Ruddlesden-Popper Type Research Articles

2D layered halide perovskite for field-effect transistors

Field-effect transistors are crucial components for modern electronics, generating significant research and profitable interest. Metal halide perovskites have recently emerged as a pioneering active material in solar cells, generating interest in their potential use in other electronic and (opto)electronic devices, including field-effect transistors and phototransistors. However, before they can be commercialized, they still face significant challenges owing to their immanent instabilities with respect to heat, moisture, and light. In contrast, due to their exceptional environmental stability, the newly emerging two-dimensional Ruddlesden–Popper type perovskites have garnered significant recognition. The current state of the field is covered in this review article, as are the problems, and a perspective for the scenarios of perovskite field-effect transistors. The effects of temperature, light, and measurement conditions are taken into account, as well as the physics of the device and the fundamental mechanisms that drive these devices, such as ion migration and ionic defects. Subsequently, the performance of perovskite transistors and phototransistors described so far is analyzed and critically evaluated. Finally, the major roadblocks to perovskite transistor advancement are identified and explored. The lessons learned from other perovskite optoelectronic devices are investigated in order to address these obstacles and bring these devices closer to industrial implementation.

Broad spectrum catalysis using Ca3Sn2S7 Ruddlesden-Popper perovskite under multi-stimulus

Advanced oxidation processes emerged as effective alternatives to traditional approaches. The efficacy of these methods significantly depends on the intrinsic properties of a catalyst material. Semiconductor materials with robust photo absorption alongside inherent polarization emerged as subjects of great interest. In this study, we synthesized two-dimensional Ca3Sn2S7 nanosheets oriented as Ruddlesden-Popper type perovskite structure. The optimal compositions of these nanosheets exhibit excellent physical and optical characteristics while varying Sn ratios significantly impact the properties. Impressively, these compositions boast a wide optical window that spans from the visible to the far infrared region, featuring a narrow bandgap of approximately 0.6 eV. Band potentials in a positive regime make it highly favorable for the oxidation process. The electrochemical analysis further validates high mobility with low resistance in both compositions. The catalytic efficiency of the synthesized nanosheets is evaluated using rhodamine-B. Three energy sources, i.e., visible light, laser, and ultrasound vibrations, decomposed 96 %, 90 %, and 78 %, respectively, in 30 min. Remarkably accelerated degradation rates are achieved under each stimulus, owing to the presence of oxidative optical bands, large surface area, and potential internal polarization within the nanosheets. Preliminary findings strongly indicate the immense potential of this compound in catalytic and photovoltaic applications.

Correction to: Karlleuite Ca2MnO4 – a first mineral with the Ruddlesden-Popper type structure from Bellerberg volcano, Germany

Correction to: Karlleuite Ca2MnO4 – a first mineral with the Ruddlesden-Popper type structure from Bellerberg volcano, Germany

New Insights into MAI Additives in 2D-Assisted 3D Controlled Crystallization Toward High-Quality α-Phase FAPbI3 Perovskites.

The highly oriented 2D perovskite templates of n = 1 have typically been created to attain controllable and oriented crystallization of 3D α-phase formamidinium lead triiodide (α-FAPbI3) perovskites. However, the role of methylammonium iodide (MAI), a widely used α-FAPbI3 phase stabilizer, in regulating the growth dynamics of 2D/3D perovskites is generally ignored. Herein, Ruddlesden-Popper type n = 1 2D octylammonium lead iodide(OA2PbI4) perovskites are added into FAPbI3 precursor solution. The template of n = 2 (OA2MAPb2I7), which is spontaneously constructed by the mixture of n = 1 2D and methylammonium chloride (MACl), acts as a skeleton to template the epitaxial growth of α-FAPbI3. However, the volatilization of MACl inevitably causes damage to the 2D structure during thermal annealing. This study reveals that small amounts of less volatile MAI additive enables the creation of stable 2D template, leading to more controlled vertical orientation crystallization. Consequently, the high-quality mixed-dimensional perovskite film delivers a high efficiency of 24.19% together with improved intrinsic stability. This work provides an in-depth understanding of 2D-assisted controlled epitaxial growth of α-FAPbI3.

Enhancing oxygen evolution reaction performance of Ruddlesden–Popper perovskite oxide through heteroatom incorporation

Prior studies have demonstrated that Ruddlesden–Popper (RP) type perovskite oxides exhibit promising electrocatalytic properties for oxygen evolution reaction (OER). However, the limited electrocatalytic activity has impeded their widespread utilization. The incorporation of heteroatoms into the Ruddlesden–Popper type structure has been identified as an effective strategy to enhance the electrocatalytic performance. This research presents the systematic design and synthesis of RP type perovskite oxide LaSrFeO4 (LSFO), followed by the optimization of electrocatalytic activity through Ni substitution at the Fe site of SLFO to create LaSrFe1-xNixO4 (LSFNO-x, x = 0.2, 0.4, and 0.5). Therein, the LSFNO-0.4 sample achieved a current density of 10 mA·cm−2 with an overpotential of only 340 mV. Additionally, it displayed exceptional stability over 120 h in a 1.0 M KOH solution. Both experimental and theoretical studies suggest that the addition of Ni cation increases the Fe − O covalency, and impacts the morphology and surface electron density of LSFO. This substitution results in a greater electrochemically active surface area, improved electron transfer, and optimization of hydrogen/water absorption free energy. This study presents a successful approach for enhancing the OER performance of Ruddlesden–Popper LSFO through the deliberate control of Fe − O covalent bonding.

Multicomponent doping realized superior triple-conducting within cobalt-free Ruddlesden-Popper-type perovskite for Proton-conducting fuel cells cathode

Proton-conducting solid oxide fuel cells (H-SOFCs) are appealing for low-to-intermediate temperature operation due to their intrinsic lower activation energy and better ionic conductivity. Unfortunately, there are currently few acceptable and high-performing cathode materials available for H-SOFCs. Herein, we present a novel high performance multicomponent Ruddlesden-Popper (RP) type cathode Pr3Ni1.5Cu0.3Nb0.05Ta0.05Zr0.05Y0.05O7-δ (PNCNTZY) with superior triple conducting property. The multicomponent co-doping of Nb, Ta, Zr, and Y brings about the enhancement of configurational entropy and cocktail effect for improving H+/O2-/e- triple conductivity and oxygen vacancy concentration. Naturally, the ORR activity is improved by facilitated charge transfer and oxygen surface exchange, exhibiting reduced polarization resistance to just 0.24 Ω cm2 at 550 °C. When used as the cathode in H-SOFC single cells, excellent power production is demonstrated at 550 °C, with a peak power density of up to 0.84 W cm-2. Furthermore, the PNCNTZY sample shows adequate thermal compatibility with proton conductor electrolytes BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) and then appropriate endurance durability (0.8 A cm-2 at 550 °C for about 250 h) due to their match of thermal expansion coefficient (TEC) brought about by the Co-free and partial replacement of larger-size elements. This extremely promising Co-free RP perovskite cathode for H-SOFCs produced by this work might provide a new type candidate and novel strategy for the commercialization of H-SOFCs.

High-Temperature Protonic Conduction in La2NiO4+ δ-Based Ruddlesden-Popper Type Oxides: Correlation with Concentration of Interstitial Oxide Ions.

Oxygen-excess La2NiO4+ δ (LNO) conducts oxide ions, electron holes, and hydroxide ions simultaneously on exposing to wet oxygen, exhibiting the potential as a cathode material in protonic ceramic fuel cells. Since the incorporation of protons in oxygen-excess LNO is via the hydration reaction assisted by interstitial oxide ions, in this work, the concentration of interstitial oxide ions is reduced and increased by substituting Ni with Cu and Co, respectively. A higher concentration of interstitial oxide ions leads to a high proton concentration, indicating the predominant role of interstitial oxide ions in the hydration reaction, different from that in the oxygen-deficient oxides, where protons are introduced by dissociative absorption of water molecules by oxygen vacancies. The theoretical calculation indicates that protons in Co-doped LNO prefer to locate between the interstitial oxide ions and unshared apical oxide ions. A trapping effect is found between protons and the oxide ions near Cu, leading to decreased proton mobility. Protonic conductivity at 400-575 °C is then directly measured by a Hebb-Wagner direct current polarization method with La0.99Ca0.01NbO4- δ as the blocking electrode, enabling the observation that Co-doped LNO has the highest protonic conductivity among the samples studied in this work.

Enhancing oxygen reaction kinetics in lanthanum nickelate Ruddlesden-Popper electrodes via praseodymium oxide infiltration for solid oxide cells.

This study explores the effect of praseodymium oxide (PrOx) impregnation in lanthanum nickelate Ruddlesden-Popper (RP) type materials for use in oxygen electrodes of solid oxide cells (SOCs). These mixed conductors are free of cobalt and strontium, which are increasingly being avoided in solid oxide cell applications. We investigate two compositions, La2NiO4+δ (L2N1) and La4Ni3O10-δ (L4N3), demonstrating distinct electrical and oxygen kinetic properties. The L2N1 material exhibits superior performance due to its higher bulk oxygen-ion diffusion, which governs the enhanced ambipolar conduction, crucial for the oxygen exchange process. In the PrOx-impregnated samples, at 700 °C, the total polarization resistance (Rpol) values decrease to ∼0.6 Ω cm2 for L2N1 + PrOx and ∼0.8 Ω cm2 for L4N3 + PrOx, representing reductions by factors of ∼7 and ∼17, respectively, compared to the non-impregnated counterparts. Electrochemical measurements as a function of oxygen partial pressure suggest that surface-exchange processes may be rate-limiting. The impregnated PrOx acts as a catalyst for the dissociative adsorption of oxygen and improves the charge transfer, leading to significant enhancements in the polarization processes. The electrochemical properties and stability of these RP phases in oxidizing conditions, combined with the oxygen transport capabilities and mixed oxidation state of praseodymium oxide (Pr4+/Pr3+), offer promising Co- and Sr-free oxygen electrodes for SOC applications.

Facile and Low Temperature Synthesis of Nd1.8Sr0.2NiO4-δ Cathode Nanofibers for Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are usually operated at high temperatures (800-1000 °C). As the overall efficiency of SOFC is governed by thermodynamics and kinetics during operation, reducing the operating temperature to 500-650 °C causes significant increase in the electrode polarization losses especially at cathode due to high activation energy towards oxygen reduction reaction (ORR) at cathode, which eventually reduces the overall cell performance. Addressing increased polarization losses at relatively low temperatures (500-650 °C) has been the key issue and a focus of many groups over past couple of decades. Conventional cathode materials lose their ORR activity and so discarded for intermediate temperature (IT)-SOFC application. In this regard, mixed ionic-electronic conductors (MIECs) offer high thermal and chemical stability along with high oxygen diffusion and good electronic conductivity at IT [1,2]. In addition, Ruddlesden-Popper (RP) type MIECs with A2BO4 structure have been demonstrated as promising cathode for IT- SOFCs due to their unique structure, rapid oxygen surface exchange kinetics, and excellent stability in atmosphere. Particularly Nd2NiO4+δ oxides offer an excellent oxygen diffusion coupled with high surface exchange coefficients and comparable thermal expansion coefficients with those of oxygen-ion conductors (YSZ and CGO) [2]. Enhanced electronic conductivity and electrochemical performance have been demonstrated for partial replacement of Nd with Sr in our previous reports [3].Albeit, the intrinsic material properties such as electrochemical catalytic activity and electrical conductivity are of primary importance, an actual electrode performance strongly depends on its microstructure. Therefore, in addition to using high catalytic and conductive materials, achieving an extremely high-power density, thus, requires a finely controlled electrode microstructure, to expand the electrochemically active triple-phase boundary (TPB). Recently, electrospinning has been considered as an efficient, cost effective and promising method for synthesis of ceramic nanofibers. Pertinent literature suggests that SOFC with cathode nanofiber has superior/better performance (nearly doubled power density and significant reduced polarization resistance) compared to powder type electrodes. Selection of suitable cathode materials with modified microstructure (nanofibers) would facilitate more electrochemically active sites for the oxygen reduction at relatively low temperatures <650 °C.In this paper we synthesized Nd1.8Sr0.2NiO4-δ (NSNO) cathode nanofibers using electrospinning technique and compared its electrochemical performance with NSNO nanopowder. Practically, 10 wt.% Polyvinylpyrrolidone (PVP) solution was prepared by dissolving it in ethanol, to which stoichiometric amounts of Nd-nitrate, Sr-nitrate, Ni-nitrate (total weight percentage of metal salts- 3 wt%) were added, together with some deionized water. The mixture was then stirred for 10 h at room temperature on a magnetic stirrer to obtain a homogeneous sol. The sol was loaded into the electrospinning syringe. Distance between the spinneret and the collector was fixed at 25 cm and the high-voltage supply was maintained at 30 kV. The spinning rate was controlled at 5 ml h- 1 by a syringe pump. The obtained nanofibers were dried at 80 °C for 12 h, then sintered at 900 °C for 2 h in air to obtain single-phase NSNO.The obtained nanofibers were characterized using room temperature XRD, XPS, SEM and dc electrical conductivity (400-650 °C). In addition, electrochemical impedance spectroscopy studies on symmetric cells were carried out as a function of temperature (400-650 °C). Improved electrochemical performance is attributed to enhanced electrochemical active sites for ORR due to nanofibers and optimum porosity.

Insights into the First Multi Transition Element Containing Ruddlesden Popper-Type Cathode for Fluoride Ion Batteries

Fluoride ion batteries (FIBs) have emerged as a recent solid-state battery technology, offering an alternative to traditional battery systems. However, the existing FIBs have encountered challenges related to their poor cycling performance and improved electrodes based on fluoride insertion chemistry are highly sought.Here, we investigate Ruddlesden-Popper type La2Ni0.75Co0.25O4+𝛿 as a promising intercalation-based active cathode material for all-solid-state FIBs. We determine the structural changes of La2Ni0.75Co0.25O4+𝛿 during Fluoride intercalation / de-intercalation by X-ray Diffraction (XRD), electrochemical measurements and X-ray absorption spectroscopy to understand the detailed complex reaction behaviour of the phase, focussing on understanding the changes of Ni, Co oxidation states and coordination environments. Under optimized operating conditions, we obtained a cycle life of 120 cycles at a critical cut-off capacity of 40 mAh/g and over 250 cycles under pressure of 452MPa. The average Coulombic efficiencies ranged from 95% to 99% for the cell running under pressure. Thus, La2Ni0.75Co0.25O4+𝛿 corresponds to the most promising cycling-stable high-energy cathode materials for all-solid-state FIBs with the improved capacity.

Synthesis and Structural Characterization of Layered Ni+1/+2 Oxides Obtained by Topotactic Oxygen Release on Nd2−xSrxNiO4−δ Single Crystals

Layered nickelate oxides containing Ni1+/Ni2+ are isoelectronic to Cu2+/Cu3+ compounds and of present interest with respect to recent findings of superconductivity in a series of different compositions. It is thereby questionable why superconductivity is still rare to find in nickelates, compared to the much larger amount of superconducting cuprates. Anisotropic dz2 vs. dx2−y2 orbital occupation as well as interface-induced superconductivity are two of the main advanced arguments. We are here interested in investigating the feasibility of synthesizing layered nickelate-type oxides, where the Ni1+/Ni2+ ratio can be tuned by oxygen and/or cation doping. Our strategy is to synthesize Sr-doped n = 1 Ruddlesden–Popper type Nd2−xSrxNiO4+δ single crystals, which are then reduced by H2 gas, forming Nd2−xSrxNiO4−δ via a topotactic oxygen release at moderate temperatures. We report here on structural studies carried out on single crystals by laboratory and synchrotron diffraction using pixel detectors. We evidence the general possibility to obtain reduced single crystals despite their increased orthorhombicity. This must be regarded as a milestone to obtain single crystalline nickelate oxides, which further on contain charge-ordering of Ni1+/Ni2+, opening the access towards anisotropic properties.

Interface engineering of Ruddlesden–Popper perovskite/CeO2/carbon heterojunction for rechargeable zinc-air batteries

The development of noble metal-based bifunctional electrocatalysts is the key to driving the sluggish oxygen reduction/evolution reaction (ORR/OER) for rechargeable zinc-air battery applications. There is an urgent need to design and construct robust and cost-efficient bifunctional electrocatalysts. Herein, an interface engineering strategy of Ruddlesden–Popper (RP) perovskite/CeO2/carbon heterojunction with core–shell nanostructures is described. Ce-based metal–organic framework derived CeO2-C nanosheets are decorated on the surface of RP type perovskite Pr3Sr(Ni0.5Co0.5)3O10−δ (PSNC) nanofibers. Benefiting from the favorable conductivity, abundant oxygen vacancies and strong interfacial coupling, the hierarchical CeO2-C/PSNC electrode delivers a half-wave potential of 0.78 V (ORR), and an OER overpotential of 370 mV at 10 mA cm−2, respectively. A liquid rechargeable zinc-air battery (ZAB) assembled with CeO2-C/PSNC electrocatalysts as the air cathode exhibits a peak power density of 161 mW cm−2 and a long-term cycling life over 219 h. In addition, the CeO2-C/PSNC-based all-solid-state cable-type ZAB provides a high open-circuit voltage (∼1.44 V), good flexibility and durability. Our study opens a new insight into the design of efficient electrocatalysts for rechargeable ZABs by constructing hierarchical heterojunctions.

Phase transition from SrRuO3 to Sr3Ru2O7 by tuning oxygen pressure at low processing temperature

The epitaxial growth of Sr3Ru2O7 thin films in the Ruddlesden-Popper type remains a challenge as the growth temperature required for a typical pulsed laser deposition (PLD) process can be as high as 1100 °C. In this study, high quality Sr3Ru2O7 epitaxial thin films were obtained using PLD with a single SrRuO3 target. By adjusting the oxygen partial pressure, the structure underwent a phase transition from SrRuO3 to Sr3Ru2O7 facilitated by the formation and development of oxygen and ruthenium vacancies. Remarkably, for the first time, high-quality Sr3Ru2O7 thin films with good crystalline quality were successfully grown at a low temperature of 680 °C on SrTiO3 substrates.

Computational and Experimental Study on the Combined Sulfur and Chromium Poisoning Mechanism of LSM, LSCF and LNO Cathodes

Prevovskite (La,Sr)MnO3 (LSM), (La,Sr)(Co,Fe)O3 (LSCF) and Ruddlesden-Popper (RP) type La2NiO4 (LNO) phase, as solid oxide fuel cell (SOFC) cathodes, has been used due to its high efficiency and low emissions. To further understand the long-term degradation mechanism of the LSM, LSCF, and LNO phase in SO2 and Cr vapor gas impurities, they were prepared, sintered, and heat-treated at 800, 900, and 1000 ℃ in the sulfur, chromium, and water vapor-containing atmospheres, respectively. Later, XRD, SEM and TEM techniques were applied to the samples and corresponding simulations were made by the CALPHAD approach based on the same experimental conditions to verify the formation of the detrimental secondary phases. Results showed good agreement among the predictions and experiments. More importantly, extensive simulations were also conducted under different conditions (temperature, the partial pressure of oxygen and impurities) to understand the long-term degradation mechanism and optimize the operating conditions.

Effect of Zn doping on Sr3Fe2O7-δ Ruddlesden-Popper oxides as active and robust cathodes for protonic ceramic fuel cells

Perovskite oxides are promising electrodes for protonic ceramic fuel cells (PCFCs) due to their good electrochemical performance and stability. However, the lower operating temperature results in a large polarization resistance and sluggish oxygen reduction reaction (ORR) in cathodes. Herein, Ruddlesden-Popper (R–P) type perovskite Sr3Fe2-xZnxO7-δ (SFZx, x = 0, 0.1, 0.5) were successfully synthesized and the partial replacement of Fe by Zn in SFZx demonstrates a superior ORR activity. At 750 °C, the polarization resistance of SFZ05 is 0.072 Ω cm−2, which is 47% lower than that of the undoped Sr3Fe2O7-δ (SFO), and the peak power density of the single-cell with an SFZ05 cathode reaches 523.56 mW cm−2. The distribution of relaxation time (DRT) results manifest that the rate-limiting steps of ORR are charge transfer and ionic migration processes. Furthermore, SFZ05 also presented better CO2 tolerance. These results show that SFZ05 is a promising cathode candidate with high activity and stability for PCFCs.

Phase Structure, Bond Features, and Microwave Dielectric Characteristics of Ruddlesden-Popper Type Sr2TiO4 Ceramics.

This work studied the phase constitution, bond characteristics, and microwave dielectric performances of Sr2TiO4 ceramics. Based on XRD and Rietveld refinement analysis, pure tetragonal Ruddlesden-Popper type Sr2TiO4 ceramic is synthesized at 1425~1525 °C. Meanwhile, the microstructure is dense and without porosity, indicating its high sinterability and densification. Great microwave dielectric performances can be obtained, namely an εr value of 39.41, and a Q × f value of 93,120 GHz, when sintered at 1475 °C. Under ideal sintering conditions, the extrinsic factors are minimized and can be ignored. Thus, the intrinsic factors are considered crucial in determining microwave dielectric performances. Based on the P-V-L complex chemical bond theory calculation, the largest bond ionicity, and proportions to the bond susceptibility from Sr-O bonds suggest that Sr-O bonds mainly determine the dielectric polarizability. However, the Ti-O bonds show lattice energy about three times larger than Sr-O bonds, emphasizing that the structural stability of Sr2TiO4 ceramics is dominated by Ti-O bonds, and the Ti-O bonds are vital in determining the intrinsic dielectric loss. The thermal expansion coefficient value of the Sr2TiO4 structure is also mainly decided by Ti-O bonds.

Tailoring the Fabrication Method of Dion–Jacobson 2D Halide Perovskites toward Highly Crystalline and Oriented Films

AbstractHalide perovskites are potential next‐generation optoelectronic devices. However, the film quality of this charming material fabricated by the conventional spin‐coating method is far from satisfactory, significantly affecting the optoelectronic devices' performance. Here, one facile slow‐evaporating solvent (SE) method is demonstrated to synthesize high‐quality organic–inorganic halide perovskite films. Compared with the conventional spin coating method, the films fabricated by this SE method show much higher crystallinity, oriented lattice, smoother surface morphology, and lower trap density. Besides, the photodetector manufactured by the SE method‐based film also performs much better than the ones by the spin‐coating method. Importantly, this universal method can be applied to different organic–inorganic halide perovskites, such as Dion–Jacobson (DJ) type and Ruddlesden–Popper type 2D halide perovskites and conventional 3D halide perovskites. This work gives an effective solution to improve the quality of the DJ‐type halide perovskites, which endows the halide perovskites with a more practical chance to be commercialized cosmically.

Regulating the Electronic Structure of Ruddlesden-Popper-Type Perovskite by Chlorine Doping for Enhanced Oxygen Evolution Activity.

Developing economical, efficient, and durable oxygen evolution catalysts is crucial for achieving sustainable energy conversion and storage. Ruddlesden-Popper-type perovskite oxides are at the forefront of oxygen evolution reaction (OER) research. However, their activity and stability are far from satisfactory. Therefore, we emphasize the paradigm shift in designing efficient perovskite-type OER catalysts through anion defect engineering. The Cl anion-doped A2BO4-type perovskite oxides, SrLaCoO4-xClx (SLCOClx), were employed as highly efficient OER catalysts, wherein Cl could tune the electronic structure of SrLaCoO4 (SLCO) to enhance the OER activity effectively. Especially, SLCOCl0.15 demonstrates significantly enhanced OER activity, and the overpotential is only 370 mV at 10 mA·cm-2, which is significantly better than that of SLCO (510 mV). As confirmed by experience results and density functional theory (DFT) calculation, due to the doping of Cl, obviously increasing the ratio of Co2+/Co3+, more abundant oxygen vacancies (O22-/O-) are generated, and the electrical conductivity is increased, which together promote the improvement of OER activity.

Synthesis of Chemically Sharp Interface in NdNiO3/SrTiO3 Heterostructures

The nickel-based superconductivity provides a fascinating new platform to explore high-T c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden–Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO3 heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO3 substrates results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra [NdO] monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.

Growth and phase transition of Sr3Zr2O7 single crystals

The melting behavior of Ruddlesden-Popper type hybrid improper ferroelectric Sr3Zr2O7 phase in the ZrO2–SrO pseudo-binary system was investigated, and its single crystals were successfully grown. A series of the slow-cooling floating zone experiments revealed that Sr3Zr2O7 melts incongruently into SrZrO3 phase and a liquid and that the compositional range where Sr3Zr2O7 and a liquid coexist is located around 70 mol% SrO composition. Based on the results, we attempted to grow Sr3Zr2O7 single crystals by the traveling solvent floating zone method using SrO-excess solvent and feed. Consequently, many small single crystals of Sr3Zr2O7 phase with several millimeters in size were discovered in the as-grown boules covered with SrO phase. The phase transition behavior of the grown crystals was investigated by differential thermal analysis with polarizing optical microscopy as well as by optical second harmonic generation measurements. We directly observed a reconstruction of orthorhombic twin domains in Sr3Zr2O7 single crystals accompanied by the first-order ferroelectric transition at about 410 °C.