Ruddlesden-Popper Family Research Articles

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials.

Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n = 2 Ruddlesden-Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors.

Local cation ordering in compositionally complex Ruddlesden–Popper n = 1 oxides

The Ruddlesden–Popper (RP) layered perovskite structure is of great interest due to its inherent tunability, and the emergence and growth of the compositionally complex oxide (CCO) concept endows the RP family with further possibilities. Here, a comprehensive assessment of thermodynamic stabilization, local order/disorder, and lattice distortion was performed in the first two reported examples of lanthanum-deficient Lan+1BnO3n+1 (n = 1, B = Mg, Co, Ni, Cu, Zn) obtained via various processing conditions. Chemical short-range order (CSRO) at the B-site and the controllable excess interstitial oxygen (δ) in RP-CCOs are uncovered by neutron pair distribution function analysis. Reverse Monte Carlo analysis of the data, Metropolis Monte Carlo simulations, and extended x-ray absorption fine structure analysis implies a modest degree of magnetic element segregation on the local scale. Further, ab initio molecular dynamics simulations results obtained from special quasirandom structure disagree with experimentally observed CSRO but confirm Jahn–Teller distortion of CuO6 octahedra. These findings highlight potential opportunities to control local order/disorder and excess interstitial oxygen in layered RP-CCOs and demonstrate a high degree of freedom for tailoring application-specific properties. They also suggest a need for expansion of theoretical and data modeling approaches in order to meet the innate challenges of CCO and related high-entropy phases.

Structural Features in Some Layered Hybrid Copper Chloride Perovskites: ACuCl4 or A2CuCl4.

We present three new hybrid copper(II) chloride layered perovskites of generic composition ACuCl4 or A2CuCl4, which exhibit three distinct structure types. (m-PdH2)CuCl4 (m-PdH22+ = protonated m-phenylenediamine) adopts a Dion-Jacobson (DJ)-like layered perovskite structure type and exhibits a very large axial thermal contraction effect upon heating, as revealed via variable-temperature synchrotron X-ray powder diffraction (SXRD). This can be attributed to the contraction of an interlayer block, via a slight repositioning of the m-PdH22+ moiety. (3-AbaH)2CuCl4 (3-AbaH+ = protonated 3-aminobenzoic acid) and (4-AbaH)2CuCl4 (4-AbaH+ = protonated 4-aminobenzoic acid) possess the same generic formula as Ruddlesden-Popper (RP) layered perovskites, A2BX4, but adopt different structures. (4-AbaH)2CuCl4 adopts a near-staggered structure type, whereas (3-AbaH)2CuCl4 adopts a near-eclipsed structure type, which resembles the DJ rather than the RP family. (3-AbaH)2CuCl4 also displays static disorder of the [CuCl4]∞ layers. The crystal structures of each are discussed in terms of the differing nature of the templating molecular species, and these are compared to related layered perovskites. Preliminary magnetic measurements are reported, suggesting dominant ferromagnetic interactions.

A DFT+U Study of the Oxygen Defect Formation in (Sr1-xPrx)2FeO4

INTRODUCTION Solid oxide fuel cell (SOFC) electrodes are commonly composed of cermet materials, a composite of ceramic and metal materials. Despite success in electrochemical performance, cermet materials suffer from sulfur poisoning at the triple-phase boundary (TPB), high operating temperatures, and metal coarsening. To mitigate these problems, single-phase mixed ionic-electronic conducting (MIEC) materials have been investigated as alternative electrode materials. One such class of materials is the Ruddlesden-Popper (RP) phase (An+1BnO3n+1), a perovskite composed of n layers of octahedra in a perovskite (ABO3) stack. The RP family of Srn+1FenO3n+1 has found recent use as a base stoichiometry in both cathodes1 and anodes2 with and without A-site trivalent dopants. In order to rationalize the design of RP electrodes, we elaborate on the relationship of MIEC properties as a function of A-site dopant concentration at the atomistic level. In the present study, we used ab initio methods to investigate the relationship between Pr3+ concentration, iron oxidation state, and charge compensation upon defect formation to better understand the role of the A-site on the ionic and electronic properties in (Sr1-xPrx)2FeO4± \U0001d6ff (x = 0, 0.125, 0.25, 0.375, 0.5). COMPUTATIONAL DETAILS All calculations presented in this work were performed using the spin-polarized DFT+U method with periodic boundary conditions as implemented in the VASP 5.4.4 code. We choose the PBE functional to describe exchange and correlation effects. Dudarev’s approach for DFT+U calculations is used to correct the inadequate description of localized 3d electrons on Fe and 4f electrons on Pr with a U-J value of 4.0 and 6.0 eV, respectively. We used the projector-augmented wave (PAW) method to represent the inner core potentials. The kinetic energy cutoff was set for all calculations to 750 eV, and integration over the Brillouin zone was performed with 5×5×3 Monkhorst-Point k-point mesh and the tetrahedron method with Blöchl corrections. RESULTS AND DISCUSSION To sample all possible structural conformations at a given dopant concentration, we tested all non-symmetrical conformers for the lowest relative energy on a model 2×2×1 supercell, as displayed in Figure 1a. A total of 7, 42, 123, and 181 structures were tested for each dopant configuration, respectively. We calculate metallic behavior for x = 0.125 and 0.25 and semi-conducting behavior for x = 0, 0.375 and 0.5, indicating electronic conductivity decreases upon increasing A-site dopant concentration. Figure 1b displays the relationship between the final optimized dopant configurations and defect stability. The interstitial formation energy decreases linearly with Pr concentration with a deviation at x = 0.125. This relationship is primarily a function of the neighboring cation elemental identity and the decreasing interatomic distance between the interstitial defect and the neighboring cations in the rocksalt regime. The vacancy formation energy appears constant with Pr except for a large energy penalty at x = 0.5. The origin of the harsh energy penalty is rooted in the over-reduction of Fe. Upon vacancy formation, the released electrons directly reduce the next-nearest Fe atoms from +3 to +2 as opposed to delocalizing upon the oxide lattice. Figure 1c displays the relationship between the degree of delocalization and the energy of vacancy formation. The degree of delocalization3 is a dimensionless quantity that describes the extent of charge delocalized upon the oxygen sublattice with a vacancy. For x = 0 to 0.375, vacancy formation delocalizes the released electrons onto the lattice as opposed to x = 0.5, where charge from the lattice directly reduces Fe. Covalence between Fe-O bonds is decreased upon the formation of Fe2+, causing a harsh penalty in vacancy formation.In conclusion, tuning MIEC behavior is a balance between reducing Fe and charge delocalization upon the oxide lattice. In terms of electronic conductivity, a small introduction of Pr yielded metallic behavior. In terms of ionic conductivity, a threshold value of Pr yields an unfavorable vacancy formation energy. Future studies will focus on the optimization of the catalytic activity by B-site doping. ACKNOWLEDGMENTS This work has been funded by the Division of Materials Research, a National Science Foundation organization under Award Number 1832809. Computing resources provided by the San Diego Supercomputer Center (SDSC) and Texas Advanced Computing Center (TACC) under XSEDE grant number TG-CTS090100 and USC High-Performance Computing clusters are gratefully acknowledged. REFERENCES T. Hong, M. Zhao, K. Brinkman, F. Chen, C. Xia, ACS Appl. Mater. Interfaces, 9, 8659–8668 (2017)C. Yang, Z. Yang, C. Jin, G. Xiao, F. Chen, Adv. Mater., 11, 1439-1443 (2012)A. M. Ritzmann, J. M. Dieterich, E. A. Carter, Phys. Chem. Chem. Phys., 17, 12260-12269 (2016) Figure 1

Separated transport relaxation scales and interband scattering in thin films of SrRuO3 , CaRuO3 , and Sr2RuO4

The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi-liquid theory. The coherence temperature ${T}_{\text{FL}}$ for normal metals is usually taken to be the temperature below which ${T}^{2}$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family frequently show non-Drude low-energy optical conductivity and unusual $\ensuremath{\omega}/T$ scaling, despite the frequent observation of ${T}^{2}$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two Lorentzian line shape at low temperature and a crossover to a one Drude peak line shape at higher temperatures. The two component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. In ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, both relaxation rates scale as ${T}^{2}$, while in ${\mathrm{CaRuO}}_{3}$ the slow relaxation rate shows ${T}^{2}$, and the fast relaxation rate generates a constant background in conductivity. We discuss three particular possibilities for the separation of rates: (a) strongly energy-dependent inelastic scattering; (b) an almost conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; and (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios requires the existence of exotic quasiparticles. However, the interpretation in terms of multiple conduction channels in particular is consistent with the existence of multiple Fermi surfaces in these compounds and with the expected relative weakness of ${\ensuremath{\omega}}^{2}$ dependent effects in the scattering as compared to ${T}^{2}$ dependent effects in the usual Fermi-liquid treatment. The results may give insight into the possible significance of Hund's coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessen's rule in this class of materials and deviations from the ``Gurzhi'' scaling relations in Fermi liquids.

Ca2MnO4 structural path: Following the negative thermal expansion at the local scale

The oxygen octahedral rotations in ${\mathrm{Ca}}_{2}\mathrm{Mn}{\mathrm{O}}_{4}$, the first member of the $\mathrm{Ca}\mathrm{O}{({\mathrm{CaMnO}}_{3})}_{n}$ Ruddlesden-Popper family, is probed through a set of complementary techniques, including temperature-dependent neutron and x-ray diffraction, combined with local probe studies and ab initio calculations. Here we demonstrate the enhancement of the uniaxial negative thermal expansion coefficient from $\ensuremath{-}1.26\ifmmode\pm\else\textpm\fi{}0.25$ to $\ensuremath{-}21\ifmmode\pm\else\textpm\fi{}1.8$ ppm/K at the second order $I{4}_{1}/acd$ to $I4/mmm$ structural phase transition, providing direct evidence for the corkscrew atomic mechanism. We establish, also, that the predicted $I4/mmm$ high symmetry is attained around 1050 K. At lower temperatures, within the 10--1000 K temperature range, our first-principles calculations and detailed analysis of the Ca local environment reveals that the reported $Aba2$ structural phase, coexisting with the $I{4}_{1}/acd$ one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical $I{4}_{1}/acd$ and $Acam$ structural phases.

Soft-Chemistry Approach To Synthesize Al3+, Ga3+, and Zr4+ Stabilized Ion-Exchangeable Layered Perovskite Oxides

Protonated oxides, H2[Sr2Nb2MO9.5] (M = Al, Ga) and H2[Sr2Nb2ZrO10] related to the Ruddlesden–Popper family of layered perovskite oxides, have been successfully synthesized at room temperature starting from Aurivillius phases Bi2Sr2Nb2MO11.5 (M = Al, Ga) and Bi2Sr2Nb2ZrO12 by the extraction of bismuth oxide sheets using acids. Synthesis of crystalline protonated oxides was also found to be possible under microwave treatment within a short time duration of 5 min. The protonated oxides have been found to undergo facile ion-exchange reactions with an aqueous solution of NaOH or KOH resulting in novel oxides A2Sr2Nb2MO9.5 (A = Na, K; M = Al, Ga) and A2Sr2Nb2ZrO10 (A = Na, K), which could not be synthesized by conventional high temperature solid state synthetic methods. Powder X-ray diffraction patterns confirmed the topochemical nature of the reactions by reflecting changes only in the c-dimensions. Rietveld refinements carried out in the case of K+ ion-exchanged products confirmed their structural similarity...

Ab initio study of Li2 CaTa2 O7 compound: electronic and optical properties forthree phases

The Li2 CaTa2 O7 compound belongs to the Ruddlesden-Popper family of layered perovskites. First principle approximation was used to investigate the electronic band structure and optical properties of the compound for three phases. Independent of the studied compound's structural type, Li2 CaTa2 O7 has semiconductor behavior and direct transition. In addition, the forbidden energy band gap of the compound decreases with rising temperature, as expected. Furthermore, the 3d orbital of Ca contributes to the conduction band due to the crystal field effect. Moreover, the optical response of the chosen axes of the compound to incoming electromagnetic rays varies with phase transition.

Control of Uniaxial Negative Thermal Expansion in Layered Perovskites by Tuning Layer Thickness.

Uniaxial negative thermal expansion (NTE) is known to occur in low n members of the An+1BnO3n+1 Ruddlesden–Popper (RP) layered perovskite series with a frozen rotation of BO6 octahedra about the layering axis. Previous work has shown that this NTE arises due to the combined effects of a close proximity to a transition to a competing phase, so called “symmetry trapping”, and highly anisotropic elastic compliance specific to the symmetry of the NTE phase. We extend this analysis to the broader RP family (n = 1, 2, 3, 4, …, ∞), demonstrating that by changing the fraction of layer interface in the structure (i.e., the value of 1/n) one may control the anisotropic compliance that is necessary for the pronounced uniaxial NTE observed in these systems. More detailed analysis of how the components of the compliance matrix develop with 1/n allows us to identify different regimes, linking enhancements in compliance between these regimes to the crystallographic degrees of freedom in the structure. We further discuss how the perovskite layer thickness affects the frequencies of soft zone boundary modes with large negative Grüneisen parameters, associated with the aforementioned phase transition, that constitute the thermodynamic driving force for NTE. This new insight complements our previous work—showing that chemical control may be used to switch from positive to negative thermal expansion in these systems—since it makes the layer thickness, n, an additional design parameter that may be used to engineer layered perovskites with tuneable thermal expansion. In these respects, we predict that, with appropriate chemical substitution, the n = 1 phase will be the system in which the most pronounced NTE could be achieved.

Synthesis and preliminary study of NdxAE2-xMnO4±δ (AE: Ca, Sr) for symmetrical SOFC electrodes

NdxAE2-xMnO4±δ (AE: Ca, Sr) manganites with x = 0.25, 0.4, 0.5 and 0.60 are part of the Ruddlesden-Popper family of generic formula An+1BnO3n+1 with n = 1. The compositions range that can be successfully synthesized by the sol-gel method in air has been identified. While Sr compounds are stable in reducing atmosphere (3% H2/Ar - 850 °C/16 h), Ca ones decompose when submitted to such atmosphere. Nd0.5Sr1.5MnO4±δ was analyzed by High-Temperature X-ray diffraction (HT-XRD) showing TEC values of 12.7 × 10−6 K−1 and 14.4 × 10−6 K−1 in air and 3% H2/N2 (25–800 °C), respectively. Using standard conditions for electrode elaboration, the material reacts with YSZ electrolyte, but is chemically compatible with GDC electrolyte. A small polaron-type behavior is observed for the electrical conductivity of Nd0.5Sr1.5MnO4±δ in air and 3% H2/Ar, with respective values of 33.4 S cm−1 and 0.9 S cm−1 at 800 °C, meeting the preliminary requirements for use as symmetrical SOFC electrode.

Comparison of Pr-Based Cathodes for IT-SOFCs in the Ruddlesden-Popper Family

The present study is focused on alternative oxygen electrodes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Among the K2NiF4 type compounds, the nickelates Ln2NiO4+δ (Ln = La, Pr or Nd) have already shown their suitability as oxygen electrode due to their mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ showed the best electrochemical properties [1] at intermediate temperature among the three nickelates: R p = 0.15 Ω∙cm² at 600 °C for instance, for screen printed electrode. Interestingly, the ageing of the Pr2NiO4+δ//CGO//8YSZ symmetrical half cells during 1 month under air shows no significant change in the R p value at i dc = 0 condition, despite the decomposition of Pr2NiO4+δ into the third term of the R.P. series (Prn+1NinO3n+1), Pr4Ni3O10+δ, the perovskite PrNiO3-δ and Pr6O11. Such decomposition was observed after one month at different SOFC operating temperatures, namely 600, 700 and 800 °C under air [2]. Aiming at further understanding the relationship between the electrochemical behavior and the phase stability, the physico-chemical and electrochemical properties of Pr4Ni3O10+δ and PrNiO3-δwere thoroughly studied. PrNiO3-δ is chemically very stable at operating temperatures up to one month while the electrochemical properties remain limited. The symmetrical half-cell with PrNiO3-δ electrode shows relatively high R p value, for instance R p = 0.9 Ω∙cm² at 600 °C, which is almost six times higher than the one of Pr2NiO4+δ [3]. On the other hand, Pr4Ni3O10+δ is a very promising material. In addition, Pr4Ni3O10+δ is highly chemically stable during long term up to 1 month under air at 600, 700 and 800 °C, contrarily to Pr2NiO4+δ [4]. Moreover, a single cell Pr4Ni3O10+δ//CGO//8YSZ//Ni-YSZ produces a very high power density of 1.60 W·cm-2 at 800 °C and 0.68 W·cm-2 at 700 °C. Hence, the electrochemical behaviors of Pr4Ni3O10+δ and Pr2NiO4+δ are similar, but Pr4Ni3O10+δis a more promising oxygen electrode since it is highly chemically stable. Herein, the comparison of Pr-based cathodes, and more especially their chemical stability, conductivity and electrochemical properties will be presented and discussed in details.

Comparison of Pr-Based Cathodes for IT-SOFCs in the Ruddlesden-Popper Family

The present study focuses on alternative oxygen electrodes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Among the K2NiF4 type compounds, the nickelates Ln2NiO4+δ (Ln = La, Pr or Nd) have already shown their suitability as oxygen electrode due to their mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ shows excellent electrochemical properties and the ageing of the Pr2NiO4+δ//CGO//8YSZ symmetrical half cells during 1 month under air does not evidence significant change in the R p value at i dc = 0 condition, despite the decomposition of Pr2NiO4+δ into the third term of the Ruddlesden-Popper, (R.P.), series (Pr n +1Ni n O3n+1), Pr4Ni3O10+δ, the perovskite PrNiO3-δ and Pr6O11. Such decomposition is also observed after one month at different SOFC operating temperatures, namely 600, 700 and 800 °C under air. Aiming at further understanding, the relationship between the electrochemical behavior and the phase stability, the physico-chemical and electrochemical properties of Pr2NiO4+δ, PrNiO3-δ and Pr4Ni3O10+δ are thoroughly studied and compared.

Activity and Stability of Ruddlesden-Popper-Type La(n+1) Ni(n) O(3n+1) (n=1, 2, 3, and ∞) Electrocatalysts for Oxygen Reduction and Evolution Reactions in Alkaline Media.

Increasing energy demands have stimulated intense research activity on cleaner energy conversion such as regenerative fuel cells and reversible metal-air batteries. It is highly challenging but desirable to develop low-cost bifunctional catalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), the lack of which is currently one of the major limiting components towards commercialization of these technologies. Here, we have conducted a systematic study on the OER and ORR performances of the Ruddlesden-Popper family of La(n+1)Ni(n) O(3n+1) (n=1, 2, 3, and ∞) in an alkaline medium for the first time. It is apparent that the Ni-O bond lengths and the hyperstoichiometric oxides in the rock-salt layers correlate with the ORR activities, whereas the OER activities appear to be influenced by the OH(-) content on the surface of the compounds. In our case, the electronic configuration fails to predict the electrocatalytic activity of these compounds. This work provides guidelines to develop new electrocatalysts with improved performances.

Novel La2-x Cu x NiO4±δ /La4Ni3O10-δ composite materials for intermediate temperature solid oxide fuel cells, IT-SOFC

Phases constituted by La2-xCuxNiO4±δ (0.01 ≤ x ≤ 0.1) of the Ruddlesden-Popper family (Lan+1NinO3n+1; n = 1) were prepared and then mixed with La4Ni3O10, in a weight ratio of 50:50 wt%, in order to be used as solid oxide fuel cell cathodes. ASR values relative to the symmetrical cells constituted by yttria-stabilized zirconia electrolyte and the following electrodes, La1.98Cu0.02NiO4+δ + La4Ni3O10 and La1.95Cu0.05NiO4+δ + La4Ni3O10, are of 11.8 Ω cm2 at 650 °C for both and, respectively, of 3.5 and 2.9 Ω cm2 at 750 °C. In the case of the second cell, the electrode material reacts with the electrolyte forming an insulating phase at the interface, contrarily to the first cell, which stability could be explained by the combination of low doping amounts of copper and the presence of La4Ni3O10 acting as a stabilizer of the material at high temperature.

Functionally graded composite La 2 NiO 4+δ and La 4 Ni 3 O 10−δ solid oxide fuel cell cathodes

Abstract Functionally graded electrodes have been fabricated from members of the (LaNiO 3 ) n LaO Ruddlesden–Popper family. These consist of a thin compact layer of La 2 NiO 4 + δ deposited on the electrolyte, topped by a thicker porous La 2 NiO 4 + δ + La 4 Ni 310 − δ composite layer, completed by a thin porous La 4 Ni 3 O 10 − δ layer acting as a current collector. The microstructure and electrochemical role of these layers are discussed in detail. It was found that using a 50:50 wt.% mix in the composite layer gave the best performance; ASRs at 500, 550, 600, 650, and 700 °C were 15.6, 5.53, 2.29, 1.06 and 0.53 Ω cm 2 respectively. This represents high performance for this class of material, particularly notable at the lower-end of the temperature range. As such this grading process is an attractive proposition for further research towards lowering the operating temperature of SOFCs.

In Situ Measurements on Solid Oxide Fuel Cell Cathodes – Simultaneous X‐Ray Absorption and AC Impedance Spectroscopy on Symmetrical Cells

AbstractA solid oxide fuel cell in operando is a complex multiphasic entity under electrical polarization and operating at high temperatures. In this work, we reproduce these conditions while studying transition metal redox chemistry in situ at the cathode. This was achieved by building a furnace that allowed for X‐ray absorption near‐edge structure and AC impedance spectroscopy data to be obtained simultaneously on symmetrical cells while at operating temperatures. The cell electrodes consisted of phases from the Ruddlesden–Popper family; La2NiO4+δ, La4Ni3O10–δ, and composites thereof. The redox chemistry of nickel in these cathodes was probed in situ through investigation of changes in the position of the X‐ray absorption K‐edge. An oxidation state reduction (Ni3+ to Ni2+) was observed on heating the cells; this was correlated to changing concentrations of ionic charge carriers in the electrode. Polarizing the cells resulted in dramatic changes to their electrical performance but not to the bulk redox chemistry of the electrode. The implications of this with respect to explaining the polarization behavior are discussed.

Relating Electrochemical Performance Measurements and Surface Analyses of Pr2-XLaxNiO4+ δ Electrode Materials

The anode in a solid oxide electrolysis cell (SOEC) is often the rate-determining component, in terms of both device performance and degradation rates. In order to improve device characteristics, better electrode materials must be developed. Interstitial oxygen conducting electrodes, such as those in the A2BO4 Ruddlesden-Popper family, have been shown to perform well as anodes in SOECs. In this work, we characterize the electrode properties of Ruddlesden-Popper structured (Pr,La)2NiO4+δ electrode materials, and relate these electrochemical properties to the surface composition characterized by Low Energy Ion Scattering (LEIS) spectroscopy.

Fluctuation of surface composition and chemical states at the hetero-interface in composites comprised of a phase with perovskite structure and a phase related to the Ruddlesden–Popper family of compounds

Surfaces and interfaces in composite solids can possess particular compositions and properties, thereby governing the functions of materials. X-ray photoelectron spectroscopy (XPS) has been used for the first time to explore the cation rearrangement between the surface and bulk material of the crystallites in the two-phase composites formed in the La–Sr–Pr–Co–O system. The two-phase composites contain a perovskite phase (major fraction) and a layered perovskite-like phase (La,Sr,Pr)2CoO4±δ related to the Ruddlesden–Popper family of compounds. The difference between the surface and nominal composition was revealed for all the composites explored. The surface concentrations of Pr and La cations are lower compared to the nominal stoichiometry, suggesting their preferable dissolution into the volume of the crystallites. Both Pr3+ and Pr4+ cations coexist at the surface. The surface of all the composites is enriched in Sr cations that could exist as SrCO3, SrO2, and Sr(OH)2 individual surface phases as well as SrO on the surface of the perovskite phase. The second phase plays an important role in balancing the surface composition at the hetero-interface in the composites. The rise in the fraction of (La,Sr,Pr)2CoO4±δ in the composites changes the “surface Sr:bulk Sr” ratio, increasing the later contribution due to the stronger accommodation of Sr cations within its crystal lattice and decreasing the surface depletion of Co cations. In addition to Co3+ and Co4+ cations, a small fraction of Co2+ cations exists in the near-surface region of the composites. The surface of all the composites is strongly enriched in oxygen (by 11.8–13.3 at %). The O1s spectra of the composites contain contributions from the lattice oxygen related to the perovskite phase and layered (La,Sr,Pr)2CoO4±δ phase as well as different surface oxygen states. The effect of surface treatment on the evolution of the surface composition has been discussed.

Mixed “cobalt-nickel” layered oxides and oxyhydroxides of n = 2 members of the Ruddlesden–Popper family Sr2.5La0.5Co1.3Ni0.7(O,OH)7−y

The crystal structure, transport and magnetic properties of Sr2.5La0.5Co1.3Ni0.7O6.4 and Sr2.5La0.5Co1.3Ni0.7Ox(OH)y·zH2O samples are detailed. Contrary to the first compound, the second one is not strictly speaking a second member of the well-known Ruddlesden–Popper (RP) series, but it exhibits very close relationships. Indeed, due to topotactical reactions with air (hydrolysis and hydration), water molecules are included within the rock salt layer of the “perfect” RP oxide, Sr2.5La0.5Co1.3Ni0.7O6.4, to form the hydrated oxyhydroxide phase presented in this paper. Cationic homogeneity of the samples was confirmed by energy dispersive spectroscopy analyses performed with a transmission electron microscope. The crystal structure of samples was determined by combined refinements of powder X-ray and electronic diffraction data. Physical properties of compounds are discussed, for instance the hydrated compound and the “perfect” RP oxide both exhibit typical magnetic frustrated systems and no long range order prevails at low temperature. Transport property measurements reveal a semi-conducting behavior for Sr2.5La0.5Co1.3Ni0.7O6.4. Finally, relationships between structure and physical properties are described herein.