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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Reconstruction-stabilized epitaxy of LaCoO 3 /SrTiO 3 (111) heterostructures by pulsed laser deposition

Unlike widely explored complex oxide heterostructures grown along [001], the study of [111]-oriented heterointerfaces is very limited thus far. One of the main challenges is to overcome the polar discontinuity that hinders the epitaxy of atomically sharp interfaces. Here, by taking LaCoO 3 /SrTiO 3 (111) as a prototype, we show that the reconstruction, which effectively compensates the surface polarity, can stabilize the epitaxy of the heterostructure with polar discontinuity. A reconstructed substrate surface is prepared, while the growth is controlled to form reconstruction on the film surface. To suppress the chemical diffusion across the interface, the growth is interrupted between each unit cell layer to allow the lattice relaxation at a lowered temperature. In this way, high quality two-dimensional growth is realized and the heterointerfaces exhibit sharpness at the atomic scale. As a result, our work provides a path to precisely control the growth of complex oxide heterostructures along polar orientations that exhibit emergent quantum phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Role of Gas in Determining Image Quality and Resolution During In Situ Scanning Transmission Electron Microscopy Experiments

As gas-solid heterogeneous catalytic reactions are molecular in nature, a full mechanistic understanding of the process requires atomic scale characterization under realistic operating conditions. While atomic resolution imaging has become a routine in modern high-vacuum (scanning) transmission electron microscopy ((S)TEM), both image quality and resolution nominally degrade when reaction gases are introduced. In this work, we systematically assess the effects of different gases at various pressures on the quality and resolution of images obtained at room temperature in the annular dark field STEM imaging mode using a differentially pumped (DP) gas cell. This imaging mode is largely free from inelastic scattering effects induced by the presence of gases and retains good imaging properties over a wide range of gas mass/pressures. We demonstrate the application of the ESTEM with atomic resolution images of a complex oxide alkane oxidation catalyst MoVNbTeOx (M1) immersed in light and heavy gas environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Interface creation on a mixed-terminated perovskite surface

In the field of complex oxide heterostructures, understanding of the initial substrate surface can be critical to fundamental studies regarding the development of emergent properties at the film–substrate interface. For this reason, a considerable amount of effort has gone into the development of techniques to achieve surfaces with single termination for a variety of perovskite single crystals. However, a decisive understanding of how an interface is created when epitaxial growth occurs on a mixed terminated surface remains lacking. Employing in situ synchrotron X-ray scattering during thin film growth by molecular beam epitaxy, we investigate the initial stages of growth on (LaAlO 3 )(Sr 2 AlTaO 6 ) (001) substrates with mixed termination. Using LaNiO 3 as a model system, we find that the surface layers of the substrate restructure during deposition such that while a NiO 2 monolayer is weakly bound, a LaO monolayer bonds strongly, effectively incorporating with (Al, Ta)O 2 from the surface and forming a La(Al, Ta)O 3 ultrathin film. <!--/abstract content--><!--fulltext content--> With regard to the synthesis of high-quality complex oxide heterostructures, it is important that the substrate meet an array of criteria related to the crystal structure, lattice parameter, thermodynamic properties, electrical and optical properties, and so on. In particular, the surface of the substrate should be atomically smooth and exhibit the step-terrace structure ideal for epitaxial thin film growth. Perovskite oxides with the formula ABO 3 have two possible (001) surfaces: AO (A-site termination) and BO 2 (B-site termination). The ability to consistently achieve one of these surface terminations on SrTiO 3 , i.e., TiO 2 , has helped SrTiO 3 become the predominant oxide in the field.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Domain- and symmetry-transition origins of reduced nanosecond piezoelectricity in ferroelectric/dielectric superlattices

Complex-oxide superlattices (SLs) with atomic-scale periodicity have dynamical properties that are distinct from thin films of uniform composition. The origins of these properties are closely related to the dynamics of polarization domains and to field-driven changes in the symmetries resulting from interfacial coupling between different components. These dynamics are apparent at timescales from a few nanoseconds to several milliseconds in experiments probing the piezoelectricity of a ferroelectric/dielectric BaTiO{sub 3}(BTO)/CaTiO{sub 3} (CTO) SL using time-resolved x-ray microdiffraction. At the 100 ns timescale, the piezoelectric distortion is approximately ten times smaller than in the millisecond regime. This reduced piezoelectricity at short timescales is not observed in previously studied PbTiO{sub 3}/SrTiO{sub 3} SLs or compositionally uniform ferroelectrics such as tetragonal compositions of Pb(Zr,Ti)O{sub 3}. The unusual behavior of the BTO/CTO SL can be linked to the switching of a nanodomain state into a uniform polarization state or to a field-induced crystallographic symmetry transition. A comparison of the results with the characteristic timescales of these two dynamical phenomena in other complex oxides with different compositions suggests that the phase transition is a more likely possibility.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

New complex bismuth oxides in the Bi{sub 2}O{sub 3}–NiO–Sb{sub 2}O{sub 5} system and their properties

Phase equilibria in the Bi{sub 2}O{sub 3}–NiO–Sb{sub 2}O{sub 5} system have been investigated. The isothermal section of the system (650°) has been constructed. For the first time the existence of the pyrochlore structure solid solution, (Bi{sub 2−x}Ni{sub x})Ni{sub 2/3−y}Sb{sub 4/3+y}O{sub 7±δ}, x=0.1−0.35, y=0−0.1, and a new compound Bi{sub 3}Ni{sub 2/3}Sb{sub 7/3}O{sub 11} of KSbO{sub 3} structure type have been discovered. The structures and properties of these compounds were studied by XRD, XAFS, diffuse reflection spectroscopy and magnetic methods. Rietveld refinement of synchrotron radiation-based powder XRD data for pyrochlore sample of 38.43Bi{sub 2}O{sub 3}–33.0NiO–28.57Sb{sub 2}O{sub 5} composition and Bi{sub 3}Ni{sub 2/3}Sb{sub 7/3}O{sub 11} compound was performed. The best possible refinement of the positional parameters for both pyrochlore and Bi{sub 3}Ni{sub 2/3}Sb{sub 7/3}O{sub 11} structures was achieved for disordered models. Magnetic, diffuse reflection and an X-ray absorption spectroscopy study confirmed that in both compounds nickel ions are mainly in a 2+ oxidation state. According to magnetic data, Bi{sub 1.84}Ni{sub 0.16}(Ni{sub 0.63}Sb{sub 1.37})O{sub 7} pyrochlore and Bi{sub 3}Ni{sub 2/3}Sb{sub 7/3}O{sub 11} are overall paramagnetic in nature. - Graphical abstract: Isothermal section of the Bi{sub 2}O{sub 3}–NiO–Sb{sub 2}O{sub 5} system at 650 °C in air and variation of magnetic susceptibility and inverse susceptibility with temperature for Bi{sub 1.84}Ni{sub 0.79}Sb{sub 1.37}O{sub 7} pyrochlore. - Highlights: • We have constructed the isothermal section of the system Bi{sub 2}O{sub 3}–NiO–Sb{sub 2}O{sub 5}. • The boundaries of pyrochlore structure solid solution have been determined. • New Bi{sub 3}Ni{sub 2/3}Sb{sub 7/3}O{sub 11} compound of KSbO{sub 3} structure type have been discovered. • The structures were studied by means of synchrotron X-ray diffraction and XAFS. • Magnetic measurements revealed that both compounds are paramagnetic at 2–350 K.

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

Hydrolytic cleavage of both CS 2 carbon–sulfur bonds by multinuclear Pd(II) complexes at room temperature

Developing homogeneous catalysts that convert CS 2 and COS pollutants into environmentally benign products is important for both fundamental catalytic research and applied environmental science. Here we report a series of air-stable dimeric Pd complexes that mediate the facile hydrolytic cleavage of both CS 2 carbon–sulfur bonds at 25 °C to produce CO 2 and trimeric Pd complexes. Oxidation of the trimeric complexes with HNO 3 regenerates the dimeric starting complexes with the release of SO 2 and NO 2 . Isotopic labelling confirms that the carbon and oxygen atoms of CO 2 originate from CS 2 and H 2 O, respectively, and reaction intermediates were observed by gas-phase and electrospray ionization mass spectrometry, as well as by Fourier transform infrared spectroscopy. We also propose a plausible mechanistic scenario based on the experimentally observed intermediates. Here, the mechanism involves intramolecular attack by a nucleophilic Pd–OH moiety on the carbon atom of coordinated µ-OCS 2 , which on deprotonation cleaves one C–S bond and simultaneously forms a C–O bond. Coupled C–S cleavage and CO 2 release to yield [(bpy) 3 Pd 3 ( µ3 -S) 2 ](NO 3 ) 2 (bpy, 2,2'-bipyridine) provides the thermodynamic driving force for the reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Novel complex ceramic oxides, Ln 2 TiO 5 (Ln = La, Sm, Gd, Tb, Dy, Ho, Er, and Yb), for polyphase nuclear waste‐forms

Abstract As part of a broader study of ceramic nuclear waste‐forms, four different lanthanide titanates were fabricated; La 0.1 Sm 0.1 Gd 0.1 Tb 0.1 Dy 0.3 Ho 0.1 Er 0.2 YbTiO 5 , Sm 0.3 Gd 0.3 Dy 0.3 Yb 1.1 TiO 5 , Sm 0.1 Gd 0.4 Dy 0.4 Yb 1.1 TiO 5 , and Sm 0.2 Gd 0.2 Dy 0.2 Yb 1.4 TiO 5 . The aim was to produce single‐phase novel materials with cubic symmetry, capable of incorporating a wide variety of cations and with acceptable radiation tolerance. The chemistry flexibility and radiation tolerance are some of the major desirable properties for nuclear waste‐form materials. By using multiple lanthanides the average lanthanide radius can be controlled and consequently the structure, along with properties such as radiation tolerance. The radiation tolerance was assessed using in situ 1 MeV krypton irradiation and transmission electron microscopy characterization. Those materials for which cubic symmetry was achieved displayed better radiation tolerance; a greater critical fluence of ions ( F c ) was required for the crystalline to amorphous transition, and a lower temperature was required to maintain crystallinity ( T c ) during irradiation.

Aughterson, Robert D.↗
DOE OSTI2021

Single-Site Metal Organic Complexes on Oxide Supports for Selective Alkane Functionalization

High levels of reaction selectivity for heterogeneous catalysis are generally difficult to achieve with traditional metal nanoparticle catalysts, due to the variety of metal binding sites available. In this project, we have developed a metal-ligand coordination strategy to form transition metal single-sites on high surface area supports as heterogeneous single-site catalysts. Metal centers can be atomically dispersed in the binding pockets of organic ligands on oxide supports. The systems have to be carefully designed so that tendencies for metal cluster formation or binding to surface defect sites are overcome by the attractive coordination environment provided by the organic ligand. This was confirmed by a comprehensive set of characterization methods, including XAS, XPS, XRD, TEM, and CO adsorption. Design and tuning of metal-support and support-ligand interactions were crucial for high structural uniformity. We demonstrate that supported metal-ligand SSCs are effective, recyclable catalysts for alkene hydrosilylation reactions. Compared with commercial homogeneous catalysts, they exhibit improved yield and selectivity, less metal aggregation and side reactions, and stronger tolerance with functionalized substrates. These results give some preliminary indication that coordination with organic ligands can improve activity and selectivity in metal/oxide heterogeneous catalysts.

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

Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines

Nickel complexes were prepared with diphosphine ligands that contain pendant amines, and these complexes catalytically oxidize primary and secondary alcohols to their respective aldehydes and ketones. Kinetic and mechanistic studies of these prospective electrocatalysts were performed to understand what influences the catalytic activity. For the oxidation of diphenylmethanol, the catalytic rates were determined to be dependent on the concentration of both the catalyst and the alcohol. The catalytic rates were found to be independent of the concentration of base and oxidant. The incorporation of pendant amines to the phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the pendant amine exhibiting the fastest rates. We thank Dr. John C. Linehan, Dr. Elliott B. Hulley, Dr. Jonathan M. Darmon, and Dr. Elizabeth L. Tyson for helpful discussions. Research by CJW, PD, DLM, and AMA was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Research by MLH was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

We introduce a new family of complexes with the general formula [Run(tda)(py)2]m+ (n = 2, m = 0, 1; n = 3, m = 1, 2+; n = 4, m = 2, 32+), with tda2– being [2,2':6',2''-terpyridine]-6,6''-dicarboxylate, including complex [RuIV(OH)(tda-κ-N3O)(py)2]+, 4H+, which we find to be an impressive water oxidation catalyst, formed by hydroxo coordination to 32+ under basic conditions. The complexes are synthesized, isolated, and thoroughly characterized by analytical, spectroscopic (UV–vis, nuclear magnetic resonance, electron paramagnetic resonance), computational, and electrochemical techniques (cyclic voltammetry, differential pulse voltammetry, coulometry), including solid-state monocrystal X-ray diffraction analysis. In oxidation state IV, the Ru center is seven-coordinated and diamagnetic, whereas in oxidation state II, the complex has an unbonded dangling carboxylate and is six-coordinated while still diamagnetic. With oxidation state III, the coordination number is halfway between the coordination of oxidation states II and IV. Species generated in situ have also been characterized by spectroscopic, computational, and electrochemical techniques, together with the related species derived from a different degree of protonation and oxidation states. 4H+ can be generated potentiometrically, or voltammetrically, from 32+, and both coexist in solution. While complex 32+ is not catalytically active, the catalytic performance of complex 4H+ is characterized by the foot of the wave analysis, giving an impressive turnover frequency record of 8000 s–1 at pH 7.0 and 50 000 s–1 at pH 10.0. Density functional theory calculations provide a complete description of the water oxidation catalytic cycle of 4H+, manifesting the key functional role of the dangling carboxylate in lowering the activation free energies that lead to O–O bond formation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Wide bandgap tunability in complex transition metal oxides by site-specific substitution

Fabricating complex transition metal oxides with a tunable bandgap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its bandgap can be narrowed by as much as 1 eV, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the bandgap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the bandgap in complex oxides for use in emerging oxide optoelectronic and energy applications.

36 MATERIALS SCIENCE↗