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Many electronic and electrochemical devices rely on the exchange of light elements such as hydrogen and oxygen with the environment. Understanding and tailoring the device functionality requires accurate information about the concentration and chemical bonding of such species inside a solid, which is particularly difficult if several species are exchanged. In LaNiO3 thin films in situ transport experiments reveal a re‐entrant metal–insulator transition upon hydrogen exposure. The origin of this unusual behavior can be understood by combining information about the stoichiometry and chemical bonding of hydrogen and oxygen as determined by neutron reflectometry and x‐ray absorption spectroscopy, respectively. In addition to the metallic parent phase, an insulating phase with composition LaNiO2.65 and a re‐entrant metallic phase with composition LaNiO2.15(OH)0.5 are identified. They can be inter‐converted by redox reactions in different external environments. The methodology employed offers new insights into the mechanisms underlying the influence of hydrogen in functional devices. [ABSTRACT FROM AUTHOR]

The recent discovery of nickelate superconductivity represents an important step toward understanding the four‐decade mastery of unconventional high‐temperature superconductivity. However, the synthesis of the infinite‐layer nickelate superconductors shows great challenges. Particularly, surface capping layers are usually unitized to facilitate the sample synthesis. This leads to an important question whether nickelate superconductors with d9 configuration and ultralow valence of Ni1+ are in metastable state and whether nickelate superconductivity can be robust? In this work, a series of redox cycling experiments are performed across the phase transition between perovskite Nd0.8Sr0.2NiO3 and infinite‐layer Nd0.8Sr0.2NiO2. The infinite‐layer Nd0.8Sr0.2NiO2 is quite robust in the redox environment and can survive the cycling experiments with unchanged crystallographic quality. However, as the cycling number goes on, the perovskite Nd0.8Sr0.2NiO3 shows structural degradation, suggesting stability of nickelate superconductivity is not restricted by the ultralow valence of Ni1+, but by the quality of its perovskite precursor. The observed robustness of infinite‐layer Nd0.8Sr0.2NiO2 up to ten redox cycles further indicates that if an ideal high‐quality perovskite precursor can be obtained, infinite‐layer nickelate superconductivity can be very stable and sustainable under environmental conditions. This work provides important implications for potential device applications for nickelate superconductors. [ABSTRACT FROM AUTHOR]

Recently, superconductivity at high temperatures is observed in bulk La3Ni2O7−δ under high pressure. However, the attainment of high‐purity La3Ni2O7−δ single crystals remains a formidable challenge. Here, the crystal structure and physical properties of single crystals of Sr‐doped La3Ni2O7 synthesized at high pressure (20 GPa) and high temperature (1400 °C) are reported. Through single crystal X‐ray diffraction, it is shown that high‐pressure‐synthesized paramagnetic Sr‐doped La3Ni2O7 crystallizes in an orthorhombic structure with Ni─O─Ni bond angles of 173.4(2)° out‐of‐plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni2+/Ni3+ in Sr‐doped La3Ni2O7, aligning them with those of "La3Ni2O7.05", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr‐doped La3Ni2O7 exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr‐doped La3Ni2O7 transits back from a metal‐like behavior to an insulator. The insulator–metal–insulator trend under high pressure dramatically differs from the behavior of the parent compound La3Ni2O7−δ, despite their similar behavior in the low‐pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates. [ABSTRACT FROM AUTHOR]

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two‐step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process. [ABSTRACT FROM AUTHOR]

Recently, superconductivity with Tc ≈ 80 K was discovered in La3Ni2O7 under extreme hydrostatic pressure (>14 GPa). For practical applications, we needed to stabilize this state at ambient pressure. It was proposed that this could be accomplished by substituting La with Ba. To put this hypothesis to the test, we used the state-of-the-art atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique to synthesize (La1−xBax)3Ni2O7 films, varying x and the distribution of La (lanthanum) and Ba (barium). Regrettably, none of the compositions we explored could be stabilized epitaxially; the targeted compounds decomposed immediately into a mixture of other phases. So, this path to high-temperature superconductivity in nickelates at ambient pressure does not seem promising. [ABSTRACT FROM AUTHOR]

Abstract Recently, superconductivity at high temperatures is observed in bulk La3Ni2O7−δ under high pressure. However, the attainment of high‐purity La3Ni2O7−δ single crystals remains a formidable challenge. Here, the crystal structure and physical properties of single crystals of Sr‐doped La3Ni2O7 synthesized at high pressure (20 GPa) and high temperature (1400 °C) are reported. Through single crystal X‐ray diffraction, it is shown that high‐pressure‐synthesized paramagnetic Sr‐doped La3Ni2O7 crystallizes in an orthorhombic structure with Ni─O─Ni bond angles of 173.4(2)° out‐of‐plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni2+/Ni3+ in Sr‐doped La3Ni2O7, aligning them with those of “La3Ni2O7.05”, thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr‐doped La3Ni2O7 exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr‐doped La3Ni2O7 transits back from a metal‐like behavior to an insulator. The insulator–metal–insulator trend under high pressure dramatically differs from the behavior of the parent compound La3Ni2O7−δ, despite their similar behavior in the low‐pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

Analog neuromorphic circuits use a range of volatile and non-volatile memristive effects to mimic the functionalities of neurons and synapses. Creating devices with combined effects is important for reducing the footprint and power consumption of neuromorphic circuits. This work presents an epitaxial SmNiO3/BaTiO3 electrical device that displays non-volatile memristive switching to either allow or block access to a volatile threshold switching regime. This behavior arises from coupling the BaTiO3 ferroelectric polarization to SmNiO3 metal-insulator transition; the polarization in the BaTiO3 layer that is in contact with the SmNiO3 layer modifies the device resistance continuously in a controllable, non-volatile manner. Additionally, the polarization state varies the threshold voltage at which the Joule-heating-driven insulator-to-metal phase transition occurs in the nickelate, which results in a negative differential resistance curve and produces a sharp, volatile threshold switch. Reliable current oscillations with stable frequencies, large amplitude, and a relatively low driving voltage are demonstrated when the device is placed in a Pearson-Ansonlike circuit. [ABSTRACT FROM AUTHOR]

Layered nickelates, Ln 2 NiO 4+δ , are promising electrode materials for many electrochemical applications, including solid oxide fuel cells and electrolysis cells. Although Ln 2 NiO 4+δ has been extensively modified by various doping strategies to tune its functional properties, the partial substitution of Ln3+ with Ba2+ remains among the least studied routes. At the same time, such substitution is found to be favorable when Ln 2 NiO 4+δ materials are used for protonic ceramic electrochemical cells based on Ba-containing proton-conducting electrolytes (i.e., BaCeO 3 , BaZrO 3 , Ba(Ce,Zr)O 3). In this work, which is the third part of a systematic study, Pr 2–x Ba x NiO 4+δ materials are used as electrodes for a proton ceramic fuel cell and as oxygen permeable membranes. The oxygen permeation experiments confirm that the compositions with x = 0.2 and 0.3 prevail over x = 0 and 0.1 in terms of their oxygen-ionic conductivity, while the electrochemical cell characterizations confirm the high electrochemical activity of the Pr 1.8 Ba 0.2 NiO 4+δ electrode in both fuel-cell- and electrolysis-cell modes. Our research thus confirms that a Ba-doping strategy is highly promising for designing new Ln 2 NiO 4+δ -based phases, simultaneously offering good chemical and thermal compatibility with state-of-the-art proton-conducting electrolytes and high electrochemical performance. [Display omitted] • Pr 2–x Ba x NiO 4+δ (PBNx, 0 ≤ x ≤ 0.3) materials were electrochemically characterized. • PBNx were used as components of protonic ceramic cells and oxygen permeable membranes. • The obtained results indicate that Ba-doping improves the ionic transport of PBNx. • PBN0.2 and BPN0.3 can be considered as promising functional materials. [ABSTRACT FROM AUTHOR]

Analog neuromorphic circuits use a range of volatile and non-volatile memristive effects to mimic the functionalities of neurons and synapses. Creating devices with combined effects is important for reducing the footprint and power consumption of neuromorphic circuits. This work presents an epitaxial SmNiO3/BaTiO3 electrical device that displays non-volatile memristive switching to either allow or block access to a volatile threshold switching regime. This behavior arises from coupling the BaTiO3 ferroelectric polarization to SmNiO3 metal–insulator transition; the polarization in the BaTiO3 layer that is in contact with the SmNiO3 layer modifies the device resistance continuously in a controllable, non-volatile manner. Additionally, the polarization state varies the threshold voltage at which the Joule-heating-driven insulator-to-metal phase transition occurs in the nickelate, which results in a negative differential resistance curve and produces a sharp, volatile threshold switch. Reliable current oscillations with stable frequencies, large amplitude, and a relatively low driving voltage are demonstrated when the device is placed in a Pearson–Anson-like circuit.

The oxygen evolution reaction (OER) is a crucial process in electrochemical water splitting, a promising technology to renewably yield hydrogen gas from water. Designing and developing earth‐abundant, efficient, and stable OER electrocatalysts to replace the most widely used but scarce RuO2 and IrO2 are thus of critical interest. Recently, ABO3‐structured perovskite oxides, especially rare‐earth nickelates, are extensively studied for their potential use as OER electrocatalysts. In particular, the epitaxial synthesis of complex oxide thin films allows flexible and precise control over the materials so that their structure–stability–property relationships can be established. Using nickelate thin films as model systems, this review illustrates how epitaxial design allows researchers to test different hypotheses and proposed descriptors, as well as formulate new design principles. Following a brief introduction to the background of OER mechanisms, proposed activity descriptors, and synthesis methods, various epitaxial design strategies are surveyed including strain tuning, composition control, surface termination/orientation selection, defect engineering, and interface design. These have led to precise control over the atomic structures and electronic properties of nickelates which in turn determine their electrochemical performance. Finally, the remaining challenges and perspectives toward a deeper understanding and use of complex oxides as OER catalysts are discussed. [ABSTRACT FROM AUTHOR]

We investigate [Formula: see text]/[Formula: see text] superlattices in which we observe a full electron transfer at the interface from Ir to Ni, triggering a massive structural and electronic reconstruction. Through experimental characterization and first-principles calculations, we determine that a large crystal field splitting from the distorted interfacial [Formula: see text] octahedra surprisingly dominates over the spin-orbit coupling and together with the Hund's coupling results in the high-spin (S = 1) configurations on both the Ir and Ni sites. This demonstrates the power of interfacial charge transfer in coupling lattice, charge, orbital, and spin degrees of freedom, opening fresh avenues of investigation of quantum states in oxide superlattices.

In rare earth nickelates, the metal‐to‐insulator transition observed as a function of temperature can be described using an electronic and a structural order parameter. The electronic one is linked to the electronic disproportionation observed below the transition temperature and the structural one characterizes the breathing mode that develops in the low temperature phase. Here SmNiO3/NdNiO3 superlattices are grown with insulating LaAlO3 spacer layers to study the unusual coupling of metal‐to‐ insulator transitions observed at SmNiO3/NdNiO3 interfaces and determine the role of the two order parameters. Temperature‐dependent transport measurements reveal that a single unit cell of LaAlO3 inserted between SmNiO3 and NdNiO3 leads to a complete decoupling of the metal‐to‐insulator transitions suggesting that the order parameter controlling the coupling is the electronic one. [ABSTRACT FROM AUTHOR]

We present a thermodynamical model based on Volmer's nucleation theory adapted to the case of the perovskite nickelate SmNiO 3 crystallization (SNO) from an amorphous (aSNO) thin film. This amorphous phase is synthesized via reactive magnetron sputtering and then subsequently annealed in air at temperatures between 725 and 925 K to crystallize it. This model allows to predict the theoretical nucleation rate of the crystallized perovskite phase according to the annealing temperature, the type of the nucleation (homogeneous and heterogeneous mechanisms) and to estimate some physical and thermodynamical data related to this transformation. A theoretical evaluation of the nucleation rate shows that the optimum temperature for crystallization is close to 800 K for which the nucleation rate can reach 1021 m-3.s-1 if considering both homogeneous and heterogeneous nucleation mechanisms. As the annealing temperature moves away from 800 K, the theoretical nucleation rate drops drastically, as observed experimentally by X-ray Diffraction (XRD) when we annealed 200 nm thick aSNO films. The good agreement between the presented model and the experimental crystallization results allows us to numerically evaluate some physical parameters not yet reported to date in the literature for SNO perovskites such as the surface energy between the amorphous and the crystallized SNO phases, the strain energy when the crystallization occurs and the enthalpy associated with crystallization. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract In rare earth nickelates, the metal‐to‐insulator transition observed as a function of temperature can be described using an electronic and a structural order parameter. The electronic one is linked to the electronic disproportionation observed below the transition temperature and the structural one characterizes the breathing mode that develops in the low temperature phase. Here SmNiO3/NdNiO3 superlattices are grown with insulating LaAlO3 spacer layers to study the unusual coupling of metal‐to‐ insulator transitions observed at SmNiO3/NdNiO3 interfaces and determine the role of the two order parameters. Temperature‐dependent transport measurements reveal that a single unit cell of LaAlO3 inserted between SmNiO3 and NdNiO3 leads to a complete decoupling of the metal‐to‐insulator transitions suggesting that the order parameter controlling the coupling is the electronic one.

In the past decade, the rapid development of modern heterointerface growth and characterization techniques has stimulated great effort to research and design the extraordinary physical properties of transition metal heterostructure composite materials. Here, new physics originates from the rearrangement of orbital, charge, spin, and lattice and the resulting rebalancing of their mutual interactions. In this paper, recent experimental and theoretical progress in the design, preparation, characterization, and physical property measurement of LaNiO3‐based heterostructure composites and the infinite‐layer Ni1 + nickelate superconducting thin films on (001) SrTiO3 substrate is reviewed, mainly by the methods of various X‐ray spectroscopy measurements, scanning transmission electron microscopy, and magneto‐transport measurements. In these materials, the electronic structure and orbital occupation around the Fermi level are modified, enabling nickelate‐based composite materials to exhibit new states of matter and physical phenomena, which are absent in the bulk constituents. Their confined structures, superconductivity, orbital polarization, charge transfer, electronic structures, magnetic properties, and X‐ray spectroscopic analysis, are therefore highlighted, aiming at understanding unconventional superconducting mechanisms and designing new high‐Tc superconducting low‐dimensional materials for device applications. [ABSTRACT FROM AUTHOR]

Layered Nickelates have gained intensive attention as potential high-temperature superconductors, showing similarities and subtle differences to well-known Cuprates. This study introduces a modelling framework to analyze the tunability of electronic structures by focusing on effective orbitals and additional Fermi pockets, mimicking doping or external pressure qualitatively. It investigates the role of the $3d_{z^2}$ orbital in interlayer hybridization, which leads to the formation of a second pocket in the Fermi surface. The resulting effective model also predicts specific charge and spin susceptibility in the form of Lindhard susceptibility at wave vector $\mathbf{q_{0}} = (\pi, \pi)$, which can be tuned by doping or pressure. These results provide valuable insights into tunable orbital contributions and their influence on potential ordering and electronic instabilities in Layered Nickelates., (© 2025 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Superconducting states onsetting at moderately high temperatures have been observed in epitaxially stabilized RE NiO 2 -based thin films. However, recently, it has also been reported that superconductivity at high temperatures is observed in bulk La 3 Ni 2 O 7-δ at high pressure, opening further possibilities for study. Here we report the reduction profile of La 3 Ni 2 O 7 in a stream of 5% H 2 /Ar gas and the isolation of the metastable intermediate phase La 3 Ni 2 O 6.45 , which is based on Ni 2+ . Although this reduced phase does not superconduct at ambient or high pressures, it offers insights into the Ni-327 system and encourages future study of nickelates as a function of oxygen content.

Among the existing material family of the correlated oxides, the rare earth nickelates (Re NiO 3) exhibit broadly adjustable metal to insulator transition (MIT) properties that enables correlated electronic applications, such as thermistors, thermochromics, and logical devices. Nevertheless, how to accurately control the critical temperature (T MIT) of Re NiO 3 via the co-occupation of the rare-earth elements is yet worthy to be further explored. Herein, we demonstrate the non-linearity in adjusting the T MIT of Re NiO 3 towards lower temperatures via introducing Pr co-occupation within Re NiO 3 (e.g., Pr x Nd 1-x NiO 3 and Pr x Sm 1-x NiO 3) as synthesized by KCl molten-salt assisted high oxygen pressure reaction approach. Although the T MIT is effectively reduced via Pr substitution, it does not strictly follow a linear relationship, in particular, when there is large difference in the ionic radius of the co-occupation rare-earth elements. Furthermore, the most significant deviation in T MIT from the expected linear relationship appears at an equal co-occupation ratio of the two different rare-earth elements, while the abruption in the variation of resistivity across T MIT is also reduced. The present work highlights the importance to use adjacent rare-earth elements with co-occupation ratio away from 1:1 for achieving more linear adjustment in designing the metal to insulator transition properties for Re NiO 3. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the electronic and superconducting properties in nickelate compounds of NiO 2 through perturbative density functional theory (P-DFT) and extended multi-band Bardeen–Cooper–Schrieffer (BCS) theory. We consider four different cases of study: (i) the crystalline structure composed by one layer of NiO 2 , with a central single atom of Nd, Yb or Pr, which is subjected to mechanical compression and tension forces (pressure), (ii) the crystalline structure composed by two layers of NiO 2 with central atom pairs of Nd–Sr, Yb–Sr or Pr–Sr, with level doping x = 0.5 , (iii) the crystalline structure composed by three layers of NiO 2 with a setup of Nd–Nd(Sr)–Nd, Yb–Yb(Sr)–Yb or Pr–Pr(Sr)–Pr (Homogeneous and doping sample), with the level doping x = 0.3 and x = 0.75 and finally (iv) through multi-band theory we study the behavior in a single-layer Nd 1 - x Sr x NiO 2 when x = 0.5 through gaps Δ χ , p , with χ (number of band or orbitals), only with the presence of inter-band coupling V χ χ ′ . As results, for (i), (ii) and (iii) we present the electronic measures of the electronic band structures e-DOS, and density of phonons (mechanical and/or optic) p-DOS. In (i), we show that under pressure there are a redistribution of the band structures for e-DOS and p-DOS, allowing the destruction of Van Hove near Fermi level E F ; thus, in (ii) and (iii) we show the presence of a single flat band when the doping level is x = 0.5 , and for (iv), the inter-band coupling between orbitals plays a key role in the construction of a superconducting phase in nickelate compounds. [ABSTRACT FROM AUTHOR]

Resistive switching effects offer new opportunities in the field of conventional memories as well as in the booming area of neuromorphic computing. Here the authors demonstrate memristive switching effects produced by a redox‐driven oxygen exchange in tunnel junctions based on NdNiO3, a strongly correlated electron system characterized by the presence of a metal‐to‐insulator transition (MIT). Strikingly, a strong interplay exists between the MIT and the redox mechanism, which on the one hand modifies the MIT itself, and on the other hand radically affects the tunnel resistance switching and the resistance states' lifetime. That results in a very unique temperature behavior and endows the junctions with multiple degrees of freedom. The obtained results bring up fundamental questions on the interplay between electronic correlations and the creation and mobility of oxygen vacancies in nickelates, opening a new avenue toward mimicking neuromorphic functions by exploiting the electric‐field control of correlated states. [ABSTRACT FROM AUTHOR]

Epitaxial LaNiO3 (LNO) thin films prepared from the sols modified with polyethyleneimine (PEI) were grown on single-crystal LaAlO3, (LaAlO3)0.3(SrAlTaO6)0.7, and SrTiO3 substrates, respectively, using a simple polymer assisted deposition (PAD). The epitaxial structure, surface morphologies and transport of the LNO films were studied by X-ray diffraction (θ/2θ symmetric scan, ω-scan, and in-plane φ-scan), the field emission scanning electron microscopy, and a standard dc four-probe method. It is found that, compared with that of LNO bulk, the c-axis parameter of the LNO film increases under compressive strain and decreases under tensile strain. All the LNO films exhibit metal properties in the temperature-dependent resistivity. The resistivity of the LNO films shows an increasing trend with the lattice mismatch strain changing from compressive to tensile. It is suggested that the oxygen vacancy compensated by more Ni2+ changed from Ni3+ in the film increases with the strain changing from compressive to tensile, which results in the increase of the resistivity. [ABSTRACT FROM AUTHOR]

In this paper, an attempt was made to synthesize LaNiO3, CeNiO3, and BaNiO3, and Ce3+ and Ba2+ co-substituted LaNiO3. These samples were further subjected to various material characterization techniques in order to evaluate their physio-chemical properties. Scanning electron microscope (SEM) images showed large chunks of aggregated nanoparticles with minute voids. The EDX-derived atomic composition deviated from the nominal composition suggesting the occurrence of multiple phases. In addition, transmission electron microscope (TEM) images revealed that the samples exhibit uneven spherical shape with a high degree of aggregation. The Fourier transformed-infrared (FT-IR) spectra of the synthesized samples show vibrations of the BO6 octahedral indicating the presence of Ni–O bonds. In addition, metal-carboxyl vibrations were identified from the peaks at 1400 and 860 cm−1. Optical diffuse reflectance spectra (DRS) showed certain peaks originating from the O2− (2p) → Ni2+ (3d) charge transfer. The X-ray powder diffraction (XRPD) analysis revealed the existence of multiple phases for the samples CeNiO3, BaNiO3, and La(Ce, Ba)NiO3. Moreover, La(Ce, Ba)NiO3 contained four phases showing that the co-substitution of Ba2+ and Ce3+ into LaNiO3 may require more sophisticated methodologies. The sample BaNiO3 showed maximum weight loss, due to the existence of carbonate phase. The dielectric properties decreased with increasing frequency, while the ac electrical conductivity enhanced with increasing frequencies obeying the Maxwell–Wagner two-layer model in accordance with Koop's phenomenological theory. [ABSTRACT FROM AUTHOR]

Superconductivity has entered the nickel age marked by enormous experimental and theoretical efforts. Notwithstanding, synthesizing nickelate superconductors remains extremely challenging, not least due to incomplete oxygen reduction and topotactic hydrogen. Here, we present density-functional theory calculations for nickelate superconductors with additional topotactic hydrogen or oxygen, namely La 1 − x Sr x NiO 2 H δ and LaNiO 2 + δ . We identify a phonon mode as a possible indication for topotactic hydrogen and discuss the charge redistribution patterns around oxygen and hydrogen impurities. [ABSTRACT FROM AUTHOR]

Lanthanide nickelate Ln2NiO4+δ (Ln = La, Pr, or Nd) based mixed ionic and electronic conducting (MIEC) materials have drawn significant attention as an alternative oxygen electrode for solid oxide cells (SOCs). These nickelates show very high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values and hence exhibit good electrocatalytic activity. Earlier reported results show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) leads to an enhancement in the transport and electrochemical properties of the material. Herein, we perform the substitution at A-site with Sr, i.e., La2−xSrxNi0.8Co0.2O4+δ, in order to further investigate the structural, physicochemical, and electrochemical properties. The structural characterization of the synthesized powders reveals a decrease in the lattice parameters as well as lattice volume with increasing Sr content. Furthermore, a decrease in the oxygen over stoichiometry is also observed with Sr substitution. The electrochemical measurements are performed with the symmetrical half-cells using impedance spectroscopy in the 700–900 °C temperature range. The total polarization resistance of the cell is increased with Sr substitution. The electrode reaction mechanism is also studied by recording the impedance spectra under different oxygen partial pressures. Finally, the kinetic parameters are investigated by analyzing the impedance spectra under polarization. A decrease in exchange current density (i0) is observed with increasing Sr content. [ABSTRACT FROM AUTHOR]

The discovery of superconductivity in hole-doped infinite-layer NdNiO2 — a transition metal (TM) oxide that is both isostructural and isoelectronic to cuprate superconductors—has lead to renewed enthusiasm in the hope of understanding the origin of unconventional superconductivity. Here, we investigate the electron-removal states in infinite-layered Ni1+ oxide, NdNiO2, which mimics hole doping, with the state-of-the-art many-body multireference quantum chemistry methods. From the analysis of the many-body wavefunction we find that the hole-doped d8 ground state of NdNiO2 is very different from the d8 ground state in isostructural cuprate analog CaCuO2, although the parent d9 ground states are for the most part identical. We show that the doped hole in NdNiO2 mainly localizes on the Ni 3dx2−y2 orbital to form a closed-shell singlet, and this singlet configuration contributes to ∼40% of the wavefunction. In contrast, in CaCuO2 the Zhang-Rice singlet configurations contribute to ∼65% of the wavefunction. With the help of the quantum information concept of entanglement entropy, we quantify the different types of electronic correlations in the nickelate and cuprate compounds, and find that the dynamic radial-type correlations within the Ni d manifold are persistent in hole-doped NdNiO2. As a result, the d8 multiplet effects are stronger and the additional hole foot-print is more three-dimensional in NdNiO2. Our analysis shows that the most commonly used three-band Hubbard model employed to express the doped scenario in cuprates represents ∼90% of the d8 wavefunction for CaCuO2, but such a model grossly approximates the d8 wavefunction for NdNiO2 as it only stands for ∼60% of the wavefunction.

Unlike their cuprate counterparts, the undoped nickelates are weak insulators without long-range antiferromagnetic order. Identifying the origin of this insulating behavior, found on both sides of the superconducting dome, is potentially a crucial step in the development of a coherent understanding of nickelate superconductivity. In this work, we study the normal-state resistivity of infinite-layer nickelates using high magnetic fields to suppress the superconductivity and examine the impact of disorder and doping on its overall temperature (T) dependence. In superconducting samples, the resistivity of Nd- and La-based nickelates continues to exhibit weakly insulating behavior with a magnitude and functional form similar to that found in underdoped electron-doped cuprates. We find a systematic evolution of the insulating behavior as a function of nominal hole doping across different rare-earth families, suggesting a pivotal role for strong electron interactions, and uncover a correlation between the suppression of the resistivity upturn and the robustness of the superconductivity. By contrast, we find very little correlation between the level of disorder and the magnitude and onset temperature of the resistivity upturn. Combining these experimental observations with previous Hall effect measurements on these two nickelate families, we consider various possible origins for this correlated insulator behavior and its evolution across their respective phase diagrams.

The discovery of superconductivity in thin films (∼10 nm) of infinite-layer hole-doped NdNiO2 has invigorated the field of high temperature superconductivity research, reviving the debate over contrasting views that nickelates that are isostructural with cuprates are either 1) sisters of the high temperature superconductors, or 2) that differences between nickel and copper at equal band filling should be the focus of attention. Each viewpoint has its merits, and each has its limitations, suggesting that such a simple picture must be superseded by a more holistic comparison of the two classes. Several recent studies have begun this generalization, raising a number of questions without suggesting any consensus. In this paper, we organize the findings of the electronic structures of n-layered NiO2 materials (n = 1 to ∞) to outline (ir)regularities and to make comparisons with cuprates, with the hope that important directions of future research will emerge.

We study the magnetic and spectral properties of a single-band Hubbard model for the infinite-layer nickelate compound LaNiO2. As spatial correlations turn out to be the key ingredient for understanding its physics, we use two complementary extensions of the dynamical mean-field theory to take them into account: the cellular dynamical mean-field theory and the dynamical vertex approximation. Additionally to the systematic analysis of the doping dependence of the non-Curie-Weiss behavior of the uniform magnetic susceptibility, we provide insight into its relation to the formation of a pseudogap regime by the calculation of the one-particle spectral function and the magnetic correlation length. The latter is of the order of a few lattice spacings when the pseudogap opens, indicating a strong-coupling pseudogap formation in analogy to cuprates.

This article summarizes recent work on the many-body (beyond density functional theory) electronic structure of layered rare-earth nickelates, both in the context of the materials themselves and in comparison to the high-temperature superconducting (high-Tc) layered copper-oxide compounds. It aims to outline the current state of our understanding of layered nickelates and to show how the analysis of these fascinating materials can shed light on fundamental questions in modern electronic structure theory. A prime focus is determining how the interacting physics defined over a wide energy range can be estimated and “downfolded” into a low energy theory that would describe the relevant degrees of freedom on the ∼0.5 eV scale and that could be solved to determine superconducting and spin and charge density wave phase boundaries, temperature dependent resistivities, and dynamical susceptibilities.

The discovery of superconductivity in infinite-layer nickelates has attracted much attention due to their association to the high-Tc cuprates. Cuprate superconductivity was first demonstrated in bulk samples and subsequently in thin films. In the nickelates, however, the situation has been reversed: although surging as a bulk phenomenon, nickelate superconductivity has only been reported in thin films so far. At the same time, the specifics of infinite-layer nickelates yield distinct interface and surface effects that determine their bulk vs thin-film behavior. In this paper, we provide an overview on these important aspects.

The emerging Ni-based superconducting oxide thin films are rather intriguing to the entire condensed matter physics. Here, we report some brief experimental results on transport measurements for a 14-nm-thick superconducting Nd0.8Sr0.2NiO2/SrTiO3 thin-film heterostructure with an onset transition temperature of ~ 9.5 K. Photoluminescence measurements reveal that there is negligible oxygen vacancy creation in the SrTiO3 substrate during thin-film deposition and post chemical reduction for the Nd0.8Sr0.2NiO2/SrTiO3 heterostructure. It was found that the critical current density of the Nd0.8Sr0.2NiO2/SrTiO3 thin-film heterostructure is relatively small, ~ 4 × 103 A·cm−2. Although the surface steps of SrTiO3 substrates lead to an anisotropy for in-plane resistivity, the superconducting transition temperatures are almost the same. The out-of-plane magnetotransport measurements yield an upper critical field of ~ 11.4 T and an estimated in-plane Ginzburg–Landau coherence length of ~ 5.4 nm. High-field magnetotransport measurements up to 50 T reveal anisotropic critical fields at 1.8 K for three different measurement geometries and a complicated Hall effect. An electric field applied via the SrTiO3 substrate slightly varies the superconducting transition temperature. These experimental results could be useful for this rapidly developing field. 摘要: 新兴镍基超导氧化物薄膜在整个凝聚态物理领域引起了很大关注。在此论文中, 我们通过电输运测量研究了一个 14 nm 厚的超导 Nd0.8Sr0.2NiO2/SrTiO3 异质结样品。这个样品的起始超导转变温度约为 9.5 K。光致荧光谱测量发现: 此异质结中 SrTiO3基片内部的氧空位含量极少。这说明纳米薄膜的沉积及后续化学氢化过程并没有在 SrTiO3 单晶基片中引入显著的氧空位。我们还发现这种超导异质结的临界电流密度相对很低, 约为 4 × 103 A·cm−2。虽然 SrTiO3 单晶基片的表面台阶诱发了面内电阻率的各向异性, 但超导转变温度却几乎不受影响。在薄膜表面的垂直方向施加磁场的电输运测量揭示出此超导样品的上临界场约为11.4 T, 对应面内Ginzburg-Landau相干长度约为 5.4 nm。高达 50 T 的强磁场电输运测量研究发现: 1.8 K 下, 完全破坏超导态使得样品达到正常态电阻率的临界磁场具有各向异性。 [ABSTRACT FROM AUTHOR]

Next-generation devices will rely on exotic functional properties not found in traditional systems. One class of materials of particular interest for applications are those possessing metal-to-insulator transitions (MITs). In this work, we probe the relationship between variations in the growth process, subsequent variations in cation stoichiometry, and the MIT in NdNiO3 thin films. Slight variations in the growth conditions, in particular the laser fluence, during pulsed-laser deposition growth of NdNiO3 produces films that are both single-phase and coherently strained to a range of substrates despite possessing as much as 15% Nd-excess. Subsequent study of the temperature-dependence of the electronic transport reveals dramatic changes in both the onset and magnitude of the resistivity change at the MIT with increasing cation nonstoichiometry giving rise to a decrease (and ultimately a suppression) of the transition and the magnitude of the resistivity change. From there, the electronic transport of nearly ideal NdNiO3 thin films are studied as a function of epitaxial strain, thickness, and orientation. Overall, transitioning from tensile to compressive strain results in a systematic reduction of the onset and magnitude of the resistivity change across the MIT, thinner films are found to possess sharper MITs with larger changes in the resistivity at the transition, and (001)-oriented films exhibit sharper and larger MITs as compared to (110)- and (111)-oriented films as a result of highly anisotropic in-plane transport in the latter.

Superconductivity has entered the nickel age marked by enormous experimental and theoretical efforts. Notwithstanding, synthesizing nickelate superconductors remains extremely challenging, not least due to incomplete oxygen reduction and topotactic hydrogen. Here, we present density-functional theory calculations for nickelate superconductors with additional topotactic hydrogen or oxygen, namely La1−xSrxNiO2Hδ and LaNiO2+δ. We identify a phonon mode as a possible indication for topotactic hydrogen and discuss the charge redistribution patterns around oxygen and hydrogen impurities.

Applying a three-band model and the random phase approximation, we theoretically study the spin excitations in nickelate superconductors, which have been newly discovered. The spin excitations were found to be incommensurate in the low energy region. The spin resonance phenomenon emerged as the excitation energy increased. The intensity can be maximized at the incommensurate or commensurate momentum, depending on the out-of-plane momentum. The spin excitations reverted to incommensurate at higher energies. We also discuss the similarities and differences in the spin excitations of nickelate and cuprate superconductors. Our predicted results can be later validated in inelastic neutron scattering experiments. [ABSTRACT FROM AUTHOR]

Abstract The perovskite oxide LaNiO3 is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO3 films (La1−x SrxNiO3) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at x = 0.5 comparable to the state‐of‐the‐art catalyst Ba0.5Sr0.5Co0.8Fe0.2O3−δ. By combining X‐ray photoemission and X‐ray absorption spectroscopies with density functional theory, it is shown that an upward energy shift of the O 2p band relative to the Fermi level occurs with increasing x in La1−xSrxNiO3. This alloying step strengthens Ni 3d–O 2p hybridization and decreases the charge transfer energy, which in turn accounts for the enhanced OER activity.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO3 (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott-Hubbard electron-electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO3 can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence. [ABSTRACT FROM AUTHOR]

We investigate SrIrO3/LaNiO3 superlattices in which we observe a full electron transfer at the interface from Ir to Ni, triggering a massive structural and electronic reconstruction. Through experimental characterization and first-principles calculations, we determine that a large crystal field splitting from the distorted interfacial IrO6 octahedra surprisingly dominates over the spinorbit coupling and together with the Hund's coupling results in the high-spin (5 = 1) configurations on both the Ir and Ni sites. This demonstrates the power of interfacial charge transfer in coupling lattice, charge, orbital, and spin degrees of freedom, opening fresh avenues of investigation of quantum states in oxide superlattices. [ABSTRACT FROM AUTHOR]

Karl Alex Mueller's intuition was that the nature of mobile charge carriers in manganates and nickelates should be like those in cuprates, which he believed were Jahn-Teller polarons, so he expected them to also host high-temperature superconductivity (HTS). Following his suggestions and persuasion, in the past, we performed several experiments that included atomic-layer-by-layer synthesis of novel, metastable manganates and nickelates. Despite our efforts and several USO ("Unidentified Superconducting Object") sightings, we failed to find any reproducible superconductivity and dropped the project fifteen years ago. Our results are presented here, for the first time — timely, since HTS was very recently discovered in nickelates (under pressure). So, whether his opinions were on the mark or not, Alex did guess it right — again. [ABSTRACT FROM AUTHOR]