Variation Of Oxygen Stoichiometry Research Articles

Evolution of 2PB/3PB Transport Pathways in Oxygen Electrodes during Degradation Test: A Perspective from Electro-Chemo-Mechanical Coupling on Microstructures

The solid oxide electrolysis cell (SOEC) has garnered significant attention over the past decades for hydrogen generation integrated with intermittent renewable energy [1]. However, a major hurdle in the commercialization of SOEC is cell degradation when operated beyond 1000 hours [2]. SOEC degrades two times more quickly than the same cell operated in fuel cell mode [3]. Perovskite oxygen electrodes (OEs), such as La1-xSrxCo1-yFeyO3-δ (LSCF), exhibit flexible oxygen stoichiometry [4-6] depending on the operating conditions. The delamination failure in perovskite OEs arises from a mismatch between the slower O2 evolution rate at the surface and the rapid O2 current in the bulk from the electrolyte (EL), which causes variations in oxygen stoichiometry. This variation in oxygen stoichiometry enhances bonding, contracts lattice volume, and leads to chemical stress at the OE/EL interface [7].In this research, we aim to construct an electro-chemo-mechanically coupled model with synthetic microstructures (Fig.1a) to explore the intricate interactions between delamination at the OE/EL interface and oxygen ion transport pathway evolution. The Dream 3D software will be utilized for microstructure generation, Matlab for mesh preparation, and Comsol for Multiphysics simulation. We will examine two OE configurations known for achieving high electrocatalytic activity [8]: single layer and bilayer OEs . For the single layer configuration, La0.6Sr0.4Co0.4Fe0.6O3 (LSCF) [9, 10] serves as the sole electrode material. For bilayer OEs, we incorporate SrTa0.1Co0.9O3 (SCT) [11, 12] as the capping layer for electrocatalytic activity, with LSCF acting as the supporting backbone. The evolution of two transport pathways (Fig.1b&c), double phase boundary (2PB) and tripe phase boundary (3PB), in different configurations during long-term degradation tests will be evaluated by coupling the models with experimental polarization curves and electrochemical impedance spectra. The investigations will focus on the effects of delamination resulting from lattice volume variations on shifting of transport pathways (Fig.1d) in different OE configurations. References Nechache, A. and S. Hody, Alternative and innovative solid oxide electrolysis cell materials: A short review. Renewable and Sustainable Energy Reviews, 2021. 149.Rashkeev, S.N. and M.V. Glazoff, Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells. International Journal of Hydrogen Energy, 2012. 37(2): p. 1280-1291.Moçoteguy, P. and A. Brisse, A review and comprehensive analysis of degradation mechanisms of solid oxide electrolysis cells. International Journal of Hydrogen Energy, 2013. 38(36): p. 15887-15902.Adler, S.B., Chemical Expansivity of Electrochemical Ceramics. Journal of the American Ceramic Society, 2001. 84(9): p. 2117-2119.Atkinson, A. and T.M.G.M. Ramos, Chemically-induced stresses in ceramic oxygen ion-conducting membranes. Solid State Ionics, 2000. 129(1): p. 259-269.Bishop, S.R., K.L. Duncan, and E.D. Wachsman, Defect equilibria and chemical expansion in non-stoichiometric undoped and gadolinium-doped cerium oxide. Electrochimica Acta, 2009. 54(5): p. 1436-1443.Cook, K., J. Wrubel, Z. Ma, K. Huang, and X. Jin, Modeling Electrokinetics of Oxygen Electrodes in Solid Oxide Electrolyzer Cells. Journal of The Electrochemical Society, 2021. 168(11): p. 114510.Laguna-Bercero, M.A., H. Monzón, A. Larrea, and V.M. Orera, Improved stability of reversible solid oxide cells with a nickelate-based oxygen electrode. Journal of Materials Chemistry A, 2016. 4(4): p. 1446-1453.Mogensen, M.B., M. Chen, H.L. Frandsen, C. Graves, J.B. Hansen, K.V. Hansen, A. Hauch, T. Jacobsen, S.H. Jensen, T.L. Skafte, and X. Sun, Reversible solid-oxide cells for clean and sustainable energy. Clean Energy, 2019. 3(3): p. 175-201.Jiang, S.P., Challenges in the development of reversible solid oxide cell technologies: a mini review. Asia-Pacific Journal of Chemical Engineering, 2016. 11(3): p. 386-391.Huang, K. and Y. Wen, Electrochemical Performance of New Bilayer Oxygen Electrode for Reversible Solid Oxide Cells. ECS Meeting s, 2021. MA2021-03(1): p. 138-138.Huang, K., Method to Make Isostructural Bilayer Oxygen Electrode. 2020, University of South Carolina: US. Figure 1

Influence of oxygen stoichiometry on [formula omitted] spin state transitions and magnetic properties in [formula omitted] cobaltites

The layered EuBaCo2O5+δ (δ= 0.47) (EBCO) cobaltite shows various magnetic phases at high temperature and magnetic glassy state at low temperature. Further, the various magnetic phases of the EBCO are coupled with the spin state transition of Co3+ octahedra. In the present study, we report the structural and magnetization properties of the EBCO by changing the oxygen stoichiometry. The various oxygen compositions were obtained after post annealing the as-prepared EBCO cobaltite under argon and oxygen atmospheres and the final stoichiometry was detected from the iodometric titrations. The δ= 0.13 and 0.67 samples show the tetragonal symmetry (space group, P4/mmm), whereas the δ= 0.41, 0.47 and 0.58 samples show the orthorhombic symmetry (space group, Pmmm). The magnetization measurements on the δ= 0.13 sample shows the paramagnetic to antiferromagnetic ordering at TN= 288 K and the electrical resistivity analysis shows the Co2+ and Co3+ charge ordering at TCO= 180 K. Further, the δ= 0.13 sample show the absence of the ferromagnetic phase and Co3+ spin state transitions. The δ= 0.58 and 0.67 samples show the coexistence of ferromagnetic and antiferromagnetic phases, however, the features of Co3+ intermediate spin (IS) to low spin (LS) state transition is suppressed, which is observed in δ= 0.47 sample at 151 K. Our results indicate a tuning of the crystal structure along with the magnetic properties and also the Co3+ spin state transitions by oxygen stoichiometry variation in EuBaCo2O5+δ cobaltites.

Structural Defects Improve the Memristive Characteristics of Epitaxial La0.8Sr0.2MnO3‐Based Devices (Adv. Mater. Interfaces 23/2022)

Memristive Properties of Perovskite-Based Planar Devices Redox-based resistive random access memories (ReRAM) show great potential to improve current electronic devices' storage capabilities. In perovskites, variations in oxygen stoichiometry are directly linked to changes in resistance, governing the devices' switching characteristics. In article number 2200498, Carlos Moncasi, Mónica Burriel, and co-workers study the memristive properties of novel perovskite-based planar devices and obtain new insights into the key role of the films' nanostructure.

Modeling Electrokinetics of Oxygen Electrodes in Solid Oxide Electrolyzer Cells

High temperature solid oxide electrolyzers operated at >600oC are advantageous from a thermodynamic perspective since heat (TDS) can be converted into chemical energy (DH) [1]. This additional heat reduces the overall electrical power (DG) requirement, allowing an electrolysis cell to achieve ~100% efficient H2 production [1]. Since the thermal energy contribution to the electrolysis reaction can also be obtained from sensible Joule heating produced within the cell, the electrical energy demand is further reduced which decreases the H2 production price. The bulk of the heat can be provided by an external source such as nuclear power, solar thermal power, or waste heat from a chemical plant. Theoretically, there is no external heat requirement if the electrolyzers operate at the thermoneutral voltage. Due to the above advantages, the research and development on solid oxide-based water electrolyzers have received significant attention [1–6].This work focuses on physics-based modeling of high temperature solid oxide electrolysis cells (SOEC). A microscale model is presented to simulate electrode kinetics of the oxygen electrode in a solid oxide electrolyzer cell (SOEC). Two mixed ionic/electronic conducting structures are examined for the oxygen producing electrode in this work: single layer porous lanthanum strontium cobalt ferrite (LSCF), and bilayer LSCF/SCT (strontium cobalt tantalum oxide) structures. An yttrium-stabilized zirconia (YSZ) electrolyte separates the hydrogen and oxygen electrodes, as well as a gadolinium doped-ceria (GDC) buffer layer on the oxygen electrode side. Electrochemical reactions [7-8] occurring at the two-phase boundaries (2PBs) and three-phase boundaries (3PBs) of single-layer LSCF and bilayer LSCF/SCT oxygen electrodes are modeled under various SOEC voltages with lattice oxygen stoichiometry as the key output. The results reveal that there exists a competition in electrode kinetics between 2PBs and 3PBs, but 3PBs are the primary reactive sites for single-layer LSCF oxygen electrode under high voltages. These locations experience the greatest oxygen stoichiometry variations and are therefore the most likely locations for dimensional changes. By applying an active SCT layer over LSCF, the 2PBs become activated to compete with the 3PBs, thus alleviating oxygen stoichiometry variations and reducing the likelihood of dimensional change. This strategy could reduce lattice structural expansion, proving to be valuable for electrode-electrolyte delamination prevention and will be the focus of future work. Figure 1

Modeling Electrokinetics of Oxygen Electrodes in Solid Oxide Electrolyzer Cells

A microscale model is presented in this study to simulate electrode kinetics of the oxygen electrode in a solid oxide electrolyzer cell (SOEC). Two mixed ionic/electronic conducting structures are examined for the oxygen producing electrode in this work: single layer porous lanthanum strontium cobalt ferrite (LSCF), and bilayer LSCF/SCT (strontium cobalt tantalum oxide) structures. A yttrium-stabilized zirconia (YSZ) electrolyte separates the hydrogen and oxygen electrodes, as well as a gadolinium doped-ceria (GDC) buffer layer on the oxygen electrode side. Electrochemical reactions occurring at the two-phase boundaries (2PBs) and three-phase boundaries (3PBs) of single-layer LSCF and bilayer LSCF/SCT oxygen electrodes are modeled under various SOEC voltages with lattice oxygen stoichiometry as the key output. The results reveal that there exists a competition in electrode kinetics between 2PBs and 3PBs, but 3PBs are the primary reactive sites for single-layer LSCF oxygen electrode under high voltages. These locations experience the greatest oxygen stoichiometry variations and are therefore the most likely locations for dimensional changes. By applying an active SCT layer over LSCF, the 2PBs become activated to compete with the 3PBs, thus alleviating oxygen stoichiometry variations and reducing the likelihood of dimensional change. This strategy could reduce lattice structural expansion, proving to be valuable for electrode-electrolyte delamination prevention and will be the focus of future work.

Tuning the resistive switching in tantalum oxide-based memristors by annealing

A key step in engineering resistive switching is the ability to control the device switching behavior. Here, we investigate the possibility to tune the resistive switching of tantalum oxide (TaOx)-based memristors from a non-switchable state to a switchable state by applying post-fabrication annealing of the devices. The switching of the devices was found to be related to: (1) the oxidation state changes in the TaOx thin film after annealing and (2) the local variations in oxygen stoichiometry in the vicinity of the interface between the TiN electrode and the TaOx active resistive layer. We further discuss the possible mechanism behind the resistive switching after annealing. This experimental approach provides a simple but powerful pathway to trigger the resistive switching in devices that do not show any resistive switching initially.

Evolutionary Optimization of a Charge Transfer Ionic Potential Model for Ta/Ta-Oxide Heterointerfaces

Tantalum, tantalum oxide and their hetero-interfaces are of tremendous technological interest in several applications spanning electronics, thermal management, catalysis and biochemistry. For example, local oxygen stoichiometry variation in TaOx memristors comprising of metallic (Ta) and insulating oxide (Ta2O5) have been shown to result in fast switching on the sub-nanosecond timescale over a billion cycles, relevant to neuromorphic computation. Despite its broad importance, an atomistic scale understanding of oxygen stoichiometry variation across Ta/TaOx hetero-interfaces, such as during early stages of oxidation and oxide growth, is not well understood. This is mainly due to the lack of a variable charge interatomic potential model for tantalum oxides that can accurately describe the ionic interactions in the metallic (Ta) and oxide (TaOx) environment as well as at their interfaces. To address this challenge, we introduce a charge transfer ionic potential (CTIP) model for Ta/Ta-oxide system by training against lattice parameters, cohesive energies, equations of state, and elastic properties of various experimentally observed Ta2O5 polymorphs. The best set of CTIP parameters are determined by employing a single-objective global optimization scheme driven by genetic algorithms followed by local Simplex optimization. Our newly developed CTIP potential accurately predicts structure, thermodynamics, energetic ordering of polymorphs, as well as elastic and surface properties of both Ta and Ta2O5, in excellent agreement with DFT calculations and experiments. We employ our newly parameterized CTIP potential to investigate the early stages of oxidation of Ta at different temperatures and atomic/molecular nature of the oxidizing species.

Magnetic field-induced switching of magnetic ordering in SrFeO3−δ

The oxygen-deficient strontium iron oxide SrFeO3−δ (SFO) exhibits richness in the phase diagram over a broad range of temperatures and for other external parameters. Room-temperature X-ray diffraction and Raman spectrum reveals that the structure of synthesized SFO system consists of two mixed phases, i.e., major orthorhombic and minor tetragonal phases. The low-temperature Raman and vibrating sample magnetometer measurements indicated a structural transition below 253 K. The magnetic property of the synthesized SFO for various external magnetic field (up to 5 T) reveals possible variation in oxygen stoichiometry. Also, the application of external H increases Neel transition temperature (T N), suppresses the hysteresis width (W H), and thus weakens the first-order nature of the transition. Our analysis revealed the vanishing of hysteresis and the first-order antiferromagnetic transition becomes a crossover above a critical magnetic field H CR ≈ 5 T. Possible switching of magnetic ordering and oxidation state observed in same system enhances interest in related compounds which may be used in magnetic sensors and other magnetic switching devices.

Tailoring high-voltage and high-performance LiNi0.5Mn1.5O4 cathode material for high energy lithium-ion batteries

Increased specific capacity and/or working potential are important prerequisites for high energy functional materials for lithium-ion batteries. Furthermore practical applications require materials with high tap density in order to maximize the loading of the electrodes, thereby optimizing the energy density on cell level.Stoichiometric and phase pure LiNi0.5Mn1.5O4 (LMNO) has been synthesised via a continuous co-precipitation and lithiation process, suited for large scale production. Powder properties have been controlled in order to obtain spherical particles consisting of a multitude of densely-packed primary crystallites. The obtained material exhibits a single plateau at 4.7 V vs. Li/Li+ with high capacity, rate capability and cycling stability.Morphological factors such as crystallite size, particle size and particle architecture, and additionally the variation of oxygen stoichiometry have been investigated. This study clearly illustrates the importance of these factors on the electrochemical performance of an optimized LMNO high-voltage material.

Ab initio modeling of oxygen-vacancy formation in doped-HfOx RRAM: Effects of oxide phases, stoichiometry, and dopant concentrations

Doping techniques have been widely investigated to improve the performance and reliability of resistive random-access memory. In this paper, oxide phases, non-stoichiometry, and dopant concentrations are identified as three critical factors that affect the characteristics of doped HfOx. Using ab initio calculations, their effects on Al, Ti, and Si dopants are thoroughly investigated and compared with experimental studies. In particular, the inconsistent observations regarding the effects of Al dopants on forming voltages are explained by the variation of oxygen stoichiometry. Only non-stoichiometric oxide will activate the dopant effects to significantly reduce the forming voltages.

Near Surface Stoichiometry in UO2: A Density Functional Theory Study

The mechanisms of oxygen stoichiometry variation in UO2at different temperature and oxygen partial pressure are important for understanding the dynamics of microstructure in these crystals. However, very limited experimental studies have been performed to understand the atomic structure of UO2near surface and defect effects of near surface on stoichiometry in which the system can exchange atoms with the external reservoir. In this study, the near (110) surface relaxation and stoichiometry in UO2have been studied with density functional theory (DFT) calculations. On the basis of the point-defect model (PDM), a general expression for the near surface stoichiometric variation is derived by using DFT total-energy calculations and atomistic thermodynamics, in an attempt to pin down the mechanisms of oxygen exchange between the gas environment and defected UO2. By using the derived expression, it is observed that, under poor oxygen conditions, the stoichiometry of near surface is switched from hyperstoichiometric at 300 K with a depth around 3 nm to near-stoichiometric at 1000 K and hypostoichiometric at 2000 K. Furthermore, at very poor oxygen concentrations and high temperatures, our results also suggest that the bulk of the UO2prefers to be hypostoichiometric, although the surface is near-stoichiometric.

Correlation between oxygen stoichiometry, structure, and opto-electrical properties in amorphous In2O3:H films

We have characterized amorphous In2O3:H (H : ∼4 at.%) transparent conducting films by Rutherford backscattering spectrometry (RBS), thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The amorphous In2O3:H films have been fabricated at room temperature by sputtering of an In2O3 ceramic target under Ar, O2, and H2O vapor with variation of a flow ratio r(O2) = O2/(O2+Ar). We observe (i) signals originating from Ar in RBS spectra for all the films and (ii) desorption of H2O and Ar gases during post thermal annealing of the films. Furthermore, O2 desorption together with H2O and Ar is observed for the films grown at r(O2) > 0.375%, whereas In desorption together with H2O and Ar is observed for the films grown at r(O2) < 0.375%. These results suggest that the films have void and/or multi-vacancy rich structures inside the amorphous network, and the variety of atoms, such as Ar, H2O, and weakly bonded O and In, is present in the void structures for the films grown at O2-rich and O2-poor conditions, respectively. Corresponding to the structural changes, optical and electrical properties also change at r(O2) = 0.375%. For the films grown at r(O2) < 0.375%, we observe a broad absorption in the visible wavelengths that cannot be explained by free carrier absorption. In this film, the carrier mobility reduces rapidly with increasing carrier density. Analysis of spectroscopic ellipsometry and Hall measurements reveals that a large decrease in mobility is due to a large increase in carrier effective mass, in addition to the effect of ionized impurity scattering. In this article, we discuss the optical and transport properties with the variation of oxygen stoichiometry and microscopic structures in the amorphous In2O3:H films.

Magnetic and electronic properties of Eu0.9Ca0.1BaCo2O5.5+δ with the variation of oxygen stoichiometry

The effect of the oxygen content on the magnetic and transport properties of the ferromagnetic Eu0.9Ca0.1BaCo2O5.5+δ has been investigated. Unlike the increase in TC with calcium content, paradoxically the TC value decreases as the oxygen content increases, as observed in the undoped phase. Assuming the Kanamori-Goodenough rules apply to this very complex magnetic system, these experiments suggest that the Co3+ disproportionation into Co4+ and Co2+, plays an important role in the appearance of ferromagnetism. The magnetic phase separation scenario of ferromagnetic domains embedded in an antiferromagnetic matrix play an important role in the ferromagnetism of the calcium doped “112” cobaltites. All the samples covering a wide range of oxygen content, exhibit p-type conductivity.

Magnetic properties of GdBaCo2O5.5−δ single crystals

The layered perovskite compounds REBaCo2O5.5−δ (RE = Rare Earth) display interesting magnetic properties arising from the variable spin states of the Co ions. There is a competition between the antiferromagnetic and ferromagnetic ordering in these materials. These compounds also allow a wide variation of oxygen stoichiometry to tune the magnetic properties. Single crystals of GdBaCo2O5.5−δ have been grown by the floating-zone technique in air. The as-grown crystal displays a twinned structure. Oriented crystals isolated from the as-grown boule were annealed in a flow of oxygen to vary the oxygen stoichiometry. Magnetization measurements have been carried out on both as-grown and annealed crystals. Results of the specific heat of GdBaCo2O5.56 crystals as well as the magnetic properties studied as a function of applied pressures (0 to 8 kbar) are presented.

Mössbauer spectroscopy analysis of 57Fe-doped YBaCo 4O 7+δ: Effects of oxygen intercalation

Mössbauer spectroscopy of layered YBaCo 3.96Fe 0.04O 7+ δ ( δ=0.02 and 0.80), where 1% cobalt is substituted with 57Fe isotope, revealed no evidence of charge ordering at 4–293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe 3+ fractions in the alternating triangular and Kagomé layers in oxidized YBaCo 3.96Fe 0.04O 7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75–80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal–oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45–50 K.

Mixed conductivity, stability and electrochemical behavior of perovskite-type (Sr 0.7Ce 0.3) 1 − xMn 1 − yCr yO 3 − δ

The total conductivity, Seebeck coefficient, thermal and chemical expansions, and steady-state oxygen permeability of Sr 0.7Ce 0.3Mn 1 − y Cr y O 3 − δ ( y = 0−0.5) with tetragonal perovskite structure were analyzed in the oxygen partial pressure range 10 − 16 to 0.3 atm at 600–1270 K. The oxygen permeation fluxes, governed by both surface exchange kinetics and bulk ionic transport limited by low oxygen-vacancy concentration, are 10–30 times higher compared to (La,Sr)MnO 3 − δ . While the perovskite lattice of (Sr 0.7Ce 0.3) 1 − x MnO 3 − δ ( x = 0−0.05) is almost intolerant with respect to A-site cation deficiency, the stability limits of Sr 0.7Ce 0.3Mn 1 − y Cr y O 3 − δ in reducing atmospheres are essentially unaffected by Cr doping and correspond to oxygen partial pressures of (0.4−1.5) × 10 − 11 atm at 1223 K. The substitution of chromium for manganese decreases p-type electronic conduction predominant in the whole phase stability domain, suppresses chemical expansion on reducing p(O 2) due to lowering oxygen stoichiometry variations, but leads also to moderately higher thermal expansion. The electrical properties indicate strong hole trapping by chromium cations and progressive charge-carrier localization. The steady-state polarization studies of porous Sr 0.7Ce 0.3Mn 0.9Cr 0.1O 3 − δ electrodes in contact with two apatite-type silicate electrolytes, La 10Si 5AlO 26.5 and La 6.83Pr 3Si 4.5Fe 1.5O 26 + δ , showed a relatively poor electrochemical performance, which may be partly associated with microstructural degradation and cation interdiffusion between the cell components.

Oxygen nonstoichiometry and mixed conductivity of SrFe 1− x[formula omitted]O 3− δ ( M=Al, Ga): Effects of B-site doping

The oxygen nonstoichiometry, electrical conductivity and Seebeck coefficient of perovskite-related SrFe 1− x Ga x O 3− δ ( x = 0 – 0.2 ) and SrFe 1− x Al x O 3− δ ( x = 0.1 – 0.3 ) were studied in the temperature range 923–1223 K at oxygen partial pressures varying from 10 −20 to 0.3 atm. The substitution of iron with gallium was found to promote long-range ordering of oxygen vacancies, resulting in the perovskite → brownmillerite transition at moderate oxygen pressures, and to decrease the mobility of the p-type electronic charge carriers. Opposite tendencies are observed for aluminum doping, which causes a higher oxygen nonstoichiometry in oxidizing atmospheres. As suggested by atomistic simulations of highly oxygen-deficient ferrite lattices, this behavior is associated with oxygen-vacancy trapping near Al 3+ having energetically favorable tetrahedral coordination, whereas Ga 3+ cations with preferential octahedral coordination in the perovskite lattice tend to repulse vacancies towards oxygen sites surrounded by iron and thus destabilize the structure. The relatively low hole mobility, with activation energies of 0.18–0.33 eV, indicates a small-polaron conduction mechanism. Under reducing conditions the oxygen content and ionic transport in brownmillerite-type polymorphs of SrFe 1− x Ga x O 3− δ , exhibiting a relatively wide range of oxygen stoichiometry variation, are both lower compared to the Al-containing materials characterized by the co-existence of perovskite- and brownmillerite-like domains.

Time resolved in-situ neutron diffraction investigation of the electrochemical reduction of BIMEVOX

An in-situ neutron diffraction experiment was performed under electrical bias on Bi 4V 2O 11 and BICOVOX membranes in order to analyse the electrochemical reduction of these electrolytes at the cathode under working conditions. For neutrons, a large amount of material is needed and thick membranes, 8 mm in diameter, 2.5 cm in thickness, were prepared. The experiments were performed at 620 °C, under air atmosphere. The sample, connected to gold wires, was placed at the centre of a furnace working under vacuum and isolated from it in a quartz tube with one end opened to air. Under polarisation, the bi-dimensional structure of the BIMEVOX compound is retained and the observed changes in unit-cell parameters are due to a slight variation in oxygen stoichiometry. This transformation is reversible and the initial diffraction pattern was fully recovered after the current was turned off and the membrane reheated up to 700 °C.

Redox behaviour of Sr 4Fe 6O 13± δ by Mössbauer spectroscopy and neutron diffraction

Mössbauer spectroscopy studies of the intergrowth Sr 4Fe 6O 13± δ phase, in combination with the structure refinement from X-ray and neutron powder diffraction data, showed that the point defects formed due to oxygen nonstoichiometry variations from approximately −0.45 up to +0.14 are accumulated in the nonperovskite layers, built of oxygen–iron polyhedra with pentacoordinated Fe cations. These defects include oxygen vacancies and Fe 2+ cations, or oxygen interstitials and Fe 4+ in the case of oxygen deficiency or hyperstoichiometry, respectively. The oxidation state of iron cations and the occupancy of oxygen sites in the perovskite-like layers, built of O 2−–Fe 3+ octahedra in the Sr 4Fe 6O 13± δ lattice, are both essentially independent of the oxygen stoichiometry variations. The perovskite layers act thus as a frame of the lattice, which provides a substantial stability and a moderate chemically induced expansion on the oxygen content variations, but leads to relatively low ionic and electronic conductivities.

Variation of oxygen stoichiometry in BaRE(Cu0.5Fe0.5)2O5+δ double perovskites upon heat treatments under ambient and high pressure

The tunability of the oxygen stoichiometry in the BaRE(Cu0.5Fe0.5)2O5+δ double-perovskite structure, stable with rare earth elements RE=Lu–Pr, was studied by both normal-pressure thermogravimetric oxidative (O2) and deoxidative (Ar) post-annealings and high-pressure heat treatments carried out in a cubic-anvil-type high-pressure apparatus. Under reducing conditions the BaRE(Cu0.5Fe0.5)2O5+δ double perovskite is obtained at δ≈0 irrespective of the size of the RE constituent. In the structures with RE=Pr and Nd the oxygen stoichiometry can be continuously varied by normal-pressure post-annealings up to δ≈0.24 and 0.35, respectively, whereas with a smaller rare earth ion, i.e., RE=Lu–Sm, the amount of excess oxygen raises from δ≈0 only upon high-pressure heat treatment.