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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Comparative investigation of Ga- and In-CHA in the non-oxidative ethane dehydrogenation reaction

Ga- and In-exchanged chabazite (CHA) zeolites with same Si/Al and metal/Al ratios were prepared via the incipient wetness impregnation method, were characterized using N 2 adsorption, electron microscopy, temperature-programed reactions and were evaluated for the ethane dehydrogenation reaction using flow microreactors. Ga-CHA has higher reaction rates and a lower activation energy of 107 kJ/mol than In-CHA (E a = 175 kJ/mol). Rietveld refinement of the X-ray powder diffraction pattern shows that the In + cation is predominantly located above the 6-ring of the CHA cage. It is proposed that the reaction proceeds through the alkyl mechanism based on stability of alkyl hydride intermediates as determined using DFT calculations. The oxidative addition of ethane to the metal shows much lower Gibbs free energy for Ga-CHA (+27.95 kJ/mol) vs In-CHA (+124.85 kJ/mol). Finally, these results indicate that oxidative addition may be the rate-limiting step of ethane dehydrogenation in these materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils

Thin films of WSe 2 have been deposited onto a conductive substrate (tungsten foil) using a relatively simple chemical-vapor-transport technique. X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy indicated that the films consisted of micron-sized single crystals of WSe 2 that were oriented perpendicular to the surface of the tungsten foil substrate. Linear sweep voltammetry was used to assess the ability of the WSe 2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions. A 350 mV overpotential (η) was required to drive the hydrogen-evolution reaction at a current density of –10 mA cm –2 in aqueous 0.5 M H 2 SO 4 , representing a significant improvement in catalytic performance relative to the behavior of macroscopic WSe 2 single crystals. The WSe 2 thin films were relatively stable under catalytic conditions, with the overpotential changing by only ~10 mV after one hour and exhibiting an additional change of ~5 mV after another hour of operation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Magnetic structure of ternary rare-earth alloy Ho 1/3 Tb 1/3 Er 1/3

Temperature dependent neutron powder diffraction measurements have been performed on the ternary rare-earth alloy Ho 1/3 Tb 1/3 Er 1/3 . Here, a dominant hexagonal closed packed structure is found, with a surface impurity phase that does not have any discernible magnetic signal. Three different magnetic phase transitions have been identified in the sample: (1) paramagnetic to helical k = (0,0,0.18) near T 1 = 90 K, (2) addition of another helix k = (0,0,0.34) near T 2 = 70 K, and (3) onset of ferromagnetism near T 3 = 45 K. There is a degree of ambiguity in the direction of the ferromagnetic moments, but we argue that a c-axis ferromagnetic moment is most plausible. At T = 3.5 K the magnetic moments are found to be m c = 2.83 μ B , m ab [k c = 0.19] = 3.96 μ B , and m ab [k c = 0.37] = 3.17 μ B .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization of U-10Zr-2Sn-2Sb and U-10Zr-2Sn-2Sb-4Ln to assess Sn+Sb as a mixed additive system to bind lanthanides

In this paper, tin and antimony are being investigated as potential additives to metallic fuel to control fuel-cladding chemical interaction (FCCI). A primary cause of FCCI is lanthanide fission products moving to the fuel periphery and interacting with the cladding. This interaction can lead to wastage of the cladding and, given enough time or burn-up, eventually to a cladding breach. The current study involves a microstructural characterization of as-cast and annealed U-10Zr-2Sn-2Sb and U-10Zr-2Sn-2Sb-4Ln, where Ln = 53Nd-25Ce-16Pr-6La, all in wt %, as alloys which could prevent FCCI. Scanning electron microscopy (SEM) analysis and X-ray powder diffraction indicates the additive-Zr compound in U-10Zr-2Sn-2Sb is Zr 5 (Sn,Sb) 3 , while the additive-Ln compound being formed in U-10Zr-2Sn-2Sb-4Ln is Ln 5 (Sn,Sb) 4 , with significantly more Sb than Sn (37 at. % versus 6 at. %, respectively). The bulk of the Sn, with a small amount of Sb, remained as Zr 5 (Sn,Sb) 3 precipitates. Finally, the potential benefits of a mixed additive system, along with relative stabilities of the intermetallic compounds, are discussed.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Solid-state phase transitions of two quaternary metallic fuel alloys (U-2.5Mo-2.5Ti-5.0Zr and U-1.5Mo-1.5Ti-7.0Zr in wt. %)

This study focuses on the solid-state phase transitions of two quaternary fuel alloys for fast reactors: U-1.5Mo-1.5Ti-7.0Zr (U-MT7Z) and U-2.5Mo-2.5Ti-5.0Zr (U-MT5Z). Here, the phase transitions were determined by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). To identify the high temperature phases, the alloys were annealed at 873K, 948K, 1023K, and 1123K (all under 72 hours). Combining those characterizations, the phase transitions are determined. In U-MT7Z, there is one phase transition observed, and it is ascribed to α + U 2 Ti → γ transition. In U-MT5Z, two transitions are found. The first transition is α → γ, while the second is U 2 Ti → γ. The γ phase onset temperature of U-MT5Z is lower than that of U-MT7Z due to the higher Mo content.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

On the product phases and the reaction kinetics of carbothermic reduction of UO 2 +C at relatively low temperatures

The synthesis of UC using carbothermic reduction of UO 2 and C mixtures has been well studied at high temperatures. However, the product phase behavior of carbothermic reduction at low temperatures (≤1773 K) is not well studied. Such a study is important as low temperatures permit single phase UC synthesis without forming secondary higher carbides, and it further supports the knowledge base of the process that needs to be used for transuranic elements such as plutonium that have high vapor pressures at elevated temperatures. Therefore, a low temperature carbothermic reduction of two different C/UO 2 molar ratios under inert and reducing environments have been studied here. Two different sample holding crucibles, alumina (Al 2 O 3 ) and graphite, were also used here to differentiate the hypostoichiometric (UC 1-a ) and oxygen dissolved (UC 1-x O x ) uranium monocarbide phases adding more details on the two systems. Also, the reaction kinetics involved in the formation of UC via the carbothermic reduction of UO 2 +C using product phases instead of evolved gases such as carbon monoxide is reported here. Under inert atmospheres but with significant oxygen partial pressures, the low temperature carbothermic reduction of UO 2 +C produced up to 90 wt.% UC 1-x O x type oxycarbides as was confirmed by Xray powder diffraction. Reducing Ar-4%H 2 environments at these temperatures were not successful in synthesizing UC as it reduces the amount of C required for the carbothermic reduction, leaving UC phase at a non-equilibrium state. Inert atmospheres with low or negligible oxygen partial pressures on the other hand produced near stoichiometric UC at high phase purity, especially at 1673 – 1773 K temperature range. An activation energy of 377±75 kJmol -1 was also calculated using product phase concentrations of the carbothermic reduction of UO 2 +C under these inert Ar (g) atmospheres.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Understanding the effect of salt concentrations on fast charging performance of Li-ion cells

Here, different lithium salt (lithium bis(fluorosulfonyl) imide (LiFSI)/LiPF 6 9/1 mol ratio) concentrations in carbonate electrolyte were studied on the fast charging performance of LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622)/graphite pouch cells. The cells with electrolyte concentration from 0.75 M to 1.50 M showed similar fast charging capabilities. Further increase of the concentration to 1.75 M and 2.00 M decreased the attainable capacity in fast charging. In the long-term cycling test, the capacity retention after 200 fast charging cycles increased with the increase of salt concentration in electrolyte. Cells with 1.5 M electrolyte showed the best overall performance in fast charging capacity and long-term cycling. Li platings were observed in the cells with 0.75 M, 1.00 M and 1.25 M electrolyte. It was improved with greatly reduced Li plating area in 1.50 M, and no Li plating at all in 1.75 M and 2.00 M electrolyte. Post-mortem analysis such as neutron powder diffraction (NPD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the electrode after cycling. It suggests that electrolyte concentration needs to be optimized for a given cell configuration with specific electrode and loading.

25 ENERGY STORAGE↗
DOE OSTI2013

Temperature-dependent framework–template interaction of |Na{sub 6}(H{sub 2}O){sub 8}|[ZnPO{sub 4}]{sub 6} sodalite

The partial dehydration followed by the decomposition of sodium-zincophosphate-hydrosodalite caused by the total water loss was examined by temperature-dependent X-ray powder diffraction, Fourier transform infrared (FTIR) and Raman spectroscopy as well as thermogravimetry combined with difference thermoanalysis. The dehydration causing the decomposition of this sodalite could be described in a two step-process. Initially two water molecules per unit-cell were lost, changing the interaction between the zincophosphate framework and the remaining water molecules as well as sodium cations as non-framework constituents. In this stage a 3+3 coordination of water molecules and sodium cations in the sodalite cage is observed. Using the autocorrelation function (AC) for the evaluation of the temperature-dependent FTIR spectra of sodalites is reported here for the first time. Calculating the changes in the AC broadness, structural features could well be analyzed due to its correlation with respective structural parameters. Temperature dependent Raman data was used to give a band assignment of bands at Raman shifts below 300 cm{sup −1}. - Graphical abstract: The thermal decomposition of |Na{sub 6}(H{sub 2}O){sub 8}|[ZnPO{sub 4}]{sub 6} sodalite is a two-step process with an initial loss of two water molecules at 400 K, intermediately stabilizing the highly stressed framework. Further heating leads to a subsequent loss of the remaining six water molecules and a breakup of the sodalite framework. A beryllonite-type NaZnPO{sub 4} is the final product of this process. Display Omitted - Highlights: • The decomposition of |Na{sub 6}(H{sub 2}O){sub 8}|[ZnPO{sub 4}]{sub 6} sodalite is a two-step process. • Initial loss of two water molecule at 400 K stabilizes the sodalite (step 1). • Further heating leads to complete loss of water and structural breakdown. • Autocorrelation of temperature dependent FTIR-spectra gives structural information.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Hydrothermal reactions: From the synthesis of ligand to new lanthanide 3D-coordination polymers

The organic ligand 2,5-piperazinedione-1,4-diacetic acid (H{sub 2}PDA) was synthesized under hydrothermal conditions starting from the iminodiacetic acid and catalyzed by oxalic acid. The X-ray powder diffraction data indicates that the compound crystallizes in the P2{sub 1}/c monoclinic system as reported in the literature. The ligand was also characterized by elemental analysis, magnetic nuclear resonance, infrared spectroscopy and thermogravimetric analysis. Two new coordination networks based on lanthanide ions were obtained with this ligand using hydrothermal reaction. In addition to single-crystal X-ray diffraction, the compounds were characterized by infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy and elemental analysis. Single-crystal XRD showed that the compounds are isostructural, crystallizing in P2{sub 1}/n monoclinic system with chemical formula [Ln(PDA){sub 1.5}(H{sub 2}O)](H{sub 2}O){sub 3} (Ln=Gd{sup 3+}(1) and Eu{sup 3+}(2)).The luminescence properties of both compounds were studied. In the compound (1), a broad emission band was observed at 479 nm, redshifted by 70 nm in comparison of the free ligand. In (2), the typical f–f transition was observed with a maximum peak at 618 nm, related with the red emission of the europium ions. Computational methods were performed to simulate the crystal structure of (2). The theoretical calculations of the intensity parameters are in good agreement with the experimental values. - Graphical abstract: Scheme of obtaining the ligand 2,5-piperazinedione-1,4-diacetic acid (H{sub 2}PDA) and two new isostructural 3D-coordination polymers [Ln(PDA){sub 1.5}(H{sub 2}O)](H{sub 2}O){sub 3} (Ln=Gd{sup 3+} and Eu{sup 3+}) by hydrothermal synthesis. Display Omitted - Highlights: • The ligand 2,5-piperazinedione-1,4-diacetic acid was synthetized using the hydrothermic method and characterized. • Two new 3D-coordination polymers with this ligand containing Gd{sup 3+} and Eu{sup 3+} ions were also obtained. • The compounds are isostructural and the typical luminescent properties were observed in both structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Synthesis and structural and magnetic characterization of the frustrated magnetic system La{sub 2}Ni{sub 4/3−x}Co{sub x}Sb{sub 2/3}O{sub 6}

We report the synthesis of double perovskites La{sub 2}Ni{sub 4/3−x}Co{sub x}Sb{sub 2/3}O{sub 6} with x=0, 1/3, 2/3 and 1 by a solid state method. Rietveld refinements of X-ray and neutron powder diffraction data show that all samples crystallize in space group P2{sub 1}/n, with almost perfect occupation of the 2d octahedral site with the transition metals, while all Sb{sup 5+} are randomly distributed in a 2c octahedral site. The saturation magnetization in hysteresis loops indicates that the samples are ferrimagnetic throughout all the series. Virgin magnetization curves lie outside hysteresis loops at low temperatures and thermal evolution of H{sub m} – defined as the inflection point of these curves – follows the de Almeida–Thouless dependence for x≠0. This spin glass like behavior below 30 K is also supported by thermal evolution of the coercivity, which follows an exponential law typical of magnetic clusters, not found in the pure Ni{sup 2+} perovskite, x=0 extreme. - Graphical abstract: Display Omitted - Author-Highlights: • We synthesized new double perovskites: La{sub 2}Ni{sub 4/3−x}Co{sub x}SbO{sub 6} (x=1/3, 2/3, 1). • The cations occupying octahedral sites are highly ordered in all samples. • Magnetic transition occurs as a consequence of superexchange paths. • Frustration is found and attributed to competition between different interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Crystal structure and electric properties of the organic–inorganic hybrid: [(CH{sub 2}){sub 6}(NH{sub 3}){sub 2}]ZnCl{sub 4}

The new organic-inorganic hybrid [(CH{sub 2}){sub 6}(NH{sub 3}){sub 2}]ZnCl{sub 4}, M{sub r}=325.406 crystallized in a triclinic, P1¯, a=7.2816 (5) Å, b=10.0996 (7) Å, c=10.0972 (7) Å, α=74.368 (4)°, β=88.046 (4)°, γ=85.974 (3)°, V=713.24 (9) Å{sup 3} and Z=2, D{sub x}=1.486 Mg m{sup −3}. Differential thermal scanning and x-ray powder diffraction, permittivity and ac conductivity indicated three phase transitions. Conduction takes place via correlated barrier hopping. - Graphical abstract: Display Omitted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Preparation and thermal stability of the spindle α-Fe{sub 2}O{sub 3}@SiO{sub 2} core–shell nanoparticles

The spindle α-Fe{sub 2}O{sub 3}@SiO{sub 2} core–shell nanoparticles (NPs) are prepared via hydrothermal synthesis and modified Stöber method. During these processes, shell thicknesses could be easily adjusted by the amount of tetraethylorthosilicate (TEOS), and the formation of core-free SiO{sub 2} could be effectively avoided. The structures and compositions of α-Fe{sub 2}O{sub 3}@SiO{sub 2} NPs are investigated by transmission electron microscope (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and ultraviolet–visible (UV–vis) absorption spectroscopy. These results reveal that the α-Fe{sub 2}O{sub 3}@SiO{sub 2} NPs with certain sizes are monodisperse and homogeneous. To estimate the thermal stability, the α-Fe{sub 2}O{sub 3}, α-Fe{sub 2}O{sub 3}@SiO{sub 2} and SiO{sub 2} NPs are annealed at 600, 800 and 1000 °C for 1 h under air atmosphere, respectively. Furthermore, the stabilities of these NPs are confirmed by thermal analysis methods. The structure and shape stabilities of these as-prepared α-Fe{sub 2}O{sub 3}@SiO{sub 2} NPs are investigated by XRD and scanning electron microscope (SEM). -- Graphical abstract: Schematic of preparation of the monodisperse spindle α-Fe{sub 2}O{sub 3}@SiO{sub 2} nanoparticles (NPs). Highlights: • The spindle α-Fe{sub 2}O{sub 3}@SiO{sub 2} nanoparticles (NPs) are successfully prepared by hydrothermal synthesis and modified Stöber method. • Optical properties are estimated and calculated by UV vis absorption spectrum. • Thermal stability of the α-Fe{sub 2}O{sub 3}, α-Fe{sub 2}O{sub 3}@SiO{sub 2} and SiO{sub 2} NPs are compared and analyzed by the SEM technique. • The structural changes of α-Fe{sub 2}O{sub 3}@SiO{sub 2} NPs are measured by XRD measurement.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Synthesis, crystal structure and electronic properties of the new iron selenide Ba{sub 9}Fe{sub 4}Se{sub 16}

The new ternary selenide Ba{sub 9}Fe{sub 4}Se{sub 16} has been synthesized from the reaction of appropriate amounts of elements at high temperature in a silica sealed tube. The compound crystallizes in the tetragonal space group I4{sub 1}/a with a=10.0068(3) Å and c=35.6415(9) Å, Z=4. It is an isostructural compound to the sulfide α-Ba{sub 9}Fe{sub 4}S{sub 15}, which is a high temperature polymorph of β-Ba{sub 9}Fe{sub 4}Se{sub 15} that belongs to the indefinitely adaptive phases series Ba{sub 3}Fe{sub 1+x}S{sub 5}, 0≤x≤1. X-ray powder diffraction and TEM analyses of the synthesized compound were used to determine the phase composition and the structure. The crystal structure can be viewed as overlapping sections along the c axis. Those sections are formed by the coordination polyhedra around barium atoms which can be described as trigonal prisms and bidisphenoids. Within the sections formed by barium polyhedra, isolated pairs of edge sharing FeSe{sub 4} tetrahedra are found. Magnetic measurements performed on Ba{sub 9}Fe{sub 4}Se{sub 16} indicate an antiferromagnetic behavior with Néel temperature of ∼13 K. Possible influence of air exposure on the magnetic properties is also discussed here. The electric measurements show an insulating behavior below 160 K and the dielectric permittivity and loss tangent at the lowest frequency measured reveal a change of slope very close to T{sub N}. However no magneto dielectric effect was evidenced for magnetic fields of up to 3 T. Activation energy, E{sub A}=0.18 eV, was extracted from the AC conductivity plot in the temperature range of 160–300 K. -- Graphical abstract: Experimental electron diffraction (ED) patterns of Ba{sub 9}Fe{sub 4}Se{sub 16} recorded along a-[010]. Highlights: • A new iron selenide material. • A structure resolution by combination of XRD and TEM. • Magnetic properties of the new compound Ba{sub 9}Fe{sub 4}Se{sub 16} are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Synthesis and characterization of two polyoxometalates consisting of different Cu-ligand hydrogen phosphate units

Two polyoxometalates [(Cu-mbpy){sub 4}(HPO{sub 4}){sub 2}(H{sub 2}O){sub 2}][PMo{sub 12}O{sub 40}]·H{sub 2}O (1) and [(Cu-mbpy){sub 6}(HPO{sub 4}){sub 4}][PW{sub 12}O{sub 40}]·4H{sub 2}O (2) (mbpy=4,4'-dimethyl-2,2'-dipyridyl in 1; 5,5″-dimethyl-2,2'-dipyridyl in 2) have been synthesized and characterized by IR, X-ray powder diffraction, TG analysis and electrochemical property. The structural features of 1–2 are in their cationic moieties consisting of different linkages of [Cu-mbpy]{sup 2+} and HPO{sub 4}{sup 2−} groups. In 1 four Cu-mbpy bridged by two HPO{sub 4}{sup 2−} ions form a discrete cluster with an interesting octahedron of (Cu{sub 4}P{sub 2}), while in 2 Cu-mbpy fragments are bridged by HPO{sub 4}{sup 2−} ions into 1D structure consisting of trigonal bipyramidal polyhedra of (Cu{sub 3}P{sub 2}). Photocatalytic experiments indicate that compounds 1 and 2 are actively photocatalytic for degradation of methyl orange in the presence of H{sub 2}O{sub 2} under UV light irradiation. -- Graphical abstract: Two polyoxometalate-based supramolecular compounds consisting of different linkages based on Cu-ligand and HPO{sub 4}{sup 2−} groups have been synthesized and characterized. The photocatalytic activity are studied. Highlights: • Two polyoxometalate-based supramolecular compounds consisting of different linkages based on Cu-ligand and HPO{sub 4}{sup 2−} groups have been synthesized. • Hydrogen bonding and π…π interactions play important roles in constructing crystal supramolecular frameworks. • Two compounds represent a high photocatalytic activity in the degradation of methyl orange.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Complex high-temperature phase transitions in Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12}

Differential scanning calorimetry measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} indicate hysteretic transformations to high-temperature phases at ≈615 K and 529 K, respectively, upon heating (1 K/min) from room temperature. X-ray and neutron powder diffraction measurements corroborate the phase-change behavior. For Li{sub 2}B{sub 12}H{sub 12}, the diffraction data are consistent with a previous study suggesting that the overall face-centered-cubic arrangement of icosahedral B{sub 12}H{sub 12}{sup 2−} anions is maintained upon transformation to the high-temperature polymorph, although the anions are now orientationally disordered and the Li{sup +} cations crystallographically disordered within an enlarged lattice. For Na{sub 2}B{sub 12}H{sub 12}, the diffraction data indicate the existence of three different high-temperature phases in addition to the known low-temperature monoclinic phase. The highest-temperature structure possesses Im3{sup -}m symmetry and exhibits a body-centered-cubic arrangement of orientationally disordered anions. The interstitial, disordered Na{sup +} cations appear to favor off-center positions within the distorted tetrahedral sites formed by the anions in this structure. An intermediate Pm3{sup -}n-symmetric phase at lower temperature is the result of a partial ordering of this higher-temperature structure. A third, minor, face-centered-cubic phase coexists with these high-temperature polymorphs. {sup 1}H NMR measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} reveal an approximately two-orders-of-magnitude increase in the reorientational jump rate of the anions in both cases upon transformation to their high-temperature structures. The enhanced anion mobilities were corroborated by neutron scattering fixed-window scans across the respective phase boundaries. The inherent cation disorder associated with these high-temperature polymorphs suggests their potential use as superionic conductors. - Graphical abstract: Na{sub 2}B{sub 12}H{sub 12} undergoes a phase transformation from an ordered monoclinic structure at low temperature to a partially disordered body-centered-cubic (bcc) structure at ≈529 K, and finally to a more-fully disordered bcc structure by ≈545 K. - Highlights: • Li{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈615 K. • Na{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈529 K. • Increased anion mobility is observed with NMR upon these H-T phase transformations. • Three high temperature phases are observed for Na{sub 2}B{sub 12}H{sub 12}, with increasing disorder.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Structural, electronic and magnetic properties of the series of double perovskites (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6}

Polycrystalline samples of the series of double perovskites Sr{sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. Their structural, electronic and magnetic properties were investigated by X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, heat capacity and electrical resistivity experiments. The compounds crystallize in a monoclinic structure and were fitted in space group P2{sub 1}/n, with a significant degree of Fe/Ir cationic disorder. As in Ca{sub 2−x}La{sub x}FeIrO{sub 6} the Sr-based system seems to evolve from an antiferromagnetic ground state for the end members (x=0.0 and x=2.0) to a ferrimagnetic order in the intermediate regions (x∼1). Since Mössbauer spectra indicate that Fe valence remains 3+ with doping, this tendency of change in the nature of the microscopic interaction could be attributed to Ir valence changes, induced by La{sup 3+} electrical doping. Upon comparing both Ca and Sr series, Sr{sub 2−x}La{sub x}FeIrO{sub 6} is more structurally homogenous and presents higher magnetization and transition temperatures. Magnetic susceptibility measurements at high temperatures on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6} indicate a very high ferrimagnetic Curie temperature T{sub C}∼700K. For the Sr{sub 2}FeIrO{sub 6} compound, electrical resistivity experiments under applied pressure suggest that this material might be a Mott insulator. - Graphical abstract: The Weiss constant as a function of La doping for the (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} series, indicating changes in Fe–Ir magnetic coupling on both families. - Highlights: • The double perovskite series (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. • Changes in the Fe-Ir magnetic coupling due to La doping on both series. • Evidence of high T{sub C} on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6}. • Indication of Mott insulator behavior on Sr{sub 2}FeIrO{sub 6}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigation into the effect of Si doping on the cell symmetry and performance of Sr{sub 1−y}Ca{sub y}FeO{sub 3−δ} SOFC cathode materials

In this paper we report the successful incorporation of silicon into Sr{sub 1−y}Ca{sub y}FeO{sub 3−δ} perovskite materials for potential applications as electrode materials for Solid Oxide Fuel Cells. It is observed that Si doping leads to a change from a tetragonal or orthorhombic structure (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). The structures of the phases, SrFe{sub 0.85}Si{sub 0.15}O{sub 3−δ}, Sr{sub 0.75}Ca{sub 0.25}Fe{sub 0.85}Si{sub 0.15}O{sub 3−δ} and Sr{sub 0.5}Ca{sub 0.5}Fe{sub 0.85}Si{sub 0.15}O{sub 3−δ}, were analysed using neutron powder diffraction. The data confirmed the cubic unit cell, with no long range oxygen vacancy ordering. Conductivity measurements showed an improvement in the conductivity on Si doping, especially for samples with high Ca content. Composite electrodes comprising 50% Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95} and 50% Sr{sub 1−y}Ca{sub y}(Fe/Si)O{sub 3−δ} on dense Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95} pellets were therefore examined in air. An improvement in the area specific resistances (ASR) values is observed for the Si-doped samples with respect to the undoped samples. Thus the results show that silicon can be incorporated into Sr{sub 1−y}Ca{sub y}FeO{sub 3−δ}-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode material in Solid Oxide Fuel Cells (SOFC). - Graphical abstract: X-ray diffraction patterns for: (left) Sr{sub 0.75}Ca{sub 0.25}Fe{sub 1−x}Si{sub x}O{sub 3−δ} (x=0, 0.05, 0.10 and 0.15) and (right) Sr{sub 0.25}Ca{sub 0.75}Fe{sub 1−x}Si{sub x}O{sub 3−δ} (x=0, 0.05, 0.10 and 0.15), showing the stabilization of the cubic form of these series through silicon doping. For the latter Sr{sub 0.25}Ca{sub 0.75}Fe{sub 1−x}Si{sub x}O{sub 3−δ} phase, the stabilisation is not quite complete at 15% Si doping. - Highlights: • In Sr{sub 1−y}Ca{sub y}Fe{sub 1−x}Si{sub x}O{sub 3−δ} materials Si doping results in a change from tetragonal or orthorhombic symmetry. • An improvement in the conductivity is observed on Si doping. • An improvement in the area specific resistances (ASR) values is observed on Si doping. • Silicon can have a beneficial effect on the performance in perovskite materials.

36 MATERIALS SCIENCE↗