The Bi3FeMo2O12 system is examined as a rare example of a transition-metal oxide wh i c h , upon heating, undergoes a symmetry lowering and a 2:1 ordering of the transition-metal cations. The compound was synthesized in the tetragonal scheelite structure (S.G. #88: I41/a) by a sol-gel method and converted into the monoclinic polymorph (S.G. #15: C2/c) by calcination above 500 degrees C. The structure of both polymorphs was analyzed using a combination of X-ray and neutron diffraction data, and the temperature-dependent phase transition between these was inves-tigated in situ using variable-temperature neutron powder diffraction and scanning transmission electron microscopy. The results show that the structural phase transition takes place at low temperatures (similar to 500 degrees C) and is first order in nature, as evident from the coexistence of both structures. The transition from tetragonal to monoclinic results in the reduction of the equivalent unit cell volume. The role of the Bi3+ 6s lone pairs in the temperature-driven phase transition has been studied using neutron pair distribution function analysis. Local structure analysis via neutron total scattering revealed the Bi3+ 6s lone pairs to be stereochemically active in both structures, with short correlation lengths in the tetragonal structure and long correlation lengths in the monoclinic structure, leading to facile phase conversion and to a more efficient packing density with highly correlated lone pairs in the monoclinic structure. Magnetization isotherms of the tetragonal structure collected at 1.8 K exhibit ferromagnetic behavior, suggesting that the interplay between the observed short-range monoclinic order, defects, and surface-to-bu l k effects alters the magnetic interaction, leading to short-range ferromagnetic interactions, which is highly unexpected given the low-temperature antiferromagnetic order observed in the monoclinic structure.

Supported, dilute Pd-in-Au alloyed nanoparticles might possess enhanced reactivity due to the altered structural and electronic properties of the Pd atoms isolated in an Au nanoparticle matrix.

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A3OhTd2O7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO2.5 compositions relative to that in the perovskite ABO3 or brownmillerite A2B2O5 structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O2- migration. The low- and high-temperature average and local structures of Ba3YGa2O7 (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P1121/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O2- ions was identified. Ionic conductivities of Ba3YGa2O7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba3YGa2O7.5 retaining proton conductivity at high temperatures. Ba2.9La0.1YGa2O7.55 and Ba3YGa1.9Ti0.1O7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.

Manganese oxides are ubiquitous marine minerals which are redox sensitive. As major components of manganese nodules found on the ocean floor, birnessite and buserite have been known to be two distinct water-containing minerals with manganese octahedral interlayer separations of ~7 Å and ~10 Å, respectively. We show here that buserite is a super-hydrated birnessite formed near 5 km depth conditions. As one of the most hydrous minerals containing ca. 34.5 wt. % water, super-hydrated birnessite, i.e., buserite, remains stable up to ca. 70 km depth conditions, where it transforms into manganite by releasing ca. 24.3 wt. % water. Subsequent transformations to hausmannite and pyrochroite occur near 100 km and 120 km depths, respectively, concomitant with a progressive reduction of Mn4+ to Mn2+. Our work forwards an abiotic geochemical cycle of manganese minerals in subduction and/or other aqueous terrestrial environments, with implications for water storage and cycling, and the redox capacity of the region.

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba3-x Sr x YGa2O7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Scanning transmission electron microscopy imaging of the MoVNbTe-oxide used as a catalyst for oxidative dehydrogenation and partial oxidation establishes anisotropic scattering projections of atom columns composed of Mo and V atoms which image the catalytically active S2 site and were predicted to be distorted by hybrid density functional theory calculations. These distortions of the S2 sites toward empty hexagonal channels created by the removal of [TeO]2+ entities experimentally corroborate that controlled partial occupancy of (TeO)n chains in the hexagonal channels of the MoVNbO-framework provides a means to introduce polarons and thereby increase the catalytic reactivity and selectivity of this catalyst.

Tuning of physical properties opens up opportunities for new applications of known materials, as the changes induced by hydrostatic pressures can sometimes be mimicked by chemical pressures created by appropriate substitutions or at the interfaces. In contrast to bulk In2O3, where photoluminescence (PL) is reduced by pressure, pressure-induced photoluminescence enhancement (PIPE) by a factor of 4.4 was found in In2O3 nanowires (NWs). Density functional theory (DFT) calculations indicate that PIPE in NWs may be ascribed to the orbital overlap between oxygen-induced defect levels and the conduction band minimum (CBM), which does not occur in bulk In2O3. Both the PL intensity and absorption spectra ofIn(2)O(3) NWs point to a discontinuity near 16 GPa. Rietveld refinements indicate that these changes in optical properties are concomitant with structural distortions, leading to strained bonds in the InO6 octahedra and trigonal prisms. The optical transmittance (T) of the NWs after compression up to 5.29 GPa is reversible after the pressure release. However, NWs compressed to 12.27 GPa and then reinvestigated at ambient pressure reveal that T increases. At compressions of up to 24.85 GPa, T at ambient conditions was enhanced by 8.29%. Synchrotron X-ray scattering and Raman spectra of NWs compressed to 24.85 GPa indicate that a local distortion of the cubic bixbyite structure is present at ambient conditions. This study establishes a correlation between changes in PL and T of In2O3 NWs and local structural distortions resulting in oxygen-induced defects, which overlap with the conduction band states as suggested by DFT calculations. Our work suggests that the optical properties of ternary metallic oxide NWs with bixbyite structure might be improved by appropriate substitutions.

Grain boundary segregation in cerium dioxide doped with varying amounts of gadolinium oxide and tantalum oxide has been measured with x-ray energy dispersive spectroscopy using a Vacuum Generators HB603 Scanning Transmission Electron Microscope (STEM). The data has been analyzed in the framework of both elastic relaxation and space charge segregation forces with a Limited number of surface sites. Results show that multiple driving forces must be taken into account to explain aliovalent solute segregation.

The controlled placement of nanoparticles (NPs) within homopolymers and block polymers is of broad interest for functional nanomaterials. This manuscript focuses on small molecule‐stabilized NPs that bring a large fraction of functionality. For such NP mixtures with block polymers, the overwhelming focus to date has been the use of attractive interactions to localize hydrophilic NPs within the hydrophilic portion of block polymers. Related lipophilic approaches often place NPs at the block polymer interface. Here, a new modality for block polymer–NP control is developed that rather relies upon repulsion via the fluorophobic effect. Fluorinated species strongly associate via repulsion from nonfluorinated media. Here, fluorinated NPs are made with ligand mixtures for granular control over the strength of the fluorophobic effect. Small‐angle X‐ray scattering data reveal that all F‐NPs readily phase separate from polystyrene whereas increasing fluorophobic strength enables dispersions within a fluorinated homopolymer. Next, the F‐NP placement within diblock polymers is investigated as a function of the fluorophobic strength. Weakly fluorophobic F‐NPs are found predominantly near the diblock interface whereas strongly fluorophobic F‐NPs are found to disperse throughout the fluorinated block. The controlled placement of NPs is an emerging way to self‐assemble materials.

A novel strategy of using hydrostatic pressures to synthesize gold-carbon (Au-C) nanohybrid materials is explored. The stable face-centered-cubic (fcc) Au undergoes a structural phase transition to a mixture of primitive orthorhombic and cubic phases as the carbon phase acquires a highly ordered onion-like carbon (OLC) structure which encapsulates the Au nanoparticles, thereby exerting an additional pressure. Increasing the pressure results in a one dimensional (1-D) chain-like structure with the primitive cubic Au nanoparticles contained in an amorphous carbon matrix. The OLC structure allows the formation of quenchable Au nanoparticle phases with the primitive close packing and Au-C hybrids with new mesoscopic structures. Under pressure, we observe the formation of a hybrid material composed of a poorly conducting matrix made of amorphous carbon and conducting OLC-encapsulated Au nanoparticles. The electrical conductivity of this hybrid material under pressure reveals a percolation threshold. We present a new synthesis approach to explore the interplay between atomic and mesoscopic structures and the electrical conductivity of metal hybrid structures.

Taking advantage of recent advances in parallel computing, we studied compositional disorder along metal-oxygen atomic columns in a complex Mo,V-oxide bronze using multislice frozen-phonon calculations. Commonly, the virtual crystal approximation (VCA) is used to model compositional disorder at crystallographic sites in a unit cell for a number of different theoretical and experimental techniques. In the VCA, a weighted linear sum of atomic properties is used to approximate the model structure. When using the VCA, the extracted V content of Mo,V-O columns from experimental high-angle annular dark-field (HAADF) images will be about half the V content estimated from simulations, considering the distinct cation ordering. This discrepancy is larger than the spread of HAADF signals of different configurational orders at a given V concentration, which can be up to 20%. Certain "isophilic" atomic arrangements along the column can be distinguished from more random ones using HAADF-STEM imaging. The trends and ratios of the simulated intensity spreads due to different compositional ordering along 11 M-O columns along the c-axis of the Mo,V oxide bronze qualitatively match those observed in experimental HAADF-STEM data. Instrumental and sample-based noise adds to the variability but does not significantly distort the relative ratios of column intensity variation. We observed that we only required seven random configurations to represent the intensity variations along columns.

Many aspects of nanostructured materials at high pressures are still unexplored. We present here, high-pressure structural behavior of two Zn2SnO4 nanomaterials with inverse spinel type, one a particle with size of ∼7 nm [zero dimensional (0-D)] and the other with a chain-like [one dimensional (1-D)] morphology. We performed in situ micro-Raman and synchrotron X-ray diffraction measurements and observed that the cation disordering of the 0-D nanoparticle is preserved up to ∼40 GPa, suppressing the reported martensitic phase transformation. On the other hand, an irreversible phase transition is observed from the 1-D nanomaterial into a new and dense high-pressure orthorhombic CaFe2O4-type structure at ∼40 GPa. The pressure-treated 0-D and 1-D nanomaterials have distinct diffuse reflectance and emission properties. In particular, a heterojunction between the inverse spinel and quenchable orthorhombic phases allows the use of 1-D Zn2SnO4 nanomaterials as efficient photocatalysts as shown by the degradation of the textile pollutant methylene blue.

Nanoscale galvanic exchange confined by metallic nanoparticles is an intriguing structure-remodeling process that transforms geometrically simple solid nanoparticles into multimetallic hollow nanoparticles with increased structural complexity and compositional diversity. Using liquid polyols with intrinsic reducing capabilities as the reaction medium for nanoparticle-templated galvanic exchange represents an interesting paradigm shift, allowing us to interface galvanic exchange with oxidative etching and seed-mediated deposition without introducing any additional oxidizing or reducing agents. By kinetically maneuvering the interplay among galvanic Cu-Pt exchange, oxidative Cu etching, and seed-mediated Pt deposition, we have been able to selectively transform AuCu3 alloy nanoparticles into two architecturally distinct multimetallic heteronanostructures, namely, Au-Pt alloy skin-covered spongy nanoparticles and Pt nanodendrite-covered hollow nanoparticles, both of which exhibit unique structural features highly desirable for high-performance electrocatalysis. Using the formic acid oxidation and hydrogen evolution reactions in acidic electrolytes as model electrocatalytic reactions, we show that the multimetallic nanoparticles derived from AuCu3 alloy nanoparticles through polyol-mediated galvanic exchange reactions markedly outperform the commercial Pt/C benchmark catalysts in terms of both activity and durability. This work not only provides important mechanistic insights on how galvanic exchange dynamically interplays with other redox processes to rigorously dictate the versatile structural transformations of multimetallic nanoparticles but also sheds light on the detailed structure-property relationships underpinning the intriguing electrocatalytic behaviors of architecturally complex multimetallic heteronanostructures.

Charge enhanced dry impregnation (CEDI) is a method to synthesize supported metal nanoparticles which combines the simplicity of incipient wetness impregnation with the small particle size obtained from electrostatic adsorption of metal precursors onto the oxide support. We have explored the utility of CEDI by applying it to a difficult to impregnate support - a largely one dimensional porous SBA-15 silica. Monometallic (N, Pd, Co, Ni, and Cu) catalysts at multiple metal loadings (1-20 wt%) as well as their bimetallic pairs were supported on mesoporous SBA-15 and characterized by high sensitivity powder XRD and in select formulations, with aberration-corrected z-contrast STEM imaging. CEDI applied to SBA-15 without washing is more effective than dry impregnation (DI) of SBA-15. Compared to amorphous silica, CEDI of SBA-15 without washing gives larger particles at lower metal loadings, while at higher metal loadings, the pores of the SBA-15 help render the particle size smaller than amorphous silica. CEDI without washing of SBA-15 with nitrate salts gives somewhat smaller particles than CEDI with chloride salts. CEDI with washing to remove residual counterions of the metal salt typically gives nanoparticles smaller than 2 nm. The smallest particles are always obtained when the counterion is removed by washing, or when a hydroxide salt is employed; ultrasmall particles are obtained without washing in this case.

Complex Oxides, pp. 157-198 (2019) No Access6: Complex Molybdenum–Vanadium Oxide Bronzes and Suboxides as Catalysts for Selective Oxidation and Ammoxidation of Light HydrocarbonsDouglas J. Buttrey, Douglas A. Blom and Thomas VogtDouglas J. ButtreyCenter for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA, Douglas A. BlomNanoCenter, Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA and Thomas VogtNanoCenter, Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USAhttps://doi.org/10.1142/9789813278585_0006Cited by:1 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: The following sections are included: Introduction Characterization of the M1 Unit Cell Characterization of Rotational Twinning in the MoVTeNbO M2 Phase Using HAADF High-Temperature and In Situ HAADF Measurements Multislice Frozen-Phonon Image Simulation Conclusions References FiguresReferencesRelatedDetailsCited By 1Probing the Positions of TeO Moieties in the Channels of the MoVNbTeO M1 Catalyst: A Density Functional Theory Model StudyJuan Manuel Arce-Ramos, Graham Rugg, Alexander Genest and Notker Rösch30 January 2021 | Catalysis Letters, Vol. 151, No. 10 Complex OxidesMetrics History PDF download

We report frozen phonon multi-slice image simulations for the complex oxidation catalyst M1. Quantitative analysis of the simulations suggests that the detailed order of the cations along the electron propagation direction in a [001] zone axis orientation can lead to different high-angle annular dark field signals from atomic columns with identical composition. The annular dark field signal varies linearly with atomic percent V, and the spread of intensities due to the atomic species order is of similar magnitude to the intensity difference due to ± 5% V.

Citation for published item: Tate, Matthew L. and Blom, Douglas A. and Avdeev, Maxim and Brand, Helen E. A. and McIntyre, Garry J. and Vogt, Thomas and Evans, Ivana Radosavljevic (2017) 'New apatite-type oxide ion conductor, Bi2La8[(GeO4)6]O3 : structure, properties, and direct imaging of low-level interstitial oxygen atoms using aberration-corrected scanning transmission electron microscopy.', Advanced functional materials., 27 (8). p. 1605625.

Seed-mediated heteroepitaxial growth provides a versatile synthetic approach to a diverse set of geometrically distinct bimetallic heteronanostructures. In bimetallic nanocrystal systems, interfacial heteroepitaxial growth typically occurs between structurally similar metals with lattice mismatch below 5%, whereas controlled epitaxial growth of bimetallic nanocrystals comprising metals with larger lattice mismatches or even dissimilar crystalline structures has long been challenging. Here, the epitaxial growth of both face-centered cubic (fcc) and hexagonal close-packed (hcp) Ni on fcc Au nanocrystal seeds in polyol solvents is systematically investigated, to shed light on the complex mechanisms underpinning the intriguing geometric evolution of lattice-mismatched bimetallic nanocrystals during seed-mediated heteroepitaxial overgrowth. The success in geometry-controlled syntheses of a series of Au-Ni bimetallic heteronanostructures, such as conformal core-shell nanoparticles, asymmetric heterodimers, and multibranched core-satellite nanocrystals, represents a significant step toward the extension of nanoscale interfacial heteroepitaxy from lattice-matched bimetallic systems to the ones exhibiting large lattice mismatches and even dissimilar crystalline structures. The insights gained from this work serve as a central design principle that guides the development of new synthetic approaches to architecturally sophisticated and compositionally diverse multimetallic heteronanostructures.

Catalytic N-H bond activation and breaking by well-defined molecular complexes or their heterogeneous analogues is considered to be a challenge in chemical science. Metal(0) nanoparticles catalytically decompose NH3; they are, however, ill defined and contain a range of contiguous metal sites with varying coordination numbers and catalytic properties. So far, no well-defined/molecular Mn+-containing materials have been demonstrated to break strong N-H bonds catalytically, especially in NH3, the molecule with the strongest N-H bonds. Recently, noncatalytic activation of NH3 with the liberation of molecular H-2 on an organometallic molybdenum complex was demonstrated. Herein, we show the catalytic activation and breaking of N-H bonds on a singly dispersed, well-defined, and highly thermally resistant (even under reducing environments) (Co1O4)-O-II site of a heterogeneous catalyst for organic (ethylamine) and inorganic (NH3, with the formation of N-2 and H-2) molecules. The single-site material serves as a viable precursor to ultrasmall (2.7 nm and less) silica-supported cobalt nanoparticles; thus, we directly compare the activity of isolated cationic cobalt sites with small cobalt nanoparticles. Density functional theory (DFT) calculations suggest a unique mechanism involving breaking of the N-H bonds in NH3 and N-N coupling steps taking place on a Co1O4 site with the formation of N2H4, which then decomposes to H-2 and N2H2; N2H2 subsequently decomposes to H-2 and N-2. In contrast, Co1N4 sites are not catalytically active, which implies that the ligand environment around a single atom of a heterogeneous catalyst largely controls reactivity. This may open a new chapter for the design of well-defined heterogeneous materials for N-H bond-activation reactions.

Ag+ -Exchanged LSX (Ag-LSX: Ag96 Al96 Si96 O384 ⋅n H2 O), a large pore low silica analogue (Si/Al=1.0) of faujasite, was prepared and post-synthetically modified using pressure and temperature in the presence of various pore-penetrating fluids. Using high-resolution synchrotron X-ray powder and single crystal diffraction we derive structural models of the as-prepared and post-synthetically modified Ag-LSX materials. In the as-prepared Ag-LSX model, we located 96 silver cations and 245 H2 O molecules distributed over seven and five distinctive sites, respectively. At 1.4(1) GPa pressure and 150 °C in ethanol the number of silver cations within the pores of Ag-LSX is reduced by ca. 47.4 %, whereas the number of H2 O molecules is increased by ca. 40.8 %. The formation of zero-valent silver nanoparticles deposited on Ag-LSX crystallites depends on the fluid present during pressurization. Ag-nanoparticle-Ag-zeolite hybrid materials are recovered after pressure release and shown to have different chemical reactivity when used as catalysts for ethylene epoxidation.