reference spectrum for Ni2+ (NiO powder), with the vertical line at 851.0 eV indicating the position of the main Ni2+ L3 peak. With Pb doping, the spectra demonstrate the appearance of an additional Ni2+ character and its gradual increase in intensity as the Pb content is increased from x = 0 to x = 0.07. b) Short-range spectra around the Ni L3 edge showing the details of the increase in the Ni2+ percentage. c) Electron energy-loss spectra around the Ni L2,3 edges for a NdNiO3 (x = 0, blue) and a Nd0.93Pb0.07NiO3 film (x = 0.07, red) show a similar trend as the XAS measurements as function of Pb doping. d) EELS measurements around the Nd M4,5 edges, showing no changes with Pb doping. e) Spatially resolved electron energy-loss spectra around the Ni L2,3 edges for the x = 0.07 film, shifted vertically for clarity. From top to bottom: top interface with SrTiO3, middle part of film, and bottom interface with NdGaO3. The position of each region labeled in (e) is indicated in Figure S6, Supporting Information. The vertical dashed lines mark the approximate position of the peak. The plots in (b)–(e) are normalized by the maximum intensity of each spectrum for easier comparison. and toward the top interface with the SrTiO3 capping layer (see between electron-doping due to A-site chemical substitution Figure S6c,d, Supporting Information for an overlay of the Ni versus doping due to oxygen vacancies or hydrogen/lithium L2,3 and Nd M4,5 spectra). This indicates that the Ni valence incorporation. is homogeneous over the whole film, due to its uniform stoi- First of all, in the case of doping with hydrogen or lithium, chiometry, consistent with the cross-sectional EDX data in the H+ and Li+ ions are incorporated as interstitials, binding Figure 1c. with oxygen from existing NiO6 octahedra without replacing These complementary spectroscopy techniques indi- existing atoms in the structure.[62,74] Additionally, oxygen vacan- cate that there is a clear shift from a nominal Ni 3+ valence cies modify the NiO6 octahedral configuration and alter the toward 2+ as the samples are doped with Pb. Similar valence crystal-field splitting accordingly.[63] In contrast, A-site substi- shifts have been observed in perovskite nickelate thin films tution in the RNiO3 structure does not alter the coordination with strain- or annealing-induced oxygen vacancies,[72,73] as environment of Ni, and for this reason, a direct comparison of well as hydrogen- and lithium-doped films.[39,62,74] Therefore the structural effects only taking into account the ionic radius we would like to discuss next the fundamental differences of the various dopants cannot be made. Adv. Electron. Mater. 2023, 9, 2201182 2201182 (7 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de Second, the appearance of Ni2+ due to doping with toward 180° with increasing Pb content x, making the octahe- hydrogen,[39,74] lithium,[62] or strain- or annealing-induced dral rotations in the Nd1−xPbxNiO3 structure less pronounced. oxygen vacancies[72,73] would lead to localization.[63] As dis- In contrast, the ionic radius of Pb4+ is smaller than Nd3+,[48] cussed in the previous section, this type of doping should not and its incorporation in the structure would cause the films to change the TMI, but will increase the resistivity of the film in the adopt smaller Ni–O–Ni bond angles due to stronger octahedral metallic phase, and above a specific threshold of dopants the rotations.[55] films will become insulating. On the other hand, the appear- To qualitatively estimate the change in the Ni–O–Ni bond ance of Ni2+ due to doping using A-site substitution will reduce angles between samples with different Pb content and to con- the TMI, as observed in Figure 3. This is expected for any type clusively determine the valence of Pb in the Nd1−xPbxNiO3 struc- of Mott insulator when we move away from half-filling by intro- ture, we use STEM. Figure 5a shows a high-resolution high- ducing holes or electrons into the system,[1] and is consistent angle annular dark-field (HAADF) image of a Nd0.93Pb0.07NiO3 with the behavior observed in the studied Nd1−xPbxNiO3 films. (x = 0.07) thin film deposited on NdGaO3, capped with 5 unit cells of SrTiO3, showing sharp and coherent interfaces and no visible dislocations or other lattice defects. Since the intensity 2.4. Scanning Transmission Electron Microscopy Studies of the in the HAADF image is roughly proportional to the square of Structural Distortions the atomic number of each atomic column, the brightest atoms in the image are the A-site cations Nd/Pb, and the second This electron doping through A-site substitution is expected brightest atoms are Ni. In this measurement configuration, the to decrease the TMI at a rate slower than hole-doped films.[47] oxygen atoms are not visible. Additionally, the suppression rate of the TMI will also depend We can extract the positions of the A-site cations as described on the change of the Ni–O–Ni bond angle of Nd1−xPbxNiO3 as in Methods, and use them as a proxy for the degree of distortion a function of doping, which in turn depends on the valence in the RNiO3 structure compared to the ideal cubic perovskite lat- of Pb. The ionic radius of Pb2+ is larger than Nd3+,[48] and tice. When imaging in the plane defined by the [001]o and [110]o would therefore lead to a shift of the Ni–O–Ni bond angle directions, we observe a projection of the antipolar ­displacements Figure 5. Visualization of increased structural distortions with scanning transmission electron microscopy (STEM). a) High-resolution HAADF image of a SrTiO3/Nd0.93Pb0.07NiO3/NdGaO3 (x = 0.07) heterostructure showing sharp and coherent interfaces (indicated by the horizontal dashed lines) and no visible dislocations or other lattice defects. The orthorhombic directions are defined by the arrows. b) Left: magnified region of the Nd0.93Pb0.07NiO3 thin film showing the projection of the antipolar cation displacements, consistent with the schematic of the NdNiO3 crystal structure sketched on the right. The magnitude of the displacements is defined as ΔZ and is proportional to the degree of structural distortion in RNiO3 (a larger ΔZ corresponds to a smaller Ni–O–Ni bond angle). Scale bar: 2 nm. c) Plot of ΔZ as a function of atomic plane position for a pure NdNiO3 film and the heterostructure in (a), showing that the structural distortions in Nd0.93Pb0.07NiO3 are larger than in pure NdNiO3 (the Ni–O–Ni bond angle is thus smaller in doped films). Note that, unlike in Figure 4c,d, here the NdNiO3 film was capped with 5 unit cells of SrTiO3. The vertical dashed line at 0 indicates the interface of the films with the NdGaO3 substrate. The red dashed line at 24 marks the interface between Nd0.93Pb0.07NiO3 and the SrTiO3 capping layer, and the blue dotted line at 33 marks the interface between NdNiO3 and the SrTiO3 capping layer. Adv. Electron. Mater. 2023, 9, 2201182 2201182 (8 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de of the rare-earth cations (as shown in Figure 5b[51]), with a pro- layer in situ, immediately after their growth. The TMI of the jected amplitude ΔZ that depends on the tolerance factor, and Nd1−xPbxNiO3 films is progressively reduced as a function of in turn the Ni–O–Ni bond angle. Due to the trilinear coupling increased Pb content; however, it is not completely suppressed between octahedral tilts and the amplitude of the A-site displace- for the maximum doping of x = 0.08. ments,[75,76] a smaller Ni–O–Ni bond angle leads to a larger value Spectroscopic measurements show that a shift from a nom- of ΔZ.[18,19] We take advantage of this coupling and use ΔZ as inally Ni3+ toward a Ni2+ valence occurs, providing direct evi- an indirect relative measurement of the Ni–O–Ni bond angles, dence for electron doping. Finally, we find that Pb doping leads rather than directly measuring the oxygen positions from to a lower Ni–O–Ni bond angle due to stronger octahedral rota- annular bright field images, which may be significantly affected tions, and therefore a smaller value of the tolerance factor with by beam aberrations and sample mistilts.[77] respect to pure NdNiO3 films, consistent with a smaller ionic We have mapped the atomic positions of the A-site cations radius for the Pb dopants compared to Nd. Thus, our work dem- and have calculated ΔZ for this x = 0.07 heterostructure, as onstrates that, as the Pb content in Nd1−xPbxNiO3 thin films is well as an undoped NdNiO3 thin film deposited on NdGaO3. increased, there is a competing effect between charge injection Figure 5c is a plot of ΔZ as a function of atomic plane posi- that lowers the transition temperature, and a reduction of the tion for these two films. First, we see that the amplitude of the Ni–O–Ni bond angles that increases the transition temperature. antipolar distortion in the center of each material is different. Our results show that A-site substitution with Pb is an appro- Figure 5c shows that the amplitude of the antipolar distortion priate method for electron doping in perovskite rare-earth nick- in the pure NdNiO3 film is ≈29 pm, consistent with the ΔZ elate systems and provide a systematic study of such doping value of bulk NdNiO3.[51] In contrast, the antipolar distortion in in epitaxial NdNiO3 films. We believe that these results will the Pb-doped sample is larger, reaching values of ≈35 pm in the contribute to further understanding of the behavior of doped center of the film. Due to the trilinear coupling between the perovskite rare-earth nickelates. Furthermore, they will hope- Ni–O–Ni bond angle and the amplitude of the antipolar distor- fully inspire future studies to investigate the possibility of tion, ΔZ, we can conclude that the Ni–O–Ni bond angle in the inducing electron-doped superconductivity in these systems Pb-doped film is smaller than in pure NdNiO3 and the octahe- through topotactic reduction to an infinite-layer phase. dral rotations become more pronounced with Pb doping. Note: After acceptance of this paper, we have learned of Additionally, we observe that the plot of ΔZ for the pure another study on electron-doped NdNiO3 thin films, in which NdNiO3 film shows an initial dip at the interface with NdGaO3 doping with Ce creates an antiferromagnetic metal phase for followed by an increase to a constant value, whereas we do not a specific range of doping, whereas above this doping range observe this drop for the Pb-doped film. A similar decrease in the films become completely metallic.[78] We note that such an the magnitude of the A-site antipolar motions has previously antiferromagnetic metallic phase has not been observed in our been seen at the interface between LaFeO3 and a NdGaO3 films, and therefore the differences between doping with Ce4+ substrate,[18] where the first 4 unit cells of LaFeO3 display a and Pb4+ merit further study. reduced octahedral tilt angle compared to the following layers. This reduction was also shown to occur simultaneously with a local expansion in the out-of-plane lattice parameter,[18] and a 4. Experimental Section similar expansion occurs at the interface for both our films (see Sample Growth: Multiple Nd1−xPbxNiO3 solid solution films were Figure S7, Supporting Information). As shown in Figure S7, deposited on (110)-oriented orthorhombic NdGaO3 substrates using Supporting Information, the out-of-plane expansion is not cor- off-axis radiofrequency magnetron sputtering. The films were deposited related with changes in the in-plane lattice parameter, with by growing sub-monolayer quantities from Pb1.1NiO3 and NdNiO3 the latter remaining constant within experimental errors. This ceramic targets and repeating multiple times. A Pb1.1NiO3 target with effect is most likely associated to the structural mismatch 10% excess lead was used to compensate for the volatility of Pb. between the layers, but its precise origin is unclear. Neverthe- Deposition from the Pb1.1NiO3 target was carried out in a 0.18 Torr atmosphere with an oxygen:argon ratio of 10:35, and RF power of 50 W. less, despite the behavior at the interface of each film and the The corresponding parameters for NdNiO3 were 0.14 Torr, 10:35, and substrate, the conclusion of the STEM measurements is clear: 50 W. During growth, the temperature of the sample was kept constant the larger amplitude of the antipolar distortion is consistent at 500 °C, measured by a thermocouple inside the heating block. For with a smaller Ni–O–Ni bond angle due to more pronounced reasons discussed in the main text, the studied films were capped with octahedral rotations, and therefore a smaller effective ionic 5 unit cells of SrTiO3 deposited in situ, immediately after the growth radius for the A-site cation in Nd0.93Pb0.07NiO3 compared to of the solid solution, at the same temperature. The growth parameters for SrTiO3 were a pressure of 0.18 Torr, oxygen:argon ratio of 20:28, NdNiO3. These measurements further validate the conclusion and RF power of 60 W. Several series of samples were deposited, with that Pb is in a 4+ valence. compositions varying from x = 0 to x = 0.08, by fixing the sub-monolayer growth time from the NdNiO3 target and varying the growth time from the Pb1.1NiO3 target. In cases where solid solutions of two materials were deposited by 3. Conclusions sequential sub-monolayer deposition from two different sputtering targets, the composition of the resulting film was controlled by the We have deposited Nd1−xPbxNiO3 thin films with high struc- precise calibration of the rates of the constituent materials.[79,80] In the tural quality and Pb doping levels up to 8%. To maintain the study crystalline films of pure PbNiO3 were not able to be grown to structural quality of the films, as well as a uniform Pb con- calibrate their growth rate, and therefore a preliminary calibration of the tent across their thickness, the Nd1−xPbxNiO3 layers need to PbNiO3 growth rate was performed by depositing an amorphous film have Pb content x ≤ 0.08 and have to be capped with a SrTiO3 and measuring its thickness with X-ray reflectivity. The Nd1−xPbxNiO3 Adv. Electron. Mater. 2023, 9, 2201182 2201182 (9 of 12) © 2023 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH 2199160x, 2023, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aelm.202201182 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advelectronicmat.de films were then deposited assuming this growth rate was valid for publication.[84] The two TEM lamellae used for the fine-structure analysis, crystalline films as well, and the precise Pb content of each film was prepared with the focused ion beam, were transferred to the same Cu determined after deposition using EDX. In the main text, the value grid so that all EELS spectra were acquired in the same session without measured by EDX was always used when referring to the Pb content changing the acquisition conditions. x. However, due to the low concentration of Pb in the x = 0.01 film, its Pb content was estimated by plotting the measured x versus Pb1.1NiO3 sputtering time for all studied films, and interpolating between x = 0 and x = 0.02. Finally, it was noted that the A/B cation ratio ((Pb+Nd)/ Supporting Information Ni) measured using EDX did not change significantly as a function of Pb doping. Supporting Information is available from the Wiley Online Library or Structural Characterization: Structural characterization of the thin from the author. films was performed using an Panalytical X’Pert Pro diffractometer with a Cu Kα source. Atomic force microscopy was used to characterize the surfaces of the thin films, using a Dimension Multimode microscope in tapping mode. Acknowledgements Transport Measurements: Low-temperature resistivity measurements The authors gratefully acknowledge Marco Lopes for technical support, were performed using a probe slowly dipped in liquid helium, as well Jennifer Fowlie and Stefano Gariglio for useful discussions during the as an Oxford Instruments Teslatron low-temperature system. For both initial stages of this work. This work was supported by the Swiss National types of measurements, a Keithley 224 or a Keithley 622X current source Science Foundation through Division II grant nos. 200020_179155 (M.H., were used to supply a current of 100 µA and a voltage was measured B.M., C.D., and J.-M.T.), PP00P2_170564 (M.G., G.D.L., J.S., and S.J.), the using a Keithley 2000 Multimeter or a Keithley 2182A Nanovoltmeter National Centers of Competence in Research MARVEL scheme under respectively. The measurements were performed in four-point van der grant no. 51NF40-182892 (M.G. and S.J.), and the European Research Pauw geometry, with the four corners of the sample contacted using Council under the European Union’s Seventh Framework Programme Al wire-bonding on sputtered Pt square pads. The resistivity values for (FP7/2007-2013)/ERC grant agreement no. 319286 (Q-MAC) (M.H., the samples were obtained using the van der Pauw formula, with the B.M., C.D., and J.-M.T.). The authors acknowledge the Paul Scherrer resistance values for the two orthogonal configurations obtained during Institute and the X-Treme (X07MA) beamline staff for support during separate heating and cooling runs. the X-ray absorption spectroscopy experiment, and the CIME at EPFL for X-ray Absorption Spectroscopy: X-ray absorption spectroscopy was access to electron microscopy facilities. used to determine the valence of Ni in the Nd1−xPbxNiO3 thin films. The measurements were performed at the EPFL/PSI X-Treme beamline at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland.[81] The measurements were carried out in total electron yield mode at room Conflict of Interest temperature. The X-ray absorption spectra were the average of linear horizontal and vertical polarization taken in normal incidence. The authors declare no conflict of interest. Scanning Transmission Electron Microscopy: Scanning transmission electron microscopy measurements were used to obtain information on the local structure of the films, using a double aberration-corrected FEI (Thermo Fisher Scientific) Titan Themis 60-300 electron microscope Data Availability Statement equipped with a double aberration-corrector (CEOS) and a high The data that support the findings of this study are openly available in brightness field emission gun with a monochromator. High quality the Yareta repository at https://doi.org/10/grrhhn. high-angle annular dark-field (HAADF) images were acquired using a convergence semi-angle of 20 mrad, a 300 kV high tension, and low beam currents (30–40 pA) to prevent electron beam damage and contamination. To correct the linear and non-linear scanning distortions, Keywords a series of 24 iterative 90° rotated HAADF frames were acquired from each analyzed area, and afterward aligned using the SmartAlign plug-in doping, metal–insulator transitions, NdNiO3, octahedral rotations, for DigitalMicrograph.[82] The coordinates associated to the central PbNiO3, rare-earth nickelates, X-ray absorption spectroscopy position of each Pb/Nd column were estimated using a 2D Gaussian fitting procedure using the Atomap script.[83] Received: October 27, 2022 Energy-Dispersive X-ray Spectroscopy: EDX maps were acquired on the Revised: January 27, 2023 same microscope, using a four quadrant Super-X windowless silicon Published online: March 16, 2023 drift detector system and beam currents of ≈100–250 pA. The maps were recorded and analyzed using Thermo Fisher Scientific Velox software. 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