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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Shifts in valence states in bimetallic MXenes revealed by electron energy-loss spectroscopy (EELS)

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti$_{2–y}$Nb$_y$CT$_x$, Nb$_{2–y}$V$_y$CT$_x$, and Ti$_{2–y}$V$_y$CT$_x$) were investigated with scanning transmission electron microscopy and electron energy-loss spectroscopy to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti–Nb and Ti–V MXenes, regardless of stoichiometry. Here, the study also explains Nb's influential role in the previously observed electronic properties in the Nb–V and Nb–Ti systems.

36 MATERIALS SCIENCE↗
NASA NTRS1977

Evolution of ion emission yield of alloys with the nature of the solute. 2: Interpretation

Solid solutions of transition elements in copper, nickel, cobalt, iron, and aluminum matrices were analyzed by observing secondary ion emissions under bombardment with 6.2-keV argon ions. Enchancement of the production of solute-element ions was observed. An ion emission model is proposed according to which the ion yield is governed by the probability of an atom leaving the metal in a preionized state. The energy distribution of the valence electrons of the solute atoms is the bases of the probability calculation.

Blaise, G.↗
DOE OSTI2018

Phase transition and luminescence properties of GdTiNbO6:Eu3+ formed by hydrothermal route

Highlights: • Phase transition and luminescence of hydrothermal GdTiNbO{sub 6}:Eu{sup 3+} were investigated. • RE{sup 3+} radius 0.1061 nm was the boundary between the aeschynite and euxenite. • GdTiNbO{sub 6}:0.30Eu{sup 3+} was preferred for UV-B excitation, 0.50Eu{sup 3+} for Vis-light excitation. • The decay lifetime of 615 nm emission (ex = 405 nm) was 570∼571 μs. - Abstract: The dependences of phase transition and luminescence properties upon both Eu{sup 3+} concentration and heating temperature in air were investigated for GdTiNbO{sub 6}:Eu{sup 3+} fine crystals formed hydrothermally at 240 °C as aeschynite-type Eu{sub x}Gd{sub 1-x}TiNbO{sub 6} solid solution. The as-prepared aeschynite solid solution in the concentration range x ≥ 0.70 did not show the phase transformation from an aeschynite- to an euxenite-type structure caused by heating at 1300 °C for 1 h in air. The present study suggested that the phase boundary between the aeschynite- and euxenite-type RETiNbO{sub 6} in the EuTiNbO{sub 6}-GdTiNbO{sub 6} system, where RE was a rare earth, lay in the average RE{sup 3+} ionic radius 0.1061 nm. The intense emissions attributed to the {sup 5}D{sub 0}→{sup 7}F{sub 1} and {sup 5}D{sub 0}→{sup 7}F{sub 2} transitions of Eu{sup 3+} for the as-prepared samples were more improved by heating at 1100–1300°C in air. The material doped with 30 mol% Eu{sup 3+} was preferred for UV-B excitation, in contrast, the material with 50 mol% Eu{sup 3+} was preferred for visible light excitation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Cobalt-doped Bi{sub 26}Mo{sub 10}O{sub 69}: Crystal structure and conductivity

A series of cobalt-doped bismuth molybdates were synthesized and investigated using X-ray powder diffraction, transmission electron microscopy and impedance spectroscopy. The ranges of solid solution were determined. Two new compounds, Bi{sub 1−x}Co{sub x}[Bi{sub 12}O{sub 14}]Mo{sub 5}O{sub 34.5±δ} (x=0.2) and Bi[Bi{sub 12}O{sub 14}]Mo{sub 5−y}Co{sub y}O{sub 34.5±δ} (y=0.2), which crystallise in monoclinic unit cells have been examined in detail by diffraction methods. Impedance spectroscopy measurements show that the studied materials are good ionic conductors with conductivity values about 5×10{sup −3} S×cm{sup −1} at 973 K and 1.7×10{sup −4} S×cm{sup −1} at 623 K, which are similar to conductivity values of yttrium substituted zirconia and (YSZ) gadolinium doped ceria (CGO). - Graphical abstract: Measured and calculated diffraction spectra for Bi{sub 12.8}Co{sub 0.2}Mo{sub 5}O{sub 34±δ} and projection of the Bi{sub 12.8}Co{sub 0.2}Mo{sub 5}O{sub 34±δ} crystal structure onto the ac plane. Highlights: • The limit of the Bi{sub 1−x}Co{sub x}[Bi{sub 12}O{sub 14}]Mo{sub 5}O{sub 34.5±δ} homogeneity range is equal to x=0.2. • The limit of the Bi[Bi{sub 12}O{sub 14}]Mo{sub 5−y}Co{sub y}O{sub 34.5±δ} homogeneity range is equal to y=0.2. • Solid solutions have monoclinic symmetry. No phase transition is observed. • The conductivity at 700° for y=0.2 solid solutions is equal to −lg σ, S×cm{sup −1}=2.23. • The conductivity at 350° for y=0.2 solid solutions is equal to −lg σ, S×cm{sup −1}=3.74.

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

Preparation and Characterization of Nanoceramics for Solid Oxide Fuel Cells

Precursor powders in the ZrO{sub 2}–HfO{sub 2}–Y{sub 2}O{sub 3}–CeO{sub 2}, In{sub 2}O{sub 3}–ZrO{sub 2}, and NiO–Nd{sub 2}O{sub 3} systems for components of solid oxide fuel cells have been prepared by liquid-phase synthesis. We have determined formation conditions and the particle size of ZrO{sub 2-} and In{sub 2}O{sub 3-}based solid solutions and neodymium nickelate (Nd{sub 2}NiO{sub 4}), demonstrated the feasibility of producing nanocrystalline powders (10–30 nm) of tailored chemical composition in the temperature range 500–900°C, and optimized powder consolidation conditions. Nanoceramics with a crystallite size from 60 to 90 nm have been obtained and their microstructure and phase composition have been investigated. We have studied the electrical properties of the ZrO{sub 2-} and In{sub 2}O{sub 3-}based solid solutions and the Nd{sub 2}NiO{sub 4} compound and established the range of their electrical conductivity at temperatures from 300 to 1000°C: 2.27 × 10{sup –3} to 2.51 S/cm for the ZrO{sub 2}-based solid solution, 8.91 × 10{sup 1} to 6.59 × 10{sup 3} S/cm for the In{sub 2}O{sub 3}-based solid solution, and 3.98 × 10{sup 2} to 5.02 × 10{sup 2} S/cm for Nd{sub 2}NiO{sub 4}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural, vibrational and luminescence properties of the (1−x)CaWO{sub 4}−xCdWO{sub 4} system

In the present work, we investigate the structural, microstructural, vibrational and luminescence properties of the system (1−x)CaWO{sub 4}−xCdWO{sub 4} with x ranging between 0 and 1. Polycrystalline samples were elaborated using a coprecipitation technique followed by thermal treatment at 1000 °C. The samples were then characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy and luminescence analyses. X-ray diffraction profile analyses using Rietveld method showed that two kinds of solid solutions Ca{sub 1−x}Cd{sub x}WO{sub 4} having scheelite and wolframite structures, with respectively tetragonal and monoclinic crystal cells, were observed, with a biphasic system for compositions x=0.6 and 0.7. The scanning electron microscopy experiments showed a complex evolution of morphologies and crystallite sizes as x increased. The vibration modes of Raman spectra were characteristic of composition-dependent disordered solid solutions with decreasing wavenumbers as x increased. Luminescence experiments were performed under UV-laser light irradiation. The energies of emission bands increased linearly with cadmium composition x. The integrated intensity of luminescence reached a maximum value for the substituted wolframite phase with composition x=0.8. - Graphical abstract: Luminescence on UV excitation (364.5 nm) of (1−x)CaWO{sub 4−x}CdWO{sub 4} system, elaborated from coprecipitation technique at 1000 °C, with 0<x<1. (a) Experimental emission bands of the Ca{sub 1−x}Cd{sub x}WO{sub 4} polycrystalline phases with 0≤x≤0.5. (b) Maximum of luminescence intensity for the composition x=0.8. - Highlights: • Solid solutions Ca{sub 1−x}Cd{sub x}WO{sub 4} are elaborated from coprecipitation technique. • The structures of two types of solid solutions are refined using Rietveld method. • A maximum of luminescence is obtained for an intermediate composition x=0.8.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

HT-solution growth and characterisation of In{sub x}Na{sub x}Mn{sub 1-2x}WO{sub 4} (0<x≤0.26)

A new solid solution system, In{sub x}Na{sub x}Mn{sub 1–2x}WO{sub 4} (0<x=≤0.26) belonging to the MnWO{sub 4}-type multiferroics was investigated. Large single crystals were grown up to several mm in length from high-temperature oxide solutions using a solvent mixture of Na{sub 2}WO{sub 4} and Na{sub 2}W{sub 2}O{sub 7}. Quantitative doping degrees were analyzed by means of electron microprobe. Single crystal X-ray diffraction studies agreed with the polar space group P2 for In{sub x}Na{sub x}Mn{sub 1–2x}WO{sub 4}. The complexity of partial (Na{sup +}/Mn{sup 2+})- and (In{sup 3+}/Mn{sup 2+})-ordering makes their atomic sites unique, explaining the dissymmetrization from P2/c down to P2. This is also valid for the InNaW{sub 2}O{sub 8}-type Mn{sub 2y}In{sub 1-y}Na{sub 1-y}W{sub 2}O{sub 8} solid solution found as co-product in this study. Specific heat and magnetisation measurements showed a paramagnetic dilution effect from (In{sup 3+}/Na{sup +})-doping while lowering the transition temperature for the multiferroic phase AF2 of In{sub x}Na{sub x}Mn{sub 1–2x}WO{sub 4} with respect to the prototype MnWO{sub 4}. - Graphical abstract: A volume expansion around Mn2 site by (In, Na)-doping is drawn by the atomic shifts in a Mn2O{sub 6} octahedron in the atomistic structure of In{sub 0.20(2)}Na{sub 0.17(2)}Mn{sub 0.63(4)}WO{sub 4} from the respective atomic positions of In-free MnWO{sub 4}. The size and direction of arrows are scaled up for indicating the consequent local distortion. - Highlights: • HT solution crystal growth of (In, Na):MnWO{sub 4}-solid-solution compounds. • Polar space group symmetry P2 for their atomistic structures. • Magnetic properties of the multiferroic phase AF2 by (In{sup 3+}, Na{sup +})-doping. • Magnetic phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Manganese-calcium intermixing facilitates heteroepitaxial growth at the 10 1 ¯ 4 calcite-water interface

In situ atomic force microscopy (AFM) measurements were performed to probe surface precipitates that formed on the (10-14) surface of calcite (CaCO3) single crystals following reaction with Mn2+-bearing aqueous solutions with a range of initial concentrations. Three-dimensional epitaxial islands were observed to precipitate and grow on the surfaces and in situ time-sequenced measurements demonstrated that their growth rates were commensurate with those obtained for epitaxial islands formed on calcite crystals reacted with Cd2+-bearing aqueous solutions of the same range in supersaturation with respect to the pure metal carbonate phase. This finding was unexpected as rhodochrosite (MnCO3) and calcite display a 10% lattice mismatch, based on the area of their (10-14) surface unit cells, whereas the lattice mismatch is only 4% for otavite (CdCO3) and calcite. Coatings of varying thicknesses were therefore synthesized by reacting calcite single crystals with calcite-equilibrated aqueous solutions with concentrations of up to 250 µM MnCl2 and analyzed to determine the composition of the surface precipitates. Ex situ X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectivity (XRR), and AFM measurements of the reacted crystals demonstrated the formation of an epitaxial (Mn,Ca)CO3 solid solution with a spatially complex composition atop the calcite surface, whereby the first few nanometers were rich in Ca and the Mn content increased with distance from the original calcite surface, culminating in a topmost region of almost pure MnCO3 for the thickest coatings. These findings explain the measured growth rates (the effective lattice mismatch was much smaller than nominal mismatch) and highlight the strong influence played by the substrate on the composition of surface precipitates in aqueous conditions.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2013

Structural and magnetic investigation of In{sub 2}Fe{sub 2−x}Ga{sub x}CuO{sub 7}

The solid solution of In{sub 2}Fe{sub 2−x}Ga{sub x}CuO{sub 7} (space group P6{sub 3}/mmc) was synthesized and investigated through X-ray diffraction and magnetic susceptibility studies. Limited changes to the lattice parameters were observed as a result of the similar ionic radii for Fe{sup 3+} and Ga{sup 3+} in the trigonal bipyramidal (TBP) crystallographic site. An increase in the Weiss temperature, along with spin glass behavior are observed from x=0 to x=2, but irregularities in the trend are apparent for x=0.75 and 1. With the highest concentration in magnetic ions, In{sub 2}Fe{sub 2}CuO{sub 7} appears to have competing nearest neighbor interactions that produce a suppression of the Curie tail and the experimental magnetic moment. In comparison to InFe{sub 1−x}Ga{sub x}CuO{sub 4}, both solid solutions show an invariable progression of the lattice parameters, but the magnetic properties are greatly affected by the distinct TBP layering schemes. - Graphical abstract: Solid solution of In{sub 2}Fe{sub 2−x}Ga{sub x}CuO{sub 7} has been synthesized and analyzed with powder X-ray diffraction and DC magnetic susceptibility studies. The magnetic ground state has been investigated through cation substitution and compared to that of InFe{sub 1−x}Ga{sub x}CuO{sub 4}. Highlights: • The complete solid solution of In{sub 2}Fe{sub 2−x}Ga{sub x}CuO{sub 7} has been synthesized. • Limited changes in the lattice parameters were observed through XRD refinements. • Spin glass magnetic interactions were found to depend on Fe{sup 3+} content. • The layering of the TBP greatly affects the observed magnetic interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Mapping growth windows in quaternary perovskite oxide systems by hybrid molecular beam epitaxy

Requisite to growing stoichiometric perovskite thin films of the solid-solution A ' 1-x A x B O 3 by hybrid molecular beam epitaxy is understanding how the growth conditions interpolate between the end members A'B O 3 and AB O 3 , which can be grown in a self-regulated fashion, but under different conditions. Using the example of La 1-x Sr x VO 3 , the two-dimensional growth parameter space that is spanned by the flux of the metal-organic precursor vanadium oxytriisopropoxide and composition, x , was mapped out. The evolution of the adsorption-controlled growth window was obtained using a combination of X-ray diffraction, atomic force microscopy, reflection high-energy electron-diffraction (RHEED), and Rutherford backscattering spectroscopy. It is found that the stoichiometric growth conditions can be mapped out quickly with a single calibration sample using RHEED. Once stoichiometric conditions have been identified, the out-of-plane lattice parameter can be utilized to precisely determine the composition x . This strategy enables the identification of growth conditions that allow the deposition of stoichiometric perovskite oxide films with random A-site cation mixing, which is relevant to a large number of perovskite materials with interesting properties, e.g., high-temperature superconductivity and colossal magnetoresistance, that emerge in solid solution A ' 1-x A x B O 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2013

Synthesis, crystal structure and properties of SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ}

The solid solution formation range in the SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ} (0≤x≤2) system examined at 1100 °C in air corresponds to 0≤x≤1.1. The crystal structure of SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ} with x≤0.5 was described as orthorhombic (space group Pmmm), and with 0.6≤x≤1.1—as tetragonal (P4/mmm). The introduction of iron into the cobalt sublattice leads to a gradual increase of the unit cell parameters and volume, accompanied with increasing oxygen content determined by thermogravimetry. The dilatometric curves collected at 25–1100 °C in air exhibit moderate nonlinearities associated with the orthorhombic→tetragonal phase transition, as confirmed by the high-temperature X-ray diffraction analysis. Temperature dependencies of the electrical conductivity measured by 4-probe dc method in the temperature range 25–1000 °C in air display maxima at 300–350 °C, whilst the conductivity decreases with iron additions. These trends were discussed in terms of the defect structure and nature of the main charge carriers in SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ}. It was also shown that SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ} is chemically stable with respect to the Ce{sub 0.8}Sm{sub 0.2}O{sub 1.9} solid electrolyte up to 1100 °S, but reacts with Zr{sub 0.85}Y{sub 0.15}O{sub 1.93} even at 900 °S. Highlights: • The solid solution formation range in the SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ} system at 1100 °C in air corresponds to 0≤x≤1.1. • The unit cell volume and oxygen content in the solid solutions increase with x. • Oxygen nonstoichiometry of SmBaCo{sub 2−x}Fe{sub x}O{sub 5+δ} solid solutions. • Temperature dependencies of the electrical conductivity exhibit maxima at the temperatures when oxygen exchange with the gaseous phase becomes significant.

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

Crystal structure and dielectric properties of a new Na2O-Nd2O3-CeO2 ceramic system at microwave frequencies

Highlights: • A large amount of Na evaporation can make τ{sub f} more negative in present ceramics. • Na-excess evaporation results in the formation of Ce{sub 1-x}Nd{sub x}O{sub 2-x/2} solid solution. • Ce{sub 1-x}Nd{sub x}O{sub 2-x/2} (x = 0.25 and 0.5) solid solutions belong to the same fluorite-type. - Abstract: Na{sub 2}O-Nd{sub 2}O{sub 3}-CeO{sub 2} ceramic systems of the form Na{sub 3x−1}Nd{sub 1−x}CeO{sub 3} (x = 0.4, 0.5, and 0.6) were prepared via a solid-state reaction method. X-ray diffraction analyses indicated that the samples with x = 0.4, 0.6 and those with x = 0.5 formed Nd{sub 0.25}Ce{sub 0.75}O{sub 1.875} and Nd{sub 0.5}Ce{sub 0.5}O{sub 1.75} solid solutions, respectively, which resulted from a large degree of Na{sup +} ion evaporation. Both the Nd{sub 0.25}Ce{sub 0.75}O{sub 1.875} and Nd{sub 0.5}Ce{sub 0.5}O{sub 1.75} phases belonged to the same cubic structure with an Fm-3m (225) space group. Additionally, the microstructures, sintering behaviors, and microwave dielectric properties were considered to be strongly related to the Na content in the Na{sub 3x−1}Nd{sub 1−x}CeO{sub 3} ceramics. A relatively large degree of Na{sup +} ion evaporation could increase the negative temperature coefficient of the resonant frequency (τ{sub f}) for these ceramic systems. Moreover, after sintering at 1380 °C for 2 h, the x = 0.6 specimen possessed a low permittivity (ε{sub r}) of approximately 9.13, a moderate quality factor (Q × f) of approximately 23,800 GHz (at 9.14 GHz) and a negative τ{sub f} value of approximately −61.7 ppm/°C. As a result, it could be a promising microwave dielectric material for achieving a near-zero τ{sub f} value by solidifying one or more microwave dielectric ceramics with a high positive τ{sub f} value.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1978

High temperature resistant cermet and ceramic compositions

High temperature oxidation resistance, high hardness and high abrasion and wear resistance are properties of cermet compositions particularly to provide high temperature resistant refractory coatings on metal substrates, for use as electrical insulation seals for thermionic converters. The compositions comprise a sintered body of particles of a high temperature resistant metal or metal alloy, preferably molybdenum or tungsten particles, dispersed in and bonded to a solid solution formed of aluminum oxide and silicon nitride, and particularly a ternary solid solution formed of a mixture of aluminum oxide, silicon nitride and aluminum nitride. Ceramic compositions comprising a sintered solid solution of aluminum oxide, silicon nitride and aluminum nitride are also described.

Phillips, W. M.↗
NASA NTRS1978

High temperature resistant cermet and ceramic compositions

Cermet compositions having high temperature oxidation resistance, high hardness and high abrasion and wear resistance, and particularly adapted for production of high temperature resistant cermet insulator bodies are presented. The compositions are comprised of a sintered body of particles of a high temperature resistant metal or metal alloy, preferably molybdenum or tungsten particles, dispersed in and bonded to a solid solution formed of aluminum oxide and silicon nitride, and particularly a ternary solid solution formed of a mixture of aluminum oxide, silicon nitride and aluminum nitride. Also disclosed are novel ceramic compositions comprising a sintered solid solution of aluminum oxide, silicon nitride and aluminum nitride.

Phillips, W. M.↗
DOE OSTI2014

Thermodynamic model for the solubility of BaSeO4(cr) in the aqueous Ba2+-SeO42--Na+-H+-OH--H2O system: Extending to high selenate concentrations

The solubility of Ba(SeO4, SO4) precipitates was determined as a function of the BaSeO4 mole fractions, ranging from 0.0015 to 0.3830, and time with an equilibration period extending to as long as 302 days. Equilibrium/steady state conditions in this system are reached in ≤ 65 days. Pitzer’s ion interaction model was used to calculate solid and aqueous phase activity coefficients. Thermodynamic analyses showed that the data do not satisfy Gibbs-Duhem equation, thereby demonstrating that a single-solid solution phase does not control both the selenate and sulfate concentrations. Our extensive data with log10 [Ba]) ranging from -3.6 to -5.9 mol.kg-1, log10 [SeO4]) ranging from -3.6 to -5.2 mol.kg-1, and log10 [SO4] ranging from -4.0 to -5.3 mol.kg-1 can be explained with the formation of an ideal BaSeO4 solid solution phase that controls the selenium concentrations and a slightly disordered/less-crystalline BaSO4(s) (log10 K0sp = -9.5 instead of -10.05 for barite) that controls the sulfate concentrations. In these experiments the BaSO4 component of the solid solution phase never reaches thermodynamic equilibrium with the aqueous phase. Thermodynamic interpretations of the data show that both the ideal BaSeO4 solid solution phase and less-crystalline BaSO4(s) phase are in equilibrium with each other in the entire range of BaSeO4 mole fractions investigated in this study.

Rai, Dhanpat↗
DOE OSTI2024

Atomic Ordering-Induced Ensemble Variation in Alloys Governs Electrocatalyst On/Off States

The catalytic behavior of a material is influenced by ensembles—the geometric configuration of atoms. Traditional approaches, mainly utilizing solid-solution alloys in electrocatalysis, have often overlooked the challenges posed by concurrent changes in the electronic structure (i.e. d-band center) when the composition is altered. Here, this study introduces a methodology that distinctly separates the geometric effects (i.e. ensembles) from the electronic structure. We compare the reactivity of compositionally identical, but structurally different Pd 3 Bi ordered intermetallic and solid-solution alloys. Remarkably, we find that Pd 3 Bi intermetallics display nearly no reactivity for the methanol oxidation (MOR), while their solid-solution counterparts have significant reactivity. This highlights a unique case where materials with identical chemical compositions demonstrate drastically different catalytic behavior underscoring the critical importance of ensembles in electrocatalysis. Specifically, Pd 3 Bi intermetallics form smaller ensembles (average coordination number: 4.5 ± 1.6) with almost no measurable MOR activity at room temperature, in contrast to the solid-solution Pd 3 Bi that exhibit larger ensembles (average coordination number: 6.8 ± 0.9) and considerable MOR reactivity (0.5 mA cm −2 Pd ). An ordered Pd 3 Bi alloy, with an intermediate ensemble size (average coordination number: 5.3 ± 1.2), displays moderate MOR activity (0.1 mA cm −2 Pd ), further confirming the direct correlation between ensemble size and catalytic activity. Notably, all Pd 3 Bi alloys maintain similar electronic structures, because the chemical composition of the alloys is fixed, indicating that the differences in reactivity are predominantly from changes to the ensemble size. Our findings offer an approach for precisely controlling catalytic activity through manipulating the geometric configuration of the atoms within an alloy, paving the way for more efficient catalyst design.

alloys↗
DOE OSTI2014

Thermodynamic Model for the Solubility of Ba(SeO 4 , SO 4 ) Precipitates

The solubility of Ba(SeO 4 , SO 4 ) precipitates was determined as a function of the BaSeO 4 mole fractions, ranging from 0.0015 to 0.3830, and time with an equilibration period extending to as long as 302 days. Equilibrium/steady state conditions in this system are reached in ≤ 65 days. Pitzer’s ion interaction model was used to calculate solid and aqueous phase activity coefficients. Thermodynamic analyses showed that the data do not satisfy Gibbs-Duhem equation, thereby demonstrating that a single-solid solution phase does not control both the selenate and sulfate concentrations. Our extensive data with log 10 [Ba] ranging from -3.6 to -5.9 mol kg -1 , log 10 [SeO 4 ] rangingfrom-3.6 to -5.2 mol kg -1 , and log 10 [SO 4 ] ranging from-4.0 to -5.3 mol kg -1 can be explained with the formation of an ideal BaSeO 4 solid solution phase that controls the selenium concentrations and a slightly disordered/less-crystalline BaSO 4 (s) (log 10 Κ° sp = -9.5 instead of -10.05 for barite) that controls the sulfate concentrations. In these experiments the BaSO 4 component of the solid solution phase never reaches thermodynamic equilibrium with the aqueous phase. Thermodynamic interpretations of the data show that both the ideal BaSeO 4 solid solution phase and less-crystalline BaSO 4 (s) phase are in equilibrium with each other in the entire range of BaSeO 4 mole fractions investigated in this study.

Rai, D.↗