Small and large polarons in nickelates, manganites, and cuprates

By comparing the optical conductivities of La_1.67Sr_0.33NiO₄ (LSNO), Sr_1.5La_0.5MnO₄ (SLMO), Nd₂CuO_4-y (NCO), and Nd_1.96Ce_0.04CuO₄ (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. Whereas in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in these cuprates the polarons appear to be large, and at lowT their binding energy decreases. This reflects an increase of the polaron radius, which may trigger coherent transport.     

Tuning Bifunctional Oxygen Electrocatalysts by Changing the A‐Site Rare‐Earth Element in Perovskite Nickelates

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, La_0.5Nd_0.5, La_0.2Nd_0.8, Nd, Nd_0.5Sm_0.5, 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 La_0.2Nd_0.8NiO₃. 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 e_g 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.     

Experimental progress on the emergent infinite-layer Ni-based superconductors

The emergence of the infinite-layer superconducting nickelate thin films marks the Ni age of superconductivity, which has excited a huge surge of studies since the first report in August of 2019. Despite of the tremendous attention drawn from the entire material science community and a large body of theoretical studies, the experimental progress has been relatively slow due to the challenging sample fabrication, which may, in turn, be holding back the fast development of theoretical research. Therefore, a timely and comprehensive review on all the up-to-date experimental progress of the emergent infinite-layer Ni-based superconductors is urgently needed. In this review, we first introduce the history of more than 30-year-long Ni-based superconductivity exploration, then summarize the sample fabrication processes, later present the experimental electrical transport and magnetic properties, and finally come up with several key issues deserving intensive studies. This review is thus expected to be helpful for researchers with diverse research background to readily capture the major progress of this emerging field.     

Electron-Doping Mottronics in Strongly Correlated Perovskite

The discovery of hydrogen-induced electron localization and highly insulating states in d-band electron correlated perovskites has opened a new paradigm for exploring novel electronic phases of condensed matters and applications in emerging field-controlled electronic devices (e.g., Mottronics). Although a significant understanding of doping-tuned transport properties of single crystalline correlated materials exists, it has remained unclear how doping-controlled transport properties behave in the presence of planar defects. The discovery of an unexpected high-concentration doping effect in defective regions is reported for correlated nickelates. It enables electronic conductance by tuning the Fermi-level in Mott–Hubbard band and shaping the lower Hubbard band state into a partially filled configuration. Interface engineering and grain boundary designs are performed for H_xSmNiO₃/SrRuO₃ heterostructures, and a Mottronic device is achieved. The interfacial aggregation of hydrogen is controlled and quantified to establish its correlation with the electrical transport properties. The chemical bonding between the incorporated hydrogen with defective SmNiO₃ is further analyzed by the positron annihilation spectroscopy. The present work unveils new materials physics in correlated materials and suggests novel doping strategies for developing Mottronic and iontronic devices via hydrogen-doping-controlled orbital occupancy in perovskite heterostructures.     

掺杂镍酸镧材料的气敏特性研究

研究了ＴｉＯ＿２掺杂ＬａＮｉＯ＿３的气敏特性，实验表明，在中等掺杂时，材料的电导适中，气敏性能最好；ＸＲＤ证实，掺杂后ＬａＮｉＯ＿３仍属于ＡＢＯ＿３钙钛矿结构，且有新相生成。还对掺杂ＳｎＯ＿２，Ｓｂ＿２Ｏ＿３及Ｖ＿２Ｏ＿５进行了研究，指出ＳｎＯ＿２中等掺杂及Ｓｂ＿２Ｏ＿３，Ｖ＿２Ｏ＿５低掺杂时，同样能使ＬａＮｉＯ＿３的气敏性质有所改善。     

Enhanced Electrocatalytic Oxygen Evolution Activity by Tuning Both the Oxygen Vacancy and Orbital Occupancy of B‐Site Metal Cation in NdNiO3

Perovskite oxides have been explored as promising electrocatalysts for the oxygen evolution reaction (OER), while a lack of understanding of key factors impacting the catalytic activity restricts their further design and development. Here, for the first time, the contributions of oxygen vacancy (V_O) and orbital occupancy of B‐site cations to the catalytic activity of NdNiO₃ films are systematically investigated. It is found that OER activity follows a typical volcano‐shaped dependence on the oxygen pressure. In the range of 0.2–10 Pa, proper concentration of V_O can provide a moderate bonding strength with intermediate hydroxyl OH* and the increased ratio of Ni³⁺/Ni²⁺ provides a more favorable occupancy of e_g orbital for the catalytic activity; while in the range of 10–60 Pa, insufficient concentration of V_O leads to an enhanced strength of hybridization between Ni 3d and O 2p band and thus deteriorated catalytic activity. The superior OER catalytic performance can be only achieved with both appropriate concentration of V_O and the ratio of B‐site metal cations with different valences.     

Oxygen Diode Formed in Nickelate Heterostructures by Chemical Potential Mismatch

Deliberate control of oxygen vacancy formation and migration in perovskite oxide thin films is important for developing novel electronic and iontronic devices. Here, it is found that the concentration of oxygen vacancies (V_O) formed in LaNiO₃ (LNO) during pulsed laser deposition is strongly affected by the chemical potential mismatch between the LNO film and its proximal layers. Increasing the V_O concentration in LNO significantly modifies the degree of orbital polarization and drives the metal–insulator transition. Changes in the nickel oxidization state and carrier concentration in the films are confirmed by soft X‐ray absorption spectroscopy and optical spectroscopy. The ability to unidirectional‐control the oxygen flow across the heterointerface, e.g., a so‐called “oxygen diode”, by exploiting chemical potential mismatch at interfaces provides a new avenue to tune the physical and electrochemical properties of complex oxides.     

Conductivity and Local Structure of LaNiO3 Thin Films

A marked conductivity enhancement is reported in 6–11 unit cell LaNiO₃ thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three‐element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6–11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth.     

Two-magnon excitations in cuprates and nickelates

We discuss two-magnon phonon-assisted infrared absorption in two-dimensional Heisenberg systems like the parent insulating high-T _c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is compared with experiments. In the case of the cuprates it explains the mid-infrared peaks observed in the insulator. In the case of the nickelates the predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.