Pseudocapacitive Properties of Isostructural Oxides Sr2 LaBMnO7 (B=Co, Fe).

Pseudocapacitors promise to fill the gap between traditional capacitors and batteries by delivering reasonable energy densities and power densities. In this work, pseudocapacitive charge storage properties are demonstrated for two isostructural oxides, Sr2 LaFeMnO7 and Sr2 LaCoMnO7 . These materials comprise spatially separated bilayer stacks of corner sharing BO6 units (B=Fe, Co or Mn). The spaces between stacks accommodate the lanthanum and strontium ions, and the remaining empty spaces are available for oxide ion intercalation, leading to pseudocapacitive charge storage. Iodometric titrations indicate that these materials do not have oxygen-vacancies. Therefore, the oxide ion intercalation becomes possible due to their structural features and the availability of interstitial sites between the octahedral stacks. Electrochemical studies reveal that both materials show promising energy density and power density values. Further experiments through fabrication of a symmetric two-electrode cell indicate that these materials retain their pseudocapacitive performance over hundreds of galvanostatic charge-discharge cycles, with little degradation even after 1000 cycles.

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction.

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A2Fe2O5 (A = Sr, Ba).

Significant enhancements of electrocatalytic activities for both half-reactions of water-electrolysis, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as pseudocapacitive charge-storage properties are demonstrated upon changing the structural order in a perovskite-type system. The structural change is prompted by the increase in the ionic radius of the A-site ion in A2Fe2O5. The structure of Sr2Fe2O5 consists of alternating layers of FeO6 octahedra and FeO4 tetrahedra, whereas Ba2Fe2O5 comprises seven different coordination geometries for Fe. We note that the catalytically active metal, i.e., iron, and the oxygen stoichiometry are the same for both materials. Nevertheless, the change in the structural order results in significantly greater electrocatalytic activity of Ba2Fe2O5, manifested in smaller overpotentials, smaller charge-transfer resistance, greater electrocatalytic current, and faster reaction kinetics. In addition, this material shows significantly enhanced pseudocapacitive properties, with greater specific capacitance and energy density compared to Sr2Fe2O5. These findings indicate the important role of structural order in directing the electrochemical properties.

Effect of the Oxygen Vacancies and Structural Order on the Oxygen Evolution Activity: A Case Study of SrMnO3-δ Featuring Four Different Structure Types.

We report the facile synthesis methods of four materials, with the general formula SrMnO3-δ, which have previously been synthesized in multiple steps, involving switching between different oxidizing and reducing gases, quenching, the use of zirconium metal as a reductant, etc. However, we have shown that it is possible to synthesize all of these materials by facile processes without unnecessary complications. In fact, we have found methods of synthesizing the oxygen-deficient phases in only one step. Given the diverse range of structures that are formed for SrMnO3-δ, we have investigated the correlations between the structural order and electrocatalytic activity for the oxygen evolution reaction (OER) of water splitting. We have uncovered a systematic trend in the OER activity, where the most oxygen-deficient compound, SrMnO2.5, which features square-pyramidal coordination geometry around manganese, shows the highest OER performance. The next OER activity belongs to SrMnO2.6, which contains both MnO5 trigonal bipyramids and MnO6 octahedra. SrMnO3(cubic), containing only corner-sharing MnO6 units, shows the third best OER performance. The least activity is observed in SrMnO3(hexagonal), featuring both face- and corner-sharing MnO6 octahedra. We have also studied the electrochemically active surface area, as well as the kinetics of OER for all four materials, and found that the trend in these properties is the same as the trend in the OER activity. These findings indicate that the electrocatalytic activity is correlated with the degree of oxygen deficiency, as well as the polyhedral connectivity.

Strategies for Enhancing Lithium-Ion Conductivity of Triple-Layered Ruddlesden-Popper Oxides: Case Study of Li2-xLa2-yTi3-zNbzO10.

We report strategies of enhancing the ionic conductivity of triple-layered Ruddlesden-Popper oxides through design and synthesis of seven compounds belonging to the series A2A'2B3O10 (A = Li, A' = La, B = Ti/Nb), investigated by neutron diffraction, impedance spectroscopy, and dielectric analyses. We demonstrate, for the first time, that lithium diffusion in triple-layered Ruddlesden-Popper oxides is a result of cooperative effect of both inter- and intrastack sites, i.e., A and A'. As shown by neutron diffraction, the structure of these materials comprises triple-layered stacks of octahedra (BO6), separated by A-site cations, while A' ions reside in intrastack spaces. We first synthesized Li2La2Ti3O10 and showed that its lithium-ion conductivity can be systematically enhanced by incorporation of cation deficiency in interstack sites through synthesis of Li1.9La2Ti2.9Nb0.1O10, Li1.8La2Ti2.8Nb0.2O10, and Li1.75La2Ti2.75Nb0.25O10. The latter represents the limit of cation deficiency on the A-site and has the highest conductivity among the A-site-deficient materials. We then investigated the enhancement of lithium-ion conductivity by incorporation of cation defects in intrastack A'-sites through synthesis of Li2La1.9Ti2.7Nb0.3O10 and Li2La1.8Ti2.4Nb0.6O10, where the latter represents the limit of cation deficiency on the A'-site and has the best conductivity among the A'-deficient materials. Finally, we hypothesized that cooperative effect of defects in both inter- and intrastack sites should have an even higher impact on ionic conductivity. This hypothesis was confirmed by synthesis of Li1.9La1.9Ti2.6Nb0.4O10, which showed the highest conductivity among all materials synthesized in this work. Detailed analysis of real and imaginary components of impedance spectroscopy, as well as dielectric and loss tangent, have been conducted. This systematic study is aimed at answering a fundamental question related to materials chemistry of Ruddlesden-Popper oxides, namely, determination of the sites that contribute to ionic conductivity.

Remarkable Oxygen-Evolution Activity of a Perovskite Oxide from the Ca2-x Srx Fe2 O6-δ Series.

Herein in we report the unprecedented catalytic activity of an iron-based oxygen-deficient perovskite for the oxygen-evolution reaction (OER). The systematic trends in OER activity as a function of composition, defect-order, and electrical conductivity have been demonstrated, leading to a methodical increase in OER catalytic activity: Ca2 Fe2 O6-δ

Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in Ca2FeCoO6-δ, CaSrFeCoO6-δ, and Sr2FeCoO6-δ.

The ability to control the electrical conductivity of solid-state oxides using structural parameters has been demonstrated. A correlation has been established between the electrical conductivity and structural order in a series of oxygen-deficient perovskites using X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrical conductivity studies at a wide temperature range, 25-800 °C. The crystal structure of CaSrFeCoO6-δ has been determined, and its stark contrast to Ca2FeCoO6-δ and Sr2FeCoO6-δ has been demonstrated. The Fe/Co distribution over tetrahedral and octahedral sites has been determined using neutron diffraction. There is a systematic increase in the structural order in progression from Sr2FeCoO6-δ (δ = 0.5) to CaSrFeCoO6-δ (δ = 0.8) and Ca2FeCoO6-δ (δ = 0.9) . The oxygen contents of these materials were determined using iodometric titration and TGA. At room temperature, there is an inverse correlation between the electrical conductivity and structural order. The ordered Ca2 and CaSr compounds are semiconductors, while the disordered Sr2 compund shows metallic behavior. The metallic nature of the Sr2 material persists up to 1073 K (800 °C), while the Ca2 and CaSr compounds undergo a semiconductor-to-metal transition above 500 and 300 °C, respectively, highlighting another important impact of the structural order. At high temperature, the CaSr compound has the highest conductivity compared to the Ca2 and Sr2 materials. There appears to be an optimum degree of structural order that leads to the highest conductivity at high temperature. Another consequence of the structural order is the observation of mixed ionic-electronic conductivity in CaSr and Ca2 compounds, as is evident from the hysteresis in the conductivity data obtained during heating and cooling cycles. The average ionic radius required for each structural transition was determined through the synthesis of 21 different materials by systematic variation of the Ca/Sr ratio. In addition, SEM and XPS were employed to gain insight into the crystallite morphology and oxidation states of transition metals, revealing an interesting redox process between Fe and Co.

Transformation of Structure, Electrical Conductivity, and Magnetism in AA'Fe2O6-δ, A = Sr, Ca and A' = Sr.

The ability to control electrical properties and magnetism by varying the crystal structure using the effect of the A-site cation in oxygen-deficient perovskites has been studied in AA'Fe2O6-δ, where A = Sr, Ca and A' = Sr. The structure of Sr2Fe2O6-δ, synthesized at 1250 °C in air, contains dimeric units of FeO5 square pyramids separated by FeO6 octahedra. Here we show that this ordering scheme can be transformed by changing the A-site cations from Sr to Ca. This leads to a structure where layers of corner-sharing FeO6 octahedra are separated by chains of FeO4 tetrahedra. Through systematic variation of the A-site cations, we have determined the average ionic radius required for this conversion to be ∼1.41 Å. We have demonstrated that the magnetic structure is also transformed. The Sr2 compound has an incommensurate magnetic structure, where magnetic moments are in spin-density wave state, aligning perpendicular to the body diagonal of the unit cell. With the aid of neutron diffraction experiments at 10 and 300 K, we have shown that the magnetic structure is converted into a long-range G-type antiferromagnetic system when one Sr is replaced by Ca. In this G-type ordering scheme, the magnetic moments align in the 001 direction, antiparallel to their nearest neighbors. We have also performed variable-temperature electrical conductivity studies on these materials in the temperature range 298-1073 K. These studies have revealed the transformation of charge transport properties, where the metallic behavior of the Sr2 compound is converted into semiconductivity in the CaSr material. The trend of conductivity as a function of temperature is reversed upon changing the A-site cation. The conductivity of the Sr2 compound shows a downturn, while the conductivity of the CaSr material increases as a function of temperature. We have also shown that the CaSr compound exhibits temperature-dependent behavior typical of a mixed ionic-electronic conducting system.

Dopant Concentration-Porosity-Li-Ion Conductivity Relationship in Garnet-Type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1 M LiCl.

Highly Li-ion conductive Y-doped garnet-type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) were studied to understand the effects of yttrium- and lithium-doping on crystal structure, porosity, and Li-ion conductivity using (7)Li MAS NMR, electrochemical ac impedance spectroscopy, and scanning electron microscopy (SEM), as well as ex situ and in situ powder X-ray diffraction (PXRD) to further explore the potential application of garnets in all-solid-state Li-ion batteries. Solid-state (7)Li MAS NMR studies showed an increase in the Li-ion mobility as a function of Y- and Li-doping in Li5+2xLa3Ta2-xYxO12, which is consistent with the results from ac impedance spectroscopy. The SEM studies on sintered pellets indicated a systematic decrease in porosity and an increase in sinterability as the Y- and Li-doping levels increase in Li5+2xLa3Ta2-xYxO12. These results are consistent with the calculated porosity and densities using the Archimedes method. Using the variable-temperature in situ PXRD in the temperature range of 30-700 °C, a thermal expansion coefficient of 7.25 × 10(-6) K(-1) was observed for Li6La3Ta1.5Y0.5O12. To further explore the possibility of a new application for the Li-stuffed garnets, the stability of these materials in aqueous LiCl solution was also studied. A high degree of structural stability was observed in these materials upon 1 M LiCl treatment, making them suitable candidates for further studies as protective layers for lithium electrodes in aqueous lithium batteries.

Evaluation of fundamental transport properties of Li-excess garnet-type Li(5+2x)La(3)Ta(2-x)Y(x)O(12) (x = 0.25, 0.5 and 0.75) electrolytes using AC impedance and dielectric spectroscopy.

The fundamental electrical transport properties including ionic conductivity, dielectric constants, loss tangent, and relaxation time constants of Li-excess garnet-type cubic (space group Ia3̄d) Li5+2xLa3Ta2-xYxO12 (x = 0.25, 0.5 and 0.75) have been studied in the temperature range of -50 to 50 °C using electrochemical AC impedance spectroscopy. A correlation has been established between the excess Li content and the Li(+) ion migration pathways. The loss tangent (tan δ) for all samples exhibits a relaxation peak corresponding to the dielectric loss because of dipolar rotations due to Li(+) migration. Comparing the modulus analysis of Li-excess garnets with fluorite-type oxygen ion conductors, we propose the local migration of Li(+) ions between octahedral sites around the "immobile" Li(+) ions in tetrahedral (24d) sites. In the samples with x = 0.25 and 0.5, Li(+) ions seem to jump from one octahedral (96h) site to another bypassing the tetrahedral (24d) site between them (path A), both in local and long-range order migration processes, with activation energies of ∼0.69 and 0.54 eV, respectively. For the x = 0.75 member, Li(+) ions exhibit mainly long-range order migration, with an activation energy of 0.34 eV, where the Li hopping between two octahedral sites occurs through the edge which is shared between the two LiO6 octahedra and a LiO4 tetrahedron (path B). The present AC impedance analysis is consistent with the ab initio theoretical analysis of Li-excess garnets that showed two conduction paths (A and B) for Li ion conduction with different activation energies.

Vacancy effect on the electrocatalytic activity of LaMn1/2Co1/2O3-δ for hydrogen and oxygen evolution reactions.

Development of efficient electrocatalysts for water splitting can be a significant step toward green hydrogen generation. In this work, a remarkable enhancement of electrocatalytic properties is achieved through the incorporation of oxygen-vacancies in a perovskite oxide, while maintaining the same structural framework. The oxygen-deficient material La2MnCoO6-δ (LaMn0.5Co0.5O3-δ) is isostructural to the parent stoichiometric material, but shows drastically enhanced electrocatalytic properties for both half-reactions of water-splitting, namely hydrogen-evolution and oxygen-evolution reactions, due to the oxygen-vacancies.

Directing the size and dispersity of silver nanoparticles with kudzu leaf extracts.

Kudzu is an abundant and invasive species in the Southeastern United States. The prospective use of kudzu as a non-toxic, green and biocompatible reducing and stabilizing agent for one-pot Ag nanoparticle synthesis was investigated. Ag nanoparticles were synthesized using aqueous and ethanolic kudzu leaf and stem extracts. The size and dispersity of the synthesized nanoparticles were found to depend on the extract used. Ultraviolet-visible and Fourier transform infrared spectroscopies were used to characterize the extracts. Surface-enhanced fluorescence and Raman scattering were used to characterize the surface species on synthesized Ag nanoparticles. The primary reducing and stabilizing agents in aqueous kudzu leaf extracts were determined to be reducing sugars and saponins which result in Ag nanoparticles with average diameters of 21.2 ± 4.8 nm. Ethanolic kudzu leaf extract was determined to be composed of chlorophyll, reducing sugars and saponins, producing Ag nanoparticles with average diameters of 9.0 ± 1.6 nm. Control experiments using a chlorophyllin standard as the reducing and stabilizing agent reveal that chlorophyll has a key role in the formation of small and monodisperse Ag nanoparticles. Experiments carried out in the absence of light demonstrate that reducing sugars and saponins also contribute to the formation of Ag nanoparticles in ethanolic kudzu leaf extracts. We propose a mechanism by which reducing sugars donate electrons to reduce Ag+ leading to the formation of Ag nanoparticles, forming carboxylic acid sugars which stabilize and partially stabilize Ag nanoparticles synthesized with aqueous and ethanolic kudzu leaf extracts, respectively. In the ethanolic extract, photoexcited chlorophyll serves as a co-reducing and co-stabilizing agent, leading to small and monodisperse Ag nanoparticles.

Systematic study of compositional and synthetic control of vacancy and magnetic ordering in oxygen-deficient perovskites Ca2Fe(2-x)Mn(x)O(5+y)and CaSrFe(2-x)Mn(x)O(5+y) (x = 1/2, 2/3, and 1; y = 0-1/2).

Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca(2)Fe(2-x)Mn(x)O(5) and CaSrFe(2-x)Mn(x)O(5+y), where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T(d) chain (vacancy) disorder. In the special case of CaSrFeMnO(5) the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ~160 Å. This reveals a systematic progression from Ca(2)FeMnO(5) (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO(5) (Icmm, disordered tetrahedral chains, overall short-range order) to Sr(2)FeMnO(5) (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T(c), decreases for the same x when Sr substitutes for one Ca. A review of the changes in T(c) for the series Ca(2)Fe(2-x)M(x)O(5), taking into account the tetrahedral/octahedral site preferences for the various M(3+) ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca(2)Fe(2-x)Mn(x)O(5) and the CaSrFe(2-x)Mn(x)O(5) series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca(2) phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ~50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) Å.

A vacancy-disordered, oxygen-deficient perovskite with long-range magnetic ordering: local and average structures and magnetic properties of Sr2Fe1.5Cr0.5O5.

The local and average crystal structures and magnetic properties of the oxygen-deficient perovskite Sr(2)Fe(1.5)Cr(0.5)O(5+y) were studied using powder X-ray and neutron diffraction, neutron-pair distribution function analysis, and electron energy-loss spectroscopy. This material crystallizes in the cubic Pm3m space group, with a = 3.94491(14) Å. The oxygen vacancies are distributed randomly throughout the perovskite-type structure, and the average coordination number of the Fe(Cr) sites is 5. Refinement of the neutron diffraction data indicates y ∼ 0.05. This is in discordance with an earlier report on a material with the same nominal composition and cell constant. Electron energy-loss Cr L(2,3)-edge spectroscopy shows that Cr(3+) is present, which is also contrary to previous speculation. Neutron-pair distribution function studies show that a brownmillerite-like model involving ordered vacancies and alternating octahedral and tetrahedral coordination at the metal sites, gives a better description of the local structure out to ∼5 Å. A remarkable phenomenon determined by neutron diffraction in Sr(2)Fe(1.5)Cr(0.5)O(5) is the occurrence of a long-range G-type antiferromagnetic ordering with T(c) ≈ 565 K because cubic oxygen-deficient perovskites with B-site disorder usually do not undergo transitions to magnetically ordered states. The observation of long-range antiferromagnetic order and the T(c) value are in accordance with previous Mössbauer spectroscopic studies.

Synthesis, crystal structure, and magnetic properties of Li3Mg2OsO6, a geometrically frustrated osmium(V) oxide with an ordered rock salt structure: comparison with isostructural Li3Mg2RuO6.

The novel osmium-based oxide Li(3)Mg(2)OsO(6) was synthesized in polycrystalline form by reducing Li(5)OsO(6) by osmium metal and osmium(IV) oxide in the presence of stoichiometric amounts of magnesium oxide. The crystal structure was refined using powder X-ray diffraction data in the orthorhombic Fddd space group with a = 5.88982(5) Å, b = 8.46873(6) Å, and c = 17.6825(2) Å. This compound is isostructural and isoelectronic with the ruthenium-based system Li(3)Mg(2)RuO(6). The magnetic ion sublattice Os(5+) (S = 3/2) consists of chains of interconnected corner- and edge-shared triangles, which brings about the potential for geometric magnetic frustration. The Curie-Weiss law holds over the range 80-300 K with C = 1.42(3) emu·K/mol [µ(eff) = 3.37(2) µ(B)] and θ(C) = -105.8(2) K. Below 80 K, there are three anomalies at 75, 30, and 8 K. Those at 75 and 30 K are suggestive of short-range antiferromagnetic correlations, while that at 8 K is a somewhat sharper maximum showing a zero-field-cooled/field-cooled divergence suggestive of perhaps spin freezing. The absence of magnetic Bragg peaks at 3.9 K in the neutron diffraction pattern supports this characterization, as does the absence of a sharp peak in the heat capacity, which instead shows only a very broad maximum at ∼12 K. A frustration index of f = 106/8 = 13 indicates a high degree of frustration. The magnetic properties of the osmium phase differ markedly from those of the isostructural ruthenium material, which shows long-range antiferromagnetic order below 17 K, f = 6, and no unusual features at higher temperatures. Estimates of the magnetic exchange interactions at the level of spin-dimer analysis for both the ruthenium and osmium materials support a more frustrated picture for the latter. Errors in the calculation and assignment of the exchange pathways in the previous report on Li(3)Mg(2)RuO(6) are identified and corrected.

Perovskite Oxides as Electrocatalysts for Hydrogen Evolution Reaction.

Hydrogen generation through electrocatalytic splitting of water, i.e., hydrogen evolution reaction (HER), is an attractive method of converting the electricity generated from renewable sources into chemical energy stored in hydrogen molecules. A wide variety of materials have been studied in an effort to develop efficient and cost-effective electrocatalysts that can replace the traditional platinum/carbon catalyst. One family of functional materials that holds promise for this application is perovskite oxides. This mini-review discusses some of the progress made in the development of HER electrocatalysts based on perovskite oxides in the past decade. Given the diverse range of possible compositions of perovskite oxides, various studies have focused on compositional modifications to develop single-phase catalysts, whereas others have investigated heterostructures and composites that take advantage of synergistic interactions of different compounds with perovskite oxides. The recent advances indicate that this family of materials have great potential for utilization in HER electrocatalysis.

Local and average structures and magnetic properties of Sr2FeMnO(5+y), y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the effect of the A-site cation.

Sr(2)FeMnO(5+y) was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm ̅3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a(0) = 3.89328(1) Å for argon (y = 0.0) and a(0) = 3.83075(3) Å for air (y = 0.5). In contrast, Ca(2)FeMnO(5) retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr(2)FeMnO(5.0) oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr(2)FeMnO(5.0)(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr(2)FeMnO(5+y)(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr(2)FeMnO(5.0)(Ar), whereas Mn(4+) is predominantly present for Sr(2)FeMnO(5.5)(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca(2)FeMnO(5) is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr(2)FeMnO(5.0)(Ar) and 100 K for Sr(2)FeMnO(5.5)(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr(2)FeMnO(5.0)(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr(2)FeMnO(5.5)(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K.

Variation of the electrocatalytic activity of isostructural oxides Sr2LaFeMnO7 and Sr2LaCoMnO7 for hydrogen and oxygen-evolution reactions.

The effect of the electronic configurations of transition metals on electrocatalytic activity, charge transport, and magnetic properties is demonstrated through the investigation of Sr2LaFeMnO7 and Sr2LaCoMnO7. The two compounds are isostructural and contain bilayer stacks of octahedrally coordinated transition metals. Despite their structural similarity, the magnetic transition temperature of Sr2LaCoMnO7 is significantly lower than that of Sr2LaFeMnO7. The electrical charge-transport properties are also different, where Sr2LaCoMnO7 shows considerably improved electrical conductivity. Importantly, the electrocatalytic activities for the two half-reactions of water-splitting, i.e., the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER), are improved in Sr2LaCoMnO7 compared to Sr2LaFeMnO7. In addition, better kinetics for the HER and OER are observed for Sr2LaCoMnO7, as evaluated by the Tafel method. Furthermore, the electrochemically active surface area (ECSA) shows an enhancement for Sr2LaCoMnO7. Therefore, the trends in electrical charge transport, the HER and OER activities, kinetics and ECSA are all similar, indicating the improved properties of Sr2LaCoMnO7. These changes are explained in the context of a greater bond covalency in this material due to the higher electronegativity of Co, which results in a better overlap between the transition metal d orbital and oxygen p orbital. The relation between the electrocatalytic performance and the optimum eg orbital occupancy in Sr2LaCoMnO7 is also discussed.

Experimental and theoretical investigation of lithium-ion conductivity in Li2LaNbTiO7.

In line with efforts to discover new classes of lithium-conducting solids, this work utilizes density functional theory (DFT), neutron diffraction, and electrochemical impedance spectroscopy to study the lithium ion mobility in the new Ruddlesden-Popper-type oxide, Li2LaNbTiO7, where bilayer stacks of (Nb/Ti)O6 octahedra are separated by layers of lithium ions. In this material, the lithium hopping distances have been shortened by incorporation of ions with smaller ionic radii, leading to a pronounced improvement in Li-ion conductivity in this family of oxides. Further enhancement of Li-ion conductivity was achieved by creation of defects in the lithium layer through the synthesis of Li1.8LaNb1.2Ti0.8O7, which contains 10% lithium-deficiency. The defects facilitate the mobility of lithium ions, leading to greater Li-ion conductivity. Detailed analysis of real and imaginary components of impedance spectroscopy and dielectric properties highlight the impact of these strategies. Neutron diffraction helps with accurate determination of lithium positions given that lithium is nearly invisible to laboratory X-ray diffraction. DFT calculations show a wide band gap, and elucidate the direction of lithium diffusion in the material lattice, as well as the energy barriers associated with Li diffusion. These calculations show that lithium diffusion occurs parallel to b and c axes of the unit cell. They also show an energy bottleneck, which is a result of close proximity of lithium and oxygen in inter-stack spaces.