Brownmillerite-type Structure Research Articles

Study of Thermal Insulation Nature of A2Fe2O6-δ (A and A’ = Ca, Sr)

Materials with low thermal conductivity have been used in thermoelectrics, electronics, solar devices, and land base and aerospace gas turbines to prevent heat dissipation and provide safety and longevity of equipment. We report Ca2Fe2O6-δ, and Sr2Fe2O6-δ for their low thermal conductivities. These compounds are vacancy-ordered oxygen-deficient perovskites but with different vacancy arrangements. Ca2Fe2O6-δ has a brownmillerite type structure while Sr2Fe2O6-δ has a different structure.

Analysis of the electrical transport, conductivity, and dielectric relaxation behavior of La0.75 Ba0.10 Sr0.15 Fe O2.875-δ (δ = 0.375 and 0.50) brownmillerite oxides

The topotactic reduction of La0.75Ba0.10Sr0.15FeO2.875 with titanium metal leads to a new isostructural material of the composition La0.75Ba0.10Sr0.15FeO2.875-δ (δ = 0.375 and 0.50). XRD analysis of phases confirms that the obtained compound adopts a brownmillerite-type structure. A slight distorted monoclinic P2/m was found to describe the crystal structure. The transport properties have been investigated by current–voltage (I–V). The electric and dielectric measurements were carried out covering a wide range of temperature (300–600 K). Our materials display semiconducting properties as well as mixed ionic and electronic conductivity. At high temperatures, the activation energy values proved to be around 907–630 meV, which refers basically to oxygen vacancies conduction. The analysis of dielectric properties and dielectric losses (ε’, tanδ) of both compounds vs temperature at different frequencies demonstrates two relaxer attitudes. A low dielectric loss and low electrical conductance were displayed. Relying upon these values, these materials stand for perfect candidates for micro-electronics devices.

Protonation-Induced B-Site Deficiency in Perovskite-Type Oxides: Fully Hydrated BaSc0.67O(OH)2 as a Proton Conductor

One of the concepts for the design of a highly proton-conductive perovskite-type oxide is to increase the proton concentration. In this study, we focused on Ba–Sc oxide known for its brownmillerite-type structure and great potential for a high proton concentration. To achieve the highly hydrated phase, high-pressure synthesis was conducted using hydroxide as the H2O source. Although the expectation was that perovskite-type BaScO2(OH) would simply be a hydrated phase, the formation of Sc vacancies increased resulting in the greater hydration of the Ba–Sc oxide. It was likely that the fully hydrated phase was perovskite-type BaSc0.67O(OH)2. One third of the Sc sites were vacant, and the effective charge of Sc vacancies were compensated by the excess protons. First principles calculation revealed that the protons residing in the Sc vacancies were orientated toward a Sc vacancy and compensated the B-site charge in a manner similar to that of hydrogarnet substitution. This B-site deficiency caused by the extensive hydration condition is a novel mechanism to increase the proton concentration of perovskite-type proton conductors. The electrical conductivity, 10–2 S·cm–1 at 450–500 °C, was comparable to that of acceptor-doped BaZrO3, although the conductivity discontinuously changed at dehydration temperature.

Oxygen storage capacity and thermal stability of brownmillerite-type Ca2(Al1-xGax)MnO5+δ oxides

Understanding the oxygen uptake/release mechanism in oxygen storage materials is of great importance in the design of energy-related materials and their corresponding applications. In this work, the effects of Ga doping amount on the oxygen storage capacity and thermal stability of Ca2(Al1-xGax)MnO5+δ (0 ≤ x ≤ 1) with a brownmillerite-type structure were investigated. Ca2AlMnO5+δ can reversibly store/release a large amount of excess oxygen (∼3.0 wt%) at low temperature (between 300 and 600 °C) under oxidative atmospheres. With the increasing Ga doping amount in Ca2(Al1-xGax)MnO5+δ, these materials uptake less oxygen at higher temperature which can be attributed to the difficulty in the oxidation of tetrahedral GaO4 blocks into octahedral GaO6 blocks under 1 atm O2. However, with the increasing of Ga-substitution amount, these Ca2(Al1-xGax)MnO5+δ (0 ≤ x < 1) can start to uptake oxygen at lower temperatures during the cooling process under flowing O2 due to the distorted structure. The results demonstrated that Ca2(Al1-xGax)MnO5+δ (0 ≤ x < 1) can reversibly store/release large amounts of oxygen via just controlling the surrounding temperature and/or oxygen partial pressure but without using reductive gases, which would enable them great potentials in many applications.

Evaluation of cobalt oxides for calcium battery cathode applications

The identification of potential cathode materials is a requirement for the development of a rechargeable calcium based battery technology. In this work, we use Density Functional Theory (DFT) calculations to explore the electrode characteristics of three ternary calcium cobalt oxides with distinct CoO dimensionality: 3D-Ca2Co2O5 (brownmillerite type structure), 2D-Ca3Co4O9 (a misfit compound) and 1D-Ca3Co2O6 (K4CdCl6 structural type). For the three compounds calculations predict Co3+/Co4+ voltages in the 3–4 V range, with a volume variation below 8% upon Ca deinsertion. Further Co4+ oxidation is predicted at too high voltages not reachable in practice. The maximum specific capacities are therefore 192 mAh/g (Ca2Co2O5), 165 mAh/g (Ca3Co4O9) and 160 mAh/g (Ca3Co2O6). The potential application of Ca2Co2O5 is discarded based on a large energy barrier for Ca diffusion (1.3 eV). With energy barriers for Ca diffusion of 0.9 eV, the 2D and 1D oxides are appealing as low rate cathode materials. To complete a previous experimental investigation, we have analyzed the reversibility of the Ca deinterclation reaction of 1D-Ca3Co2O6. It is found that a phase transformation takes place upon decalciation driven by the change in the electronic configuration of Co ions (from High Spin-trigonal prismatic Co3+ to octahedral Low Spin-Co4+) and involving the Ca diffusion pathways.

Transition-Metal Distribution in Brownmillerite Ca2FeCoO5.

Ca2Fe2-xCoxO5 (0 ≤ x ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral (Oh) and tetrahedral (Td) sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in Ca2FeCoO5 by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the Td and Oh sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the Td site shows the better OER activity, and the optimum annealing temperature for more OER active Ca2FeCoO5 should be higher than the crossover temperature.

Electrical properties of the ordered oxygen-deficient perovskite Ca2Fe0.5Ga1.5O5

Ca2Fe0.5Ga1.5O5 is an oxygen-deficient perovskite, where the defects generated due to oxygen-deficiency are distributed in an ordered fashion. Neutron diffraction experiments indicate that the defect-order results in the formation of alternating (Ga)O4 tetrahedral and (FeGa)O6 octahedral units, forming the so-called brownmillerite-type structure. This material represents the highest degree of Ga-doping in the brownmillerite compound Ca2Fe2O5, which can be achieved using solid-state synthesis method. X-ray photoelectron spectroscopy (XPS) combined with iodometric titration was employed to determine the Fe oxidation state and the oxygen-content in Ca2Fe0.5Ga1.5O5. The XPS studies show that Fe is predominantly in trivalent state, and the iodometric titrations indicate that the oxygen stoichiometry is 5.07 per formula unit, consistent with primarily trivalent Fe. Variable-temperature electrical conductivity studies of Ca2Fe0.5Ga1.5O5 have been performed in a wide temperature range, 25–800 °C, indicating semiconducting behavior and significant contribution of ionic conductivity to total conductivity of this material.

Structural and electrical characterization of the Co-doped Ca2Fe2O5 brownmillerite: Evaluation as SOFC -cathode materials

Brownmillerite-type oxides Ca2Fe2-xCoxO5-δ (x = 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4) have been explored as possible cathodes for solid oxide fuel cells (SOFC). The samples have been prepared, characterized and tested as cathode materials in single solid-oxide fuel cells. As shown in a neutron powder diffraction (NPD) study at RT, for x = 0 and 0.2 the compounds crystallized in a single phase with brownmillerite-type structure (s.g. Pcmn), whereas for x = 0.4, 0.6, 0.8, 1, 1.2, and 1.4 the samples crystallized in a supercell, two times the c-axis and the volume of that for a typical brownmillerite (s.g Pcmb). This superstructure consists of Fe1O4 and Co1O4 tetrahedral layers containing Fe1 and Co1 atoms, which alternate with (Fe,Co)2O6 and (Fe,Co)3O6 octahedral layers. In an “in situ” NPD experiment of Ca2Fe0.8Co1.2O5-δ at the working temperature of the SOFC, this compound shows the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms that suggest a large lability and mobility. In single test cells these cathode materials generated a maximum power of 412 mW/cm2 at 850 °C with pure H2 as a fuel. The electrodes were supported on a 300-mm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM). The measured thermal expansion coefficients between 300 and 850 °C exhibit an excellent chemical compatibility with the electrolyte. The obtained compounds display a semiconductor-like behavior with conductivity values at the SOFCs working temperatures (650–850 °C) that are sufficient to yield a good performance in IT-SOFC.

HR-TEM study of oxygen vacancy ordered Sr4+nMn4+nO10+3n compounds

The study of oxygen vacancy ordering in the LaxSr1-xMnOy system has shown a strong correlation between Mn formal valence and coordination to oxygen. The Mn2+ was found forming octahedra (Oc) and tetrahedra (Te), Mn3+ octahedra and pyramids (Py) and Mn4+ only octahedra. This tendency was derived from the presence of three kinds of vacancy orderings in the system. For y&gt;2.5 and the average Mnm+ charge of 2&lt;m&lt;3 (x&gt;0.5), brownmillerite-type structures are found with Mn2+/3+ Oc and Mn2+ Te. [1] For 3&lt;m&lt;4 (x&lt;0.5) structures of the homologous Sr4+nMn4+nO10+3n –type series are found with Mn3+ Py and Mn3+/4+ Oc. [2] For y&lt;2.5 and 2&lt;m&lt;3 (x&lt;0.5) a complex layered structure with OcTeOcOcTe'Oc ordering and step defects of Mn3+ Py and Mn2+ Te [3] is observed. Brownmillerite-type structure is absent in the Sr-rich region since mostly Mn3+ is present, which does not show tendency to form Te. Compounds of the Sr4+nMn4+nO10+3n series have been described as arrangements of groups of four Py and n Oc in symmetrical patterns. [2] This description did not elucidate the crystal chemistry reasons for specific pattern (n=0, 1 and 3) since it neglected the coordination stabilization associated with oxygen vacancy, charge and orbital ordering observed in the structures.[2] Using high-resolution transmission electron microscopy (HR-TEM) for SrMnOy with y values located between corresponding n=0, 1 and 3 compositions, we have determined that the oxygen vacancy ordering directs the formation of these patterns. The structural patterns can be described as perovskites with lines of oxygen vacancies along [001] with nearest lines of vacancies in the cubic (310) plane. Successive (310) planes are n perovskite blocks (Oc) apart in the [010] direction. This ordering pattern allows the coherent growth of phases with different n at the sides of (310) plane as observed in grains of a sample of SrMnO2.668 where n=1 and n=3 phases grown coherently one at each side of the vacancy plane.

Stoichiometry dependent Co3+spin-state in LaxSr2−xCoGaO5+δbrownmillerite phases

Phases in the LaxSr2-xCoGaO5+δ (0.5 ≤ x ≤ 1) series adopt anion-vacancy ordered brownmillerite-type structures. The x = 0.5 and x = 0.6 phases show only small deviations from the ideal A2B2O5 brownmillerite stoichiometry and exhibit paramagnetic behaviour above 100 K, with magnetic susceptibilities indicative of a high-spin, S = 2 configuration for the Co(3+) centres present. In contrast, lanthanum-rich phases (x ≥ 0.7) incorporate increasing amounts of 'additional' oxygen and show much smaller paramagnetic susceptibilities, consistent with the Co(3+) centres present adopting low-spin S = 0 spin states. The evolution with lanthanum concentration of both the oxygen stoichiometry and the magnetic susceptibility of phases is rationalised on the basis of the lattice strain which builds across the compositional series. This lattice strain acts to couple the Co(3+) spin state to the oxygen stoichiometry in the range 0.65 < x < 0.85 giving materials oxygen sensing capability.

Crystal structure and high-temperature electrical conductivity of novel perovskite-related gallium and indium oxides

Novel complex oxides Sr2Ga1+x In1−x O5, x = 0.0–0.2 with brownmillerite-type structure were prepared in air at T = 1,273 K, 24 h. Study of the crystal structure of Sr2Ga1.1In0.9O5 refined using X-ray powder diffraction data (S.G. Icmm, a = 5.9694(1) A, b = 15.2091(3) A, c = 5.7122(1) A, χ 2 = 2.48, R F 2 = 0.0504, R p = 0.0458) revealed ordering of Ga3+ and In3+ cations over tetrahedral and octahedral positions, respectively. A partial replacement of Sr2+ by La3+ according to formula Sr1−y La y Ga0.5In0.5O2.5+y/2, leads to the formation of a cubic perovskite (a = 4.0291(5) A) for y = 0.3. No ordering of oxygen vacancies or cations was observed in Sr0.7La0.3Ga0.5In0.5O2.65 as revealed by electron diffraction study. The trace diffusion coefficient (D T) of oxygen for cubic perovskite Sr0.7La0.3Ga0.5In0.5O2.65 is in the range 2.0 × 10−9–6.3 × 10−8 cm2/s with activation energy 1.4(1) eV as determined by isotopic exchange depth profile technique using secondary ion mass spectrometry at 973–1,223 K. These values are close to those reported for Ca-doped ZrO2. High-temperature electrical conductivity of Sr0.7La0.3Ga0.5In0.5O2.65 studied by AC impedance was found to be nearly independent on oxygen partial pressure. Calculated values of activation energy at T < 1,073 K for hole and oxide-ion conductivities are 0.96 and 1.10 eV, respectively.

Oxygen Storage Capability of Brownmillerite-type Ca2AlMnO5+δ and Its Application to Oxygen Enrichment

The oxygen storage capability was investigated for Ca2(AlxMn1–x)2O5+δ (0.50 ≤ x ≤ 0.67) with a Brownmillerite-type structure. This oxide can store/release a large amount of excess oxygen (∼3.0 wt %) topotactically in response to variations in temperature and the surrounding atmosphere in a highly reversible manner. The capacity and response of oxygen storage are remarkable only in the vicinity of x = 0.50, that is, Ca2AlMnO5+δ, and rapidly deteriorated as the Al content increases. Owing to the high sensitivity in terms of oxygen nonstoichiometry, Ca2AlMnO5+δ exhibits oxygen intake/release ability when temperature swing between 500 and 700 °C is applied. With this characteristic feature of this oxide, a facile method for oxygen enrichment is demonstrated.

Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: a Play in the Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites

Three different perovskite-related phases were isolated in the SrGa(1-x)Sc(x)O(2.5) system: Sr(2)GaScO(5), Sr(10)Ga(6)Sc(4)O(25), and SrGa(0.75)Sc(0.25)O(2.5). Sr(2)GaScO(5) (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å] with complete ordering of Sc(3+) and Ga(3+) over octahedral and tetrahedral positions, respectively. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. I4(1)/a, a = 17.517(1) Å, c = 32.830(3) Å]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) can be described as a stacking of eight perovskite layers along the c axis ...[-(Sc/Ga)O(1.6)-SrO(0.8)-(Sc/Ga)O(1.8)-SrO(0.8)-](2).... Similar to Sr(2)GaScO(5), this structure features a complete ordering of the Sc(3+) and Ga(3+) cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr(10)Ga(6)Sc(4)O(25) is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc(3+) by Ga(3+) leads to the formation of the cubic perovskite phase SrGa(0.75)Sc(0.25)O(2.5) (x = 0.25) with a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa(0.75)Sc(0.25)O(2.5)·xH(2)O hydrate, which exhibits a proton conductivity of ∼2.0 × 10(-6) S/cm at 673 K.

The n = 3 member of the SrCoO (3 n − 1)/ n series of layered oxygen-defect perovskites

Topotactic oxygen deintercalation from cubic perovskite oxide SrCoO 3 − δ ( δ < 0.25) is realized in a controlled way at low temperatures. Annealing in air at 200 < T < 275 °C results in a three-phase region within 0.25 < δ < 0.50. Two of the phases are known previously, namely SrCoO 2.5, a phase with an orthorhombic brownmillerite-type (BM) structure, and SrCoO 2.75 (T1), a phase with a slight tetragonal distortion from cubic symmetry. The third phase (T2) has a strong tetragonal distortion, and appears to be the missing n = 3 member of the SrCoO (3 n − 1)/ n series. Once the overall oxygen content reaches the value of (3 − δ) aver = 2.68(1), the T1-to-T2 phase conversion proceeds at constant (3 − δ) aver. A more pronounced evolution of the c T2 lattice parameter as compared to the a T2 parameter implies a layered oxygen-vacancy-ordered structure for T2 with some flexibility for oxygen-content variation, in the style of the BM phase.

A novel one step synthesized Co-free perovskite/brownmillerite nanocomposite for solid oxide fuel cells

Cobalt-free perovskite oxides Pr1−xCaxFe0.8Ni0.2O3 (PCFN) were investigated as novel cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). Ca and Ni substitution in the PrFeO3 material shows that a wide range of perovskites Pr1−xCaxFe0.8Ni0.2O3 (0 < x < 0.9) can be prepared by sintering in air at 600 °C. Perovskites with 0 < x < 0.4 exhibit orthorhombic single phases (Pnma space group), whereas 0.4 < x < 0.9 show a coexistence of the perovskite and the brownmillerite-type structure (Ca2Fe2O5). The structure of the polycrystalline powders was analyzed by X-ray powder diffraction, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The analysis on the synthesized powders shows the presence of clusters formed by 30–100 nm nanoparticles. High-Resolution TEM (HRTEM) studies were carried out to confirm the existence of Ca2Fe2O5. The dc four-probe measurement exhibits a total electrical conductivity, over 100 S cm−1 at T ≥ 600 °C when 0 < x < 0.6, pointing out that strontium can be substituted for calcium without modifying the electrochemical properties. The Pr0.4Ca0.6Fe0.8Ni0.2O3/Ca2Fe2O5 composite cathode presents the better performance in electrochemical measurements, showing an area specific resistance value of 0.09 ohm cm2 at 850 °C.

Oxyanion doping strategies to enhance the ionic conductivity in Ba2In2O5

In this paper we report the successful incorporation of phosphate and sulfate groups into the ionic conductor, Ba2In2O5, with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. The results show that such oxyanion incorporation leads to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, and hence increases the conductivity at temperatures <800 °C. In wet atmospheres, there is evidence for a significant enhancement of the conductivity through a protonic contribution.

On the role of lattice dynamics on low-temperature oxygen mobility in solid oxides: a neutron diffraction and first-principles investigation of $$ {\hbox{L}}{{\hbox{a}}_2}{\hbox{Cu}}{{\hbox{O}}_{{4 + \delta }}} $$

The structure of oxygen-intercalated La2CuO4.07 has been investigated at 20 and 300 K by neutron diffraction on an electrochemically oxidized single crystal. At 20 K, reconstruction of the nuclear density by maximum entropy method shows strong displacements of the apical oxygen atoms towards [100] with respect to the F-centred unit cell, whilst displacements towards [110] and [100] were both found to be present at ambient temperature. Combining structural studies with first-principles lattice dynamical calculations, we interpret the displacements of the apical oxygen atoms to be at least partially of dynamic origin already at ambient temperature. Strong displacements of the apical oxygen atoms of stoichiometric and oxygen-doped \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) and corresponding associated lattice instabilities, i.e. low-energy phonon modes, are considered as a general prerequisite of low-temperature oxygen diffusion mechanisms. Lattice dynamical calculations on \( {\hbox{L}}{{\hbox{a}}_{{2}}}{\hbox{Cu}}{{\hbox{O}}_{{{4} + \delta }}} \) suggest that the oxygen species diffusing at low temperature are not the interstitial but, more prominently, the apical oxygen atoms. The presence of interstitial oxygen atoms is, however, important to amplify via specific, low-energy phonon modes, a dynamic exchange mechanism between apical and vacant interstitial oxygen sites, thus allowing a dynamically triggered, shallow potential oxygen diffusion pathway. The crucial role of lattice dynamics to enable low-temperature oxygen mobility in K2NiF4-type oxides is discussed on a microscopic scale and compared to similar low-temperature oxygen diffusion mechanisms, recently proposed for non-stoichiometric oxides with Brownmillerite-type structure.

Time Resolved in Situ XAFS Study of the Electrochemical Oxygen Intercalation in SrFeO2.5 Brownmillerite Structure: Comparison with the Homologous SrCoO2.5 System

Low temperature oxygen ionic conductors are key materials for the development of the next generation solid oxide fuel cells. In this regard, SrMO2.5 (M = Fe,Co) systems with a Brownmillerite-type structure are able to reversibly intercalate oxygen in an electrochemical reaction at room temperature to reach SrMO3 stoichiometry. To understand and characterize this behavior, at the atomic level, in situ X-ray absorption spectroscopy experiments during the electrochemical oxidation reaction were performed for both SrFeO2.5 and SrCoO2.5 compounds at the Fe and Co K-edge, respectively. The comparative analysis of the two experiments allowed us to emphasize the similarities and differences observed during electrochemical oxidation of the two parent compounds. The data were analyzed both in XANES and EXAFS regions to extract both electronic configuration and local order information. To extract as much information as possible from collected data, the standard linear combination of spectra was complemented by the principal component analysis advanced method, which allowed us to clarify some aspects of the reaction process that were otherwise hidden. As for SrCoO2.5+x ordered intermediates (x = 0.25, 0.375) that could have been identified by neutron diffraction and XAFS experiments [J. Am. Chem. Soc. 2006, 128, 13161], no ordered intermediates of the homologous SrFeO2.5+x different from an oxygen deficient perovskite phase have been reported to occur during the oxygen intercalation reaction, except on a very local level. However, a detailed fit of the EXAFS signals for starting and final phases showed that the final fully oxidized compound (with stoichiometry SrCoO3) has been obtained for SrCoO2.5, whereas for SrFeO2.5, the reaction ended before the expected charge transfer. We interpret the formation of SrFeO3 to be accompanied by the parasitic formation of an unknown phase, containing Fe(III). Furthermore we were able to highlight that, at a local level, the most probable space group for SrCoO2.5 at ambient temperature is Imma, as the average local environments extracted from Pnma and I2mb models were not able to reproduce the experimental EXAFS spectrum.

Silicon Doping in Ba2In2O5: Example of a Beneficial Effect of Silicon Incorporation on Oxide Ion/Proton Conductivity

In the solid oxide fuel cell (SOFC) field, the presence of silicon impurities is commonly considered to be detrimental to the performance due to segregation as silica at the grain boundaries. In this paper we demonstrate, however, that silicon doping into Ba2In2O5 leads to a significant enhancement in the oxide ion conductivity. The results indicate that silicon is incorporated into the structure leading to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, with the oxygen vacancy disordering leading to the conductivity enhancement. In wet atmospheres, the conductivity is further enhanced through a protonic contribution, leading to conductivities (2.4 × 10−3 Scm−1 at 400 °C) comparable to the best perovskite proton conductors. Thus, the results show that silicon can be incorporated into the perovskite structure, suggesting further studies in this area are warranted, particularly related to electrode materials.

Negative pressure driven phase transformation in Sr dopedSmCoO3

Atomistic computer simulation techniques based on energy minimizationprocedures are utilized for the structural investigation of perovskite-typeSmCoO3. Areliable potential model is derived which reproduces both cubic as well as orthorhombic phases ofSmCoO3. We observe a negative chemical pressure induced structural phase transformation fromdistorted perovskite (orthorhombic) to perfect perovskite (cubic) due to the substitution ofSr2 + atthe Sm3 + sites. However, external hydrostatic pressure shows isotropic compression and nopressure-induced structural transformation is observed up to 100 GPa. To maintain theelectroneutrality of the system, charge compensation is through oxygen vacancies whichresults in the brownmillerite-type structure. A defect model is proposed, which is consistentwith experimental results. The solution energies for divalent and trivalent cations are alsocalculated. These results show that the cations having ionic radii less than 0.75 Å will occupythe Co sites and those with ionic radii larger than 0.75 Å will substitute at the Sm sites.