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Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO3 by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO3 film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO3 single crystalline film. The obtained current density of 1038 μΑ cm-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.

In strongly correlated materials, lattice, charge, spin and orbital degrees of freedom interact with each other, leading to emergent physical properties such as superconductivity, colossal magnetic resistance and metal-insulator transition. Quasi-2D square planar nickelates, Rn+1NinO2n+2 (R=rare earth, n=2, 3...), are of significant interest and long sought for cuprate analogue due to the 3d9 electronic configuration of Ni+, the same as the active ion Cu2+ in the high-Tc superconducting cuprates. The field has attracted intense attention since 2019 due to the discovery of superconductivity in thin films of Nd0.8Sr0.2NiO2, although no superconductivity has been reported in bulk polycrystalline powders. Herein, we review the synthesis of polycrystalline powders of quasi-2D square planar nickelates through topotactic reduction of parent compounds that are synthesized via solid state reaction, precursor method, high pressure floating zone method and high-pressure flux method. We emphasize single crystal preparation using the high-pressure floating zone techniques. We discuss their crystal structure and physical properties including resistivity, magnetic susceptibility and heat capacity. We highlight the cuprate-like physics, including charge/spin stripes and large orbital polarization, identified in single crystals of R4Ni3O8 (R=La and Pr) combining synchrotron X-ray/neutron single crystal diffraction and density functional theory calculations. Furthermore, the challenges and possible research directions of this fast-moving field in the future are briefly discussed., 22 pages and 9 figures

In the past decade, intensive research on proton-conducting oxide materials has provided a basis for the development of intermediate-temperature protonic ceramic electrochemical cells, which constitute a real alternative to conventional cells based on oxygen-conducting electrolytes. To achieve both high efficiency and excellent performance, not only electrolytes but also electrode materials should be carefully selected considering their functional properties. Compared to the traditional ABO3 perovskite electrode materials, Ln2NiO4+δ with a layered structure has unique advantages (high chemical stability, mechanical compatibility, improved oxygen transport, and hydration ability), and thus is now becoming a hot topic in this field, offering both scientific and practical interests. However, a comprehensive and in-depth review is still lacking in the literature to date. Accordingly, this work presents a comprehensive overview of the prospects of layered nickelates (Ln2NiO4+δ, where Ln = La, Nd, and Pr) as one of the most attractive oxygen (steam) electrode materials for protonic ceramic electrochemical cells. In particular, the crystalline features, defect structure, stability, chemical properties, and mechanical compatibility of this class of materials, contributing to their transport functionality, are discussed with the primary emphasis on revealing the relationship between the composition of the materials and their properties. The presented systematic results reveal the main strategies regarding the utilisation of Ln2NiO4+δ-based electrodes and existing gaps related to fundamental and applied research aspects.

A-site deficient Ruddlesden–Popper phases now attract a lot of attention as promising materials for intermediate temperature solid oxide fuel cells' cathodes and oxygen separation membranes. However, controversial information on the A-site deficiency effect on structural, transport and electrochemical properties of these materials apparently requires further studies of such systems. In this work, structural and transport features of (Nd2-xCax)0.975NiO4+δ system were studied. (Nd2-xCax)0.975NiO4+δ (x = 0–0.4) samples were synthesized via a solution-assisted solid state reaction method and characterized by X-ray diffraction. Oxygen transport was studied by the temperature programmed isotope exchange of oxygen with C18O2 in the flow reactor. A high oxygen mobility of materials (D* ~ 10−7 cm2/s at 700 °C) is provided by the cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. As typical for Ruddlesden–Popper phases, doping with Ca leads to increase of total conductivity due to holes' formation and decrease of oxygen mobility due to hampering cooperative mechanism, however, this effect is less pronounced compared to A-site stoichiometric Nd nickelates which can be attributed to oxygen vacancies formation and their participation in cooperative migration as well as to other features of A-site deficiency. High mixed ionic-electronic conductivity of materials makes them promising in using as solid oxide fuel cells’ cathodes and oxygen separation membranes.

One of the main directions for the attainment of high-performance solid oxide electrochemical cells, including those based on proton-conducting electrolytes, consists in the rational engineering of functional materials. Herein, we examine the use of Nd2–xBaxNiO4+δ (NBNx, 0 ≤ x ≤ 0.4) materials as potential oxygen electrodes for protonic ceramic fuel cells (PCFCs) based on conventional Ba(Ce,Zr)O3-electrolytes. A Ba-doping strategy is proposed for Nd2NiO4+δ for reducing chemical interdiffusion of barium-ions from electrolyte to electrode during long-term operation. It is experimentally confirmed that a moderate Ba-content is beneficial in terms of reducing the quantity of impurity phases formed at the NBNx/Ba(Ce,Zr)O3 interface during deliberately-imposed extreme temperature conditions (1250 °C for 20 h). In addition, materials with x = 0.1 and 0.2 exhibit the highest electrical conductivity with no adverse effects on their thermomechanical properties. Thus, when characterising symmetrical cells in wet air conditions, NBN0.1 and NBN0.2 electrodes demonstrates lower polarisation resistances (down to 1.7 Ω cm2 at 700 °C) compared with NBN0, NBN0.3 and NBN0.4 electrodes. Based on these findings, the Ba-doping of nickelates can be recommended as a promising modification for the design of electrodes having good performance and chemical compatibility.

Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates have been characterized as prospective oxygen electrode materials for solid electrolyte cells. XRD studies showed that these oxides retain tetragonal K2NiF4-type structure in air until at least 900°C. Average thermal expansion coefficients of Nd0.8Sr1.2Ni0.8M0.2O4-δ calculated from the structural data are in the range 14.5-15.8 ppm/K. TGA studies revealed that these nickelates are oxygen-deficient in air at temperature above 700°C but tends to oxygen stoichiometry or minor excess on cooling. Incorporation of cobalt or iron into nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency and electrical conductivity. Electrochemical impedance spectroscopy studies of symmetrical cells showed that porous Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ electrolyte exhibit quite similar performance, with lowest values of polarization resistance (0.8 Ω⸱cm2 at 800°C) observed for M = Ni. The polarization resistance can be further decreased (down to 0.04 Ω⸱cm2 at 800°C for M = Ni) by surface modification with PrOx. published

Perovskite rare-earth nickelates (RNiO3) have attracted much attention because of their exotic physical properties and rich potential applications. Here, we report systematic tuning of the electronic structures of RNiO3 (R = Nd, Sm, Gd, and Lu) by isovalent A-site substitution. By integrating RNiO3 thin films with Nb-doped SrTiO3 (NSTO), p-n heterojunction photovoltaic cells have been prepared and their performance has been investigated. The open-circuit voltage increases monotonically with decreasing A-site cation radius. This change results in a downward shift of the Fermi level and induces an increase in the built-in potential at the RNiO3/NSTO heterojunction, with LuNiO3/NSTO showing the largest open-circuit voltage. At the same time, the short-circuit current initially increases upon changing the A-site element from Nd to Sm. However, the larger bandgaps of GdNiO3 and LuNiO3 reduce light absorption which in turn induces a decrease in the short-circuit current. A power conversion efficiency of 1.13% has been achieved by inserting an ultrathin insulating SrTiO3 layer at the SmNiO3/NSTO interface. Our study illustrates how changing the A-site cation is an effective strategy for tuning photovoltaic performance and sheds light on which A-site element is the best for photovoltaic applications, which can significantly increase the applicability of nickelates in optoelectric devices. MOE (Min. of Education, S’pore) Accepted version

There is growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components and competing quantum phases is needed to grab the key physics of high critical temperature ( T c ) superconductivity in layered cuprates. While it is known that defect self-organization controls T c , the mechanism remains an open issue. Here we focus on the theoretical prediction of the multiband electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials O i atomic wires in the spacer layers in HgBa 2 CuO 4 &plusmn, &delta, La 2 CuO 4 &plusmn, and La 2 NiO 4 &plusmn, by means of self-consistent Linear Muffin-Tin Orbital (LMTO) calculations. The electronic structure of a first phase of ordered O i atomic wires and of a second glassy phase made of disordered O i impurities have been studied through supercell calculations. We show the common features of the influence of O i wires in the electronic structure in three types of materials. The ordering of O i into wires leads to a separation of the electronic states between the O i ensemble and the rest of the bulk. The wire formation first produces quantum confined localized states near the wire, which coexist with, Second, delocalized states in the Fermi surface (FS) of doped cuprates. A new scenario emerges for high T c superconductivity, where Kitaev wires with Majorana bound states are proximity-coupled to a 2D d-wave superconductor.

Rare-earth nickelates ${R}^{3+}{\mathrm{Ni}}^{3+}{\mathrm{O}}_{3}$ ($R=\mathrm{Lu}\text{\ensuremath{-}}\mathrm{Pr}$, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer $R\mathrm{Ni}{\mathrm{O}}_{2}$ superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in $R\mathrm{Ni}{\mathrm{O}}_{3}$ materials. Using first-principles calculations based on density functional theory (DFT) we study the effect of hole and electron doping in a prototypical nickelate $\mathrm{SmNi}{\mathrm{O}}_{3}$. We perform calculations without Hubbard-like $U$ potential on Ni $3d$ levels but with a meta--generalized gradient approximation better amending self-interaction errors. We find that at low doping, polarons form with intermediate localized states in the band gap resulting in a semiconducting behavior. At larger doping, the intermediate states spread more and more in the band gap until they merge either with the valence (hole doping) or the conduction (electron doping) band, ultimately resulting in a metallic state at 25% of $R$ cation substitution. These results are reminiscent of experimental data available in the literature and demonstrate that DFT simulations without any empirical parameter are qualified for studying doping effects in correlated oxides and exploring the mechanisms underlying the superconducting phase of rare-earth nickelates.

The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high quality nickelate films using molecular beam epitaxy (MBE). We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction (RHEED) indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction (XRD) data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in superconducting nickelate family., Comment: 12 pages, 3 figures

We demonstrate a practical way to identify the presence of a perovskite phase in rare-earth nickelates (RNiO3) using X-ray photoelectron spectroscopy (XPS). By varying the calcination temperature, we prepared RNiO3 powders with different degrees of chemical reaction. We found that perovskite RNiO3 becomes predominant after high-temperature calcination (≥1,000 °C) in X-ray diffraction and XPS (at Ni 3p and O 1s edges) measurements. While the observed spectra at the Ni 3p edge are similar for all powders, a sizable difference was observed in the O 1s-edge spectra depending on the calcination temperature. With the formation of a perovskite phase with a trivalent Ni3+ state, an XPS peak corresponding to oxygen ions in the perovskite lattice distinctly emerges. Our work shows that the Ni3+ state cannot be determined by analyzing the Ni 3p edge solely and rather, the O 1s edge should be simultaneously monitored for explicit identification.

The d-band electron correlations shed a light on bridging multiple functionalities within one material system, and this further extends the horizon in material designs and their emerging device applications. Herein, we demonstrate the combination of thermoelectric and thermistor functionalities within the perovskite family of correlated rare-earth nickelates (ReNiO3) having small rare-earth elements (i.e., YNiO3 and DyNiO3), in addition to their already known metal-to-insulator transitions. In contrast to conventional semiconductive materials, the electronic band structure of ReNiO3 split within the hybridized Ni3d-O2p is closely coupled to the structure of NiO6 octahedron. Based on such a distinguished feature, it is possible to achieve the coexistence of a large magnitude of thermopower (S) and negative temperature coefficient of resistance (NTCR) in the insulating phase of ReNiO3 with small Re and more distorted NiO6 octahedron. This establishes a thermoelectric thermistor that can be used for sensing the thermal perturbations by integrating the two distinguished detection modes within one system: the active mode utilizing the high NTCR, and the passive mode utilizing the large S. It is worth noticing that as-achieved S-NTCR relationship in ReNiO3 differs form the one for conventional semiconductors, in which cases enlarging the band gap enlarges S but reduces NTCR. As achieved thermoelectric thermistor combing thermistor and thermoelectric functionalities via electron correlation opens up a new direction to explore emerging energy/electronic devices for sensing the thermal perturbations. The temperature range that keeps a high thermoelectric thermistor performance (i.e., |TCR | >2%K-1 and meanwhile S > 100 μVK-1) of ReNiO3 with a small rare-earth radius is possible to cover most of the outdoor conditions on earth (i.e., -50 to 150 °C).

The application of doped lanthanum nickelate type perovskites as Solid Oxide Cell (SOC) materials has proven to be an area of significant study in recent years. These materials, in particular LaNi1-xFexO3, have gained interest due to their apparent resilience to certain degradation effects. There is, therefore, a suggestion in the academic literature that these materials may find application as oxygen electrode materials for future generations of SOC, with lower degradation rates and longer life-times. However, most of the discussion surrounding these materials does not consider the relative performance of such cells with respect to the state-of-the-art. Several commentators have suggested that the popular La‑nickelates are not feasible as replacements for current materials. Clearly, a material with stable but inferior performance is unlikely to offer a realistic alternative to the current crop of cell materials. However, to contradict this, there have also been numerous studies observing how cell performance may be improved via an appropriate tailoring of the electrode architecture. Here, a consideration on the wide body of literature on ABO3-type lanthanum nickelates is provided. This includes an assessment of studies at the fundamental level and with reference to their application in SOC technology. Primarily, this is with reference to LaNi0.6Fe0.4O3 where the literature is most substantial.

Perovskite-structured (ABO3) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare-earth nickelates (RNiO3) thin films is investigated with controlled A-site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three-electrode system in O2-saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A-site element ionic radius which lowers the conductivity of RNiO3 (R = La, La0.5Nd0.5, La0.2Nd0.8, Nd, Nd0.5Sm0.5, Sm, and Gd) films, with LaNiO3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La0.2Nd0.8NiO3. Moreover, the OER activity remains comparable within error through Sm-doped NdNiO3. Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni3+ to Ni2+ as a result of oxygen vacancies, which increases the average occupancy of the eg antibonding orbital to more than one. The work highlights the importance of tuning A-site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts. MOE (Min. of Education, S’pore)

While electron correlations were previously recognized to result in direct current (DC) transportation properties beyond-conventional (e.g., metal-to-insulator transitions, bad metal, thermistors), their respective influences on the alternating current (AC) transport are largely overlooked. Herein, active regulation of the electronic functionalities of d-band correlated rare-earth nickelate (ReNiO3) thin films, by simply utilizing their electronic responses to AC-frequencies (fAC) is demonstrated. Assisted by temperature dependent near edge X-ray absorption fine structure analysis, positive temperature dependences in the Coulomb viscosity of ReNiO3 were discovered, which moderate their AC impedance (R′ + iR′′). Distinguished crosslinking among R′–fAC measured in nearby temperatures is observed that differs to conventional oxides. This enables active adjustability in correlated transports of ReNiO3, among NTCR-, TDelta- and PTCR-thermistors, via fAC from the electronic perspective without varying materials or device structures. Furthermore the TDelta–fAC relationship can be widely adjusted via the Re composition and interfacial strains. The AC-frequency sensitivity discovered in ReNiO3 leads the way to a new freedom in regulating and switching the device working states beyond the present semiconductor technologies. It opens up a new model for enriching novel electronic applications in automatic transmission, artificial intelligence in sensing temperatures or thermal perturbations.

A substantial amount of interest has been focused on ABO3-type perovskite oxides over the past decade as oxygen electrocatalysts. Despite many studies on various compositions, the correlation between the structure of the oxygen octahedra and electrocatalytic property has been overlooked, and there accordingly have been a very limited number of attempts regarding control of atomistic structure. Utilizing epitaxial LnNiO3 (Ln = La, Pr, Nd) thin films, here we demonstrate that simple electrochemical exchange of Fe in the surface region with several-unit-cell thickness is notably effective to boost the catalytic activity for the oxygen evolution reaction by different orders of magnitude. Furthermore, we directly establish that strong distortion of oxygen octahedra at the angstrom scale is readily induced during the Fe exchange, and that this structural perturbation permits easier charge transfer. The findings suggest that structural alteration can be an efficient approach to achieve exceptional electrocatalysis in crystalline oxides., While perovskite oxides are well-studied for their oxygen electrocatalysis performances, the impact of oxygen octahedra is less studied. Here, authors show electrochemical exchange of lanthanide nickelate surfaces to distort oxygen octahedra, facilitating charge transfer and improving catalysis.

Eu doped La2NiO4 powders, with the general formula La2-xEuxNiO4+δ denoted as LENOx (for x = 0, 0.2, 0.4, 0.6 and 0.8), were synthesized via the mechanical milling reaction method. The Eu3+ doping content has a remarkable influence on structural and electrochemical properties. The phase identification and morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, Infrared spectroscopy (IR), A laser size analyzer and scanning electron microscopy (SEM). Lattice parameters were calculated using the Rietveld method. It was observed that the lattice parameter values in LENOx systems varied with the amount of Eu3+. The latter was symmetrically deposited by spin coating on both surfaces of an Ce0.8Sm0.2O1.9 (SDC) electrolyte and studied using AC impedance spectroscopy. The electrochemical properties were studied using two-probe impedance spectroscopy and results showed that the ASR of LENOx was enhanced by the Eu3+ dopant content x. Results also showed that LNEO0.2 had the lowest Area specific resistance (ASR) at 700 °C and it was therefore concluded that doping with the appropriate amount of Eu3+ can further improve the properties of a nickelate cathode.

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) were successfully grown using an optical-image floating zone furnace under oxygen pressure $(p{\mathrm{O}}_{2})$ of 20 bar for ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ and 140 bar for ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$. A combination of synchrotron and laboratory x-ray single-crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ ($R=\mathrm{La},\mathrm{Pr}$) crystallizes in the monoclinic $P{2}_{1}/a$ ($Z=2$) space group at room temperature, and that a metastable orthorhombic phase (Bmab) can be trapped by postgrowth rapid cooling. Both ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ and ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, the MMT is found at 157.6 K. For ${\mathrm{La}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, the MMT depends on the lattice symmetry: 147.5 K for Bmab vs 138.6 K for $P{2}_{1}/a$. Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between Bmab and $P{2}_{1}/a$ phases, demonstrate a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.

Correlated electrons give rise to both exotic electronic and magnetic properties in rare-earth nickelates. Here we present evidence of the interfacial coupling between two nickelate systems, EuNiO_{3} (ENO) and LaNiO_{3} (LNO), with different electronic and magnetic properties but with compatible structural registry giving rise to an electrostructural transition, unobserved in each constituent. Nominally, LNO remains in a paramagnetic-metallic R3[over ¯]c phase while orthorhombic ENO undergoes antiferromagnetic and insulating transitions. However, the ENO/LNO heterostructure displays a uniform rotational symmetry set by an entwined interface. This leads to an anomalous reduction of bond disproportionation in the ENO layer through the metal to insulator transition and concomitantly charge disproportionation opens the gap accompanied by antiferromagnetic ordering. Our results resolve a long-standing question in the physics of rare-earth nickelates, herein demonstrating that charge and bond disproportionation are competing mechanisms for the charge localization process in the rare-earth nickelate system.

The dimensionality of the crystal structure plays an important role in the electronic structures of materials. Ruddlesden-Popper perovskite oxides offer an attractive platform for studying this role due to dimensional flexibility. The effects of dimensionality on physical properties in those oxides have been widely reported. However, the study of dimensional dependence on the chemical properties is still lacking. Here, we synthesized a series of Ruddlesden-Popper perovskite nickelates LanSrNinO3n+1 (n = 1, 2, 3, and ∞) to explore the role of dimensionality on oxygen-evolution reaction (OER) performance. As the dimensionality increased with n, the nickelates exhibited an enhanced OER activity. We found that the weakening of electron correlations among Ni 3d electrons by increasing the dimensionality induced an insulator-to-metal transition and a strengthened Ni-O hybridization, both of which accelerated the OER kinetics. This work sets up a bridge between the dimensionality and electrocatalysis, which provides guidance for designing highly efficient oxygen-evolving catalysts.

We proved that the critical thickness for metal-to-insulator transition (MIT) of LaNiO3 could be controlled by substrate orientation. By means of density functional theory calculations, films grown on SrTiO3 substrates with (001), (110) and (111) orientations have different amount of charge transfer across the interface. Different charge transfer induces different interfacial conductivity behavior and at the same time modifies the carrier density of adjacent LaNiO3 films. The manipulation of MIT by substrate orientation can be achieved through interfacial charge transfer induced interfacial conductive layer with the modified conductivity of LNO layer. Published version

The search for new electrode materials that can be applied to the solid oxide fuel cell (SOFC) technology is an active field of research. In continuing our studies of Ca-substituted layered nickelates, in this work we investigated Nd2−xCaxNiO4+δ materials synthesized via the solution assistant solid state reaction method to evaluate their prospective usage as cathode materials for the intermediate temperature SOFC. It was found that Ca doping resulted in the temperature decreasing for the Fmmm → I4/mmm transition in Nd2NiO4+δ (NNO) and the tetragonal structure stabilizing at room temperature at x ≥ 0.2 in ambient air. The coefficient of thermal expansion calculated from the linear expansion under heating and cooling was different; however there was a common tendency for it to decrease when the Ca content was increased above 5 mol% (x = 0.1). The dc four-probe measurements revealed an increase in total electrical conductivity with doping. The polarization resistance of the Nd2−xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 depended non-linearly on the Ca content with a moderate maximum at x = 0.1. Ca-doping was found to decrease the interaction of the electrode materials with the electrolyte. It resulted in the polarization resistance of the composite electrodes containing doped materials decreasing compared to the composite electrodes based on Nd2NiO4+δ. The polarization resistance of the Nd1.7Ca0.3NiO4+δ-Ce0.8Sm0.2O1.9 (NCNO3-SDC) electrode formed at 1200 °C is equal to 0.56 Ω cm2 at 700 °C in ambient air and comparable with the resistance of the NNO electrode (0.51 Ω cm2). Due to its superior stability and lower interaction with the electrolyte this electrode composition can be recommended for preferential usage in IT-SOFCs.

Description, Perovskite nickelate single crystals are topotactically reduced to the infinite-layer phase and show metallic properties., Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such “soft chemistry” experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals, the gold standard for physical property measurements on quantum materials, has been studied only sparsely. Here, we report the topotactic reduction of La1−xCaxNiO3 single crystals to La1−xCaxNiO2+δ using CaH2 as the reducing agent. The transformation from the three-dimensional perovskite to the quasi–two-dimensional infinite-layer phase was thoroughly characterized by x-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.

The recent observation of superconductivity in infinite-layer Nd$_{1-x}$Sr$_x$NiO$_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of LaNiO$_3$ embedded in insulating LaGaO$_3$, and used layer-selective topotactic reactions to reduce the nickelate layers to LaNiO$_{2}$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction and found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition metal oxides., 12 pages, 10 figures

Nickel oxides appear to have a fundamentally different superconductivity from that of copper oxides, according to new calculations.

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material., Qikai Guo and Saeedeh Farokhipoor acknowledge financial support from a China Scholarship Council (CSC) grant and a VENI grant (016.veni.179.053) of the Netherlands Organisation for Scientific Research (NWO), respectively. Francisco Rivadulla acknowledges support by the Ministry of Science of Spain (Project No. MAT2016-80762-R), the Conselleria de Cultura, Educacion e Ordenacion Universitaria. Xunta de Galicia (ED431F 2016/008, and Centro singular de investigación de Galicia accreditation 2016–2019, ED431G/09), the European Union (European Regional Development Fund (ERDF)) and the European Commission through the project 734187- SPI-COLOST (H2020-MSCA-RISE-2016).

Effective synthesis of meta-stable materials challenging the thermodynamic limits will play a significant role in broadening the horizon in material designs and further explorations of their functionalities. Although d-band correlated rare-earth nickelate perovskites (ReNiO3) have achieved promising applications, e.g., metal-to-insulator transition, artificial intelligence, and memory/logical devices, the thermodynamic instability and high vacuum-dependence in material synthesis have largely caused bottlenecks in these applications. Herein we demonstrate a vacuum-free and low cost chemical route to effectively synthesize single-crystalline ReNiO3 thin films that further promote their device applications. It achieves high flexibility and convenience by adjusting the A-site compositions within the perovskites via single (i.e. Nd, Sm, Eu, and Gd), binary (i.e., Sm1−xNdx and Sm1−xEux) and triple (i.e. Sm1−x−yNdxEuy and Sm1−x−yNdxGdy) rare-earth elements. The respective regulations in electronic structures, as probed via near edge X-ray absorption fine structure analysis, result in sharper metal-to-insulator transitions within a broad temperature range of 400 K, compared with their reported performances. Furthermore, we discover an overlooked thermistor transport behavior of ReNiO3 within the binary A-site elements, which exhibits large temperature coefficients of resistance (>2%) across a broad range of temperatures (5–470 K). By overcoming the bottlenecks in material synthesis of ReNiO3, the present work profoundly paves the way for device fabrication.

Many-body effects produce deviations from the predictions of conventional band theory in quantum materials, leading to strongly correlated phases with insulating or bad metallic behavior. One example is the rare-earth nickelates RNiO3, which undergo metal-to-insulator transitions (MITs) whose origin is debated. Here, we combine total neutron scattering and broadband dielectric spectroscopy experiments to study and compare carrier dynamics and local crystal structure in LaNiO3 and NdNiO3. We find that the local crystal structure of both materials is distorted in the metallic phase, with slow, thermally activated carrier dynamics at high temperature. We further observe a sharp change in conductivity across the MIT in NdNiO3, accompanied by slight differences in the carrier hopping time. These results suggest that changes in carrier concentration drive the MIT through a polaronic mechanism, where the (bi)polaron liquid freezes into the insulating phase across the MIT temperature., Rare-earth nickelates undergo a metal-to-insulator transition accompanied by the formation of ordered lattice distortions, but the role of the lattice in the metallic phase remains unclear. Here the authors provide evidence that the metal is a polaronic liquid that freezes into the insulating state.

Progress in hydrogen energy and promising directions for its modern development are closely related to design of fuel cells, including solid oxide fuel cells, and solid state membranes for hydrogen, oxygen and synthesis gas production. A necessary condition for fabrication of economically competitive devices in this area is the use of cheap electrode materials combining high electrochemical activity and long-term stability. Ln2NiO4+δ oxides with the Ruddlesden–Popper layered structure possessing a high mixed ion-electron conductivity and moderate values of the coefficients of thermal expansion are promising materials for development of oxygen-conducting membranes and cathodes of intermediate-temperature solid oxide fuel cells. In this paper structural characteristics, electrical conductivity, oxygen mobility and electrochemical properties of Ln2-xCaxNiO4+δ (Ln = La, Pr, Nd; x = 0; 0.3) samples were studied to determine factors, which have the most significant effect on the electrochemical activity of electrodes and their stability. It was found that doping with calcium lead to stabilization of the structure and increased the electrical conductivity of materials. However, addition of calcium decreased the electrochemical activity of electrodes in varying degrees depending on the nature of lanthanide. There is no direct interrelation of such a decrease of activity with either the electrical properties or the interstitial oxygen content. We have revealed correlation of the polarization resistance of electrodes with characteristics of oxygen transfer in the electrode material (self-diffusion coefficient, surface exchange constant). Using the C18O2 SSITKA method, the total oxygen mobility in the doped materials was shown to fall with doping due to a decrease in the content of highly mobile interstitial oxygen and hampering of the cooperative oxygen transport mechanism. In the case of La1.7Ca0.3NiO4+δ, this leads to the appearance of a slow diffusion channel and a substantial decrease in the total diffusion coefficient value which results in a sharp increase in the polarization resistance of the electrodes. This phenomenon is not observed in materials with praseodymium and neodymium. The electrodes based on Pr1.7Ca0.3NiO4+δ and Nd1.7Ca0.3NiO4+δ, developed in this work, have an acceptable level of the electrochemical activity along with a high electrical conductivity and increased stability in comparison with undoped compositions and can be recommended for use as cathodes for intermediate temperature fuel cells.

Progress in hydrogen energy and promising directions for its modern development are closely related to the development of fuel cells, including solid oxide fuel cells, and solid state membranes for hydrogen, oxygen and synthesis gas production. A necessary condition for fabrication the economically competitive devices in this area is the use of cheap electrode materials combining high electrochemical activity and long-term stability. Ln 2 NiO 4+ δ oxides with the Ruddlesden–Popper layered structure with a high mixed ion-electron conductivity and moderate values of the coefficients of thermal expansion are promising materials for the development of oxygen-conducting membranes and cathodes of intermediate-temperature solid oxide fuel cells. The paper studies the structure, electrical conductivity, oxygen mobility and electrochemical properties of Ln 2-x Ca x NiO 4+ δ (Ln = La, Pr, Nd; x = 0; 0.3) in order to determine the factors that have the most significant effect on the electrochemical activity of electrodes and their stability. We have found that doping with calcium leads to stabilization of the structure and an increase in the electrical conductivity of materials. However, addition of calcium decreases the electrochemical activity of the electrodes in varying degrees depending on the nature of the lanthanide. There is no direct interrelation of such a decrease of activity with either the electrical properties or the interstitial oxygen content. We have revealed correlation of the polarization resistance of electrodes between characteristics of oxygen transfer in the electrode material (self-diffusion coefficient, surface exchange constant). Using the C 18 O 2 SSITKA method, the total oxygen mobility in the doped materials is shown to fall due to a decrease in the content of highly mobile interstitial oxygen and hampering of the cooperative oxygen transport mechanism. In the case of La 1.7 Ca 0.3 NiO 4+ δ , this leads to the appearance of a slow diffusion channel and a substantial decrease in the total diffusion coefficient value which leads to a sharp increase in the polarization resistance of the electrodes. This phenomenon is not observed in materials with praseodymium and neodymium. The electrodes based on Pr 1. La 1.7 Ca 0.3 NiO 4+ δ and Nd 1.7 La 1.7 Ca 0.3 NiO 4+ δ , developed in this work, have an acceptable level of the electrochemical activity along with a high electrical conductivity and increased stability in comparison with undoped compositions and can be recommended for use as cathodes for intermediate temperature fuel cells.

We report the crystal structures and physical properties of trilayer nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$. Measurements of magnetization and electrical resistivity display contrasting behaviors in the two compounds. Nd$_4$Ni$_3$O$_{10}$ shows a paramagnetic metallic behavior with a metal to metal phase transition($T^{\ast}$) at about 162 K, as revealed by both magnetic susceptibility and resistivity. Further magnetoresistance and Hall coefficient results show a negative magnetoresistance at low temperatures and the carrier type of Nd$_4$Ni$_3$O$_{10}$ is dominated by hole-type charge carriers. The significant enhancement of Hall coefficient and resistivity below $T^{\ast}$ suggest that effective charge carrier density decreases when cooling through the transition temperature. In contrast, Nd$_4$Ni$_3$O$_8$ shows an insulating behavior despite small value of resistivity at room temperature. The compound shows paramagnetic behavior, with the similar magnetic moments as in Nd$_4$Ni$_3$O$_{10}$ derived from the Curie-Weiss fitting. This may suggest that the magnetic moments in both systems are contributed by the Nd ions. By applying pressures up to about 49 GPa, the insulating behavior is still present and becomes even stronger. Our results suggest that the different Ni configurations ($Ni^{1+/2+}$ or $Ni^{2+/3+}$) and competition between localized and itinerant electrons may account for the contrasting behaviors in trilayer nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$.

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, RNiO(3) (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 NdNiO(3) 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.

The K2NiF4-type nickelates i.e. Ln2NiO4+δ (Ln = La, Pr, Nd) and La1.5Pr0.5NiO4+δ have shown promising behaviour as oxygen electrodes for solid oxide cells [1]. These compounds show high anionic bulk diffusion (D*) as well as surface exchange coefficients (k*), combined with good electrical conductivity and thermal expansion properties matching with those of other components (electrolyte, interconnect etc.) of the cell. In order to further enhance the physico-chemical properties, electrochemical performance of these nickelates as oxygen electrode, we performed the substitution of nickel by other transition element such as cobalt. Two compositions of Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ (x=0.1 and 0.2) were considered (as higher cobalt containing nickelates are difficult to synthesize under air atmosphere) and completely characterized using several methods. Finally, the symmetrical half-cells (8YSZ/GDC/electrode) as well as single cells (Ni-YSZ/8YSZ/GDC/electrode) were prepared and electrochemically characterized using DC- and AC-techniques in the temperature range 700 - 900 °C. The current-voltage characteristics of the single cells show an increase in the performance with increasing cobalt content in both SOFC and SOEC operation for Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ electrodes. The best cell performance is obtained with 20% cobalt substitution for all four nickelates. However, the performance of Pr2Ni0.8Co0.2O4+ δ (PNCO20), La1.5Pr0.5Ni0.8Co0.2O4+ δ (LPNCO20) and Nd2Ni0.8Co0.2O4+ δ (NNCO20) are similar and far better than La2Ni0.8Co0.2O4+ δ (LNCO20) [2,3]. For instance under SOFC operation at 900 °C, the maximum power densities obtained at 0.6 V are 1.8, 1.9 and 1.91 W.cm-2 for NNCO20, LPNCO20 and PNCO20 single cells, respectively. Under SOEC conditions, the cell current densities obtained at 900 °C with an applied voltage of 1.5 V are 3.0, 2.84 and 2.8 A.cm-2, respectively for PNCO20, LPNCO20 and NNCO20 single cells with 50% H2O and 50% H2 feed gas mixture. The performance of these electrodes are better than the commercial single cell containing state of art (La,Sr)(Co,Fe)O3 oxygen electrodes. Moreover a large improvement in durability is also observed for the single cells containing cobalt substituted nickelates. Results including physico-chemical and electrochemical properties of these materials, electrochemical performance as oxygen electrode, long term stability under electrolysis conditions and post-test analysis (using SEM-EDX, XRD) will be presented and discussed in detail. References: [1] V.Vibhu et al., Journal of Energy Chemistry, 46 (2020) 62. [1] V.Vibhu et al., Journal of Power Sources, 444 (2019) 227292. [2] V.Vibhu et al., Journal of Power Sources, 482 (2021) 228909.

The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) further strengthens these efforts.With a crystal structure similar to the infinite-layer cuprates - transition metal oxide layers separated by a rare-earth spacer layer - formal valence counting suggests that these materials have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly-interacting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx2-y2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics, well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo- or Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the reference state from which superconductivity emerges upon doping., 28 pages, 5 figures

We compute the nominal magnetic penetration depth of $R$NiO$_2$ ($R =$ La, Nd) from first principles calculations and discuss the results in relation to the superconducting $T_c$. We find a marked discrepancy with the well established phenomenology that correlates these two quantities in cuprates (Uemura plot). We also consider the 2D ultrathin limit and estimate the maximum attainable $T_c$ to be $\sim 180$ K according to the Nelson-Kosterliz universal relation between the superfluid density and the transition temperature., Comment: 3 pages, 1 table, 2 figures

The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ calls for further investigation and optimization of the synthesis of this metastable infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO$_{3}$ (001) substrates, and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, it is essential that the precursor perovskite Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ thin film is stabilized with high crystallinity and no impurity phases; in particular, a Ruddlesden-Popper-type secondary phase is often observed. We have further investigated the evolution of the soft-chemistry topotactic reduction conditions to realize full transformation to the infinite-layer structure with no film decomposition or formation of other phases. We find that capping the nickelate film with a subsequent SrTiO$_{3}$ layer provides an epitaxial template to the top region of the nickelate film, much like the substrate. Thus, for currently optimized growth conditions, we can stabilize superconducting single-phase Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ (001) epitaxial thin films up to ~ 10 nm.

Recently, superconductivity at about 9-15K was discovered in Nd$_{1-x}$Sr$_x$NiO$_2$ infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate the RNiO$_3$ phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni$^{3+}$ oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physical properties of polycrystalline perovskites Sm$_{1-x}$Sr$_x$NiO$_3$ (x = 0, 0.2) in which the ionic radius of Sm$^{3+}$ is smaller than that of Pr$^{3+}$ and Nd$^{3+}$ in related superconducting thin films. The structural and compositional analyses conducted by X-ray diffraction and energy dispersive X-ray spectrum reveal that the samples mainly contain the perovskite phase of Sm$_{1-x}$Sr$_x$NiO$_3$ with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO$_3$ undergoes a paramagnetic-antiferromagnetic transition at about 224K on a global insulating background. In contrast, the Sr-doped sample Sm$_{0.8}$Sr$_{0.2}$NiO$_3$ shows a metallic behavior from 300K down to about 12K, while below 12K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12K in Sm$_{0.8}$Sr$_{0.2}$NiO$_3$. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm$_{0.8}$Sr$_{0.2}$NiO$_2$ with square NiO$_2$ planes. The compound shows an insulating behavior which can be described by the three-dimensional variable-range-hopping model. And superconductivity is still absent in the polycrystalline Sm$_{0.8}$Sr$_{0.2}$NiO$_2$., Comment: 13 pages, 6 figures

International audience; The present study is focused on the development of alternative oxygen electrodes for Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolyzers Cells (SOECs). Rare earth nickelates with general formula Ln2NiO4+δ (Ln = La, Pr or Nd) have shown good performance as oxygen electrodes with various electrolytes. To further enhance the physico-chemical properties, electrochemical performance as oxygen electrode and durability of Solid Oxide Cells (SOCs), herein, we have performed the substitution of nickel with cobalt in these nickelates. Two compositions (x=0.1 and 0.2) were mainly considered and completely characterized using several techniques. The single cells were then prepared and electrochemically characterized using DC- and AC-techniques in the temperature range 700-900 °C. The durability test up to 250h were also investigated at 1 A.cm-2 current density at 800 °C under both SOFC (dry conditions) and SOEC conditions (with 50% H2 and 50% H2O feed gas mixture) indicating different degradation behavior.

Competing interactions in complex materials tend to induce multiple quantum phases of comparable energetics close to the ground state stability. This requires novel strategies and tools to segregate such phases with desired control to manipulate the properties relevant for contemporary technologies. Here, we demonstrate 'quenched disorder' as a predominant control parameter to realize a broad range of the quantum phases of bulk RNiO3 (R = rare-earth ion) phase diagram in a LaxEu1-xNiO3 compounds by systematic introduction of quenched disorder. Using static and terahertz dynamic transport studies on epitaxial thin films, various phases such as Fermi to non-Fermi liquid crossover, bad metallic behavior, quantum criticality, preservation of orbital and charge order symmetry and increased electronic inhomogeneity responsible for Maxwell-Wagner type of dielectric response, etc., are demonstrated. The underlying mechanisms are unveiled by the anomalous responses of microscopic quantities such as scattering rate, plasma frequency, spectral weight, effective mass, and disorder. The results and methodology implemented here can be a generic pursuit of disorder based unified control to extract quantum phases submerged in competing energetics in all complex materials.

A strong coupling of the lattice to functional properties is observed in many transition metal oxide systems, such as the ABO(3) perovskites. In the quest for tailor-made materials, it is essential to be able to control the structural properties of the compound(s) of interest. Here, thin film solid solutions that combine NdNiO3 and LaNiO3, two materials with the perovskite structure but distinct space groups, are analyzed. Raman spectroscopy and scanning transmission electron microscopy are combined in a synergistic approach to fully determine the mechanism of the structural crossover with chemical composition. It is found that the symmetry transition is achieved by phase coexistence in a way that depends on the substrate selected. These results carry implications for analog-tuning of physical properties in future functional materials based on these compounds.

This work presents the results from a study of the structure and transport properties of Ca-doped La2NiO4+δ. La2−xCaxNiO4+δ (x = 0–0.4) materials that were synthesized via combustion of organic-nitrate precursors and characterized by X-ray diffraction (XRD), in situ XRD using synchrotron radiation, thermogravimetric analysis (TGA) and isotope exchange of oxygen with C18O2. The structure was defined as orthorhombic (Fmmm) for x = 0 and tetragonal (I4/mmm) for x = 0.1–0.4. Changes that occurred in the unit cell parameters and volume as the temperature changed during heating were shown to be caused by the excess oxygen loss. Typical for Ruddlesden–Popper phases, oxygen mobility and surface reactivity decreased as the Ca content was increased due to a reduction in the over-stoichiometric oxygen content with the exception of x = 0.1. This composition demonstrated its superior oxygen transport properties compared to La2NiO4+δ due to the enhanced oxygen mobility caused by structural features. Electrochemical data obtained showed relatively low polarization resistance for the electrodes with a low Ca content, which correlates well with oxygen transport properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. The materials synthesis as well as electrochemical study were performed in a framework of the budget task of the IHTE UB RAS with using the equipment of the shared access center “Composition of compounds”. The TGA and isotope exchange studies were supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for Boreskov Institute of Catalysis (projects АААА‐А21‐121011390007‐7, АААА‐А21‐ 121011390009‐1). The SXRD experiments were performed at the shared research center SSTRC on the basis of the Novosibirsk VEPP‐3 complex at BINP SB RAS, using equipment supported by pro‐ ject RFMEFI62119X0022.

We report the epitaxial growth of Ruddlesden–Popper nickelates, Lan+1NinO3n+1, with n up to 5 by reactive molecular beam epitaxy. X-ray diffractions indicate high crystalline quality of these films, and transport measurements show strong dependence on the n values. Angle-resolved photoemission spectroscopy reveals the electronic structure of La5Ni4O13, showing a large hole-like pocket centered around the Brillouin zone corner with a (π, π) back-folded copy.

The rare-earth nickelates are a rich playground for transport properties, known to host non-Fermi liquid character, resistance saturation and metal-insulator transitions. We report a study of transport in LaNiO3 in the presence of tunable disorder induced by irradiation. While pristine LaNiO3 samples are metallic, highly irradiated samples show insulating behaviour at all temperatures. Using irradiation fluence as a tuning handle, we uncover an intermediate region hosting a metal-insulator transition. This transition falls within the Mott-Ioffe-Regel regime wherein the mean free path is comparable to lattice spacing. In the high temperature metallic regime, we find a transition from non-Fermi liquid to a Fermi-liquid-like character. On the insulating side of the metal-insulator transition, we find behaviour that is consistent with weak localization. This is reflected in magnetoresistance that scales with the square of the field and in resistivity. In the highly irradiated insulating samples, we find good agreement with variable range hopping, consistent with Anderson localization. We find qualitatively similar behaviour in thick PrNiO3 films as well. Our results demonstrate that ion irradiation can be used to tailor transport, serving as an excellent tool to study the physics of localization., Comment: 14 pages, 4 figures

Strongly correlated materials show unique solid-state phase transitions with rich nanoscale phenomenology that can be controlled by external stimuli. Particularly interesting is the case of light–matter interaction in the proximity of the metal–insulator transition of heteroepitaxial nickelates. In this work, we use near-infrared laser light in the high-intensity excitation regime to manipulate the nanoscale phase separation in NdNiO3. By tuning the laser intensity, we can reproducibly set the coverage of insulating nanodomains, which we image by photoemission electron microscopy, thus semipermanently configuring the material state. With the aid of transport measurements and finite element simulations, we identify two different timescales of thermal dynamics in the light–matter interaction: a steady-state and a fast transient local heating. These results open interesting perspectives for locally manipulating and reconfiguring electronic order at the nanoscale by optical means.

International audience; Oxygen migration is increasingly acknowledged as playing an important role in the ionic transport in mixed conductors and influencing the electrode electrochemical performance. The aim of this work was to establish correlations between the structural and electrical properties of undoped (Ln2NiO4 + δ, Ln = La, Pr) and doped (La1.7M0.3NiO4 + δ, M = Ca, Sr, Ba, La0.85Pr0.85Ca0.3NiO4 + δ, Pr1.7Ca0.3NiO4 + δ) layered nickelates and the oxygen diffusion in these materials to determine what influences their electrochemical response. A new technique for temperature programmed isotope exchange of oxides with C18O2 in a flow reactor was applied to investigate oxygen mobility and surface reactivity in the polycrystalline powder samples which provided the means to experimentally demonstrate the appearance of two channels of oxygen migration in the doped materials via cooperative mechanism and via near-dopant position. The electrochemical performance of the electrodes based on the developed materials was found to exhibit a strong dependence on their oxygen transport characteristics.