SEARCH · Search NASA
Results for “structure”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Dominance of interface chemistry over the bulk properties in determining the electronic structure of epitaxial metal/perovskite oxide heterojunctions
We show that despite very similar crystallographic properties and work function values in the bulk, epitaxial Fe and Cr metallizations on Nb:SrTiO3(001) generate completely different heterojunction electronic properties. Cr is Ohmic whereas Fe forms a Schottky barrier with a barrier height of 0.50 eV. This contrast arises because of differences in interface chemistry. In contrast to Cr [Chambers, S. A. et al., Adv. Mater. 2013, 25, 4001.], Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Cr and Fe are understood by performing first-principles calculations of the energetics of defect formation which corroborate the observed interface chemistry and structure.
Low-Temperature Synthesis, Structural Characterization, and Electrochemistry of Ni-Rich Spinel-like LiNi 2– y Mn y O 4 (0.4 ≤ y ≤ 1)
The thermal conversion of chemically delithiated layered Li 0.5 Ni 1-y Mn y O 2 (0.2 ≤ y ≤ 0.5) into spinel-like LiNi 2-y Mn y O 4 (0.4 ≤ y ≤ 1) has been systematically investigated. The formed spinel-like phases are metastable and cannot be accessed by a conventional high-temperature solid-state method. The layered-to-spinel transformation mechanism has been studied by the Rietveld refinement of in situ neutron diffraction as a function of temperature (25–300 °C). In particular, the ionic diffusion of Li and M ions is quantified at different temperatures. Electrochemistry of the metastable spinel-like phases obtained has been studied in lithium-ion cells. A bond valence sum map has been performed to understand the ionic diffusion of lithium ions in the Ni-rich layered, spinel, and rock-salt structures. The study can aid the understanding of the possible phases that could be formed during the cycling of Ni-rich layered oxide cathodes.
Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interactions
Designing optimal electrolytes is key to enhancing the performance of energy storage devices,especially relating to cycle life, efficiency, and stability.1 Specifically, for future beyond-Li ion energy storage, there is still ample room for electrolyte improvements. Among the candidates for higher gravimetric energy storage, the Li-S battery is considered quite promising, owing to its theoretical specific energy density (2600 Wh/kg) and specific capacity (1675 mAh/g) and significantly lower cost as compared to state-of-art lithium-ion batteries.
Influence of Copper Oxidation State on the Bonding and Electronic Structure of Cobalt-Copper Complexes
Heterobimetallic complexes that pair cobalt and copper were synthesized and characterized by a suite of physical methods, including X-ray diffraction, X-ray anomalous scattering, cyclic voltammetry, magnetometry, electronic absorption spectroscopy, electron paramagnetic resonance, and quantum chemical methods. Both Cu(II) and Cu(I) reagents were independently added to a Co(II) metalloligand to provide (py 3 tren)CoCuCl (1-Cl) and (py 3 tren)CoCu(CH 3 CN) (2-CH 3 CN), respectively, where py3tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. Complex 2-CH 3 CN can lose the acetonitrile ligand to generate a coordination polymer consistent with the formula “(py3tren)CoCu” (2). One-electron chemical oxidation of 2-CH 3 CN with AgOTf generated (py 3 tren)CoCuOTf (1-OTf). The Cu(II)/Cu(I) redox couple for 1-OTf and 2-CH 3 CN is reversible at -0.56 and -0.33 V vs Fc + /Fc, respectively. The copper oxidation state impacts the electronic structure of the heterobimetallic core, as well as the nature of the Co–Cu interaction. Quantum chemical calculations showed modest electron delocalization in the (CoCu) +4 state via a Co–Cu σ bond that is weakened by partial population of the Co–Cu σ antibonding orbital. By contrast, no covalent Co–Cu bonding is predicted for the (CoCu) +3 analogue, and the d-electrons are fully localized at individual metals.
Nuclear Magnetic Resonance Measurements and Electronic Structure of Pu(IV) in [(Me)4N]2PuCl6
The synthesis, electronic structure, and characterization via single crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and magnetic susceptibility of (Me4N)2PuCl6 are reported. NMR measurements were performed to both search for the direct 239Pu resonance and to obtain local magnetic and electronic information at the Cl site through 35Cl and 37Cl spectra. No signature of 239Pu NMR was observed. The temperature dependence of the Cl spectra was simulated by diagonalizing the Zeeman and quadrupolar Hamiltonians for 35Cl, 37Cl and 14N isotopes. Electronic structure calculations predict a magnetic ?5 triplet ground state of Pu(IV) in the crystalline electric field of the undistorted PuCl6 octahedron. A tetragonal distortion would result in a very small splitting (20 cm-1) of the triplet ground state into a nonmagnetic singlet and a doublet state. The Cl shifts have an inflection point at T = 15 K, differing from the bulk susceptibility, indicating a nonmagnetic crystal field ground state. The Cl spin-lattice relaxation time is constant down to T = 15 K, below which it rapidly increases, also supporting the nonmagnetic crystal field ground state.
Crystal Structure Evolution and Notable Thermal Expansion in Hybrid Perovskites Formamidinium Tin Iodide and Formamidinium Lead Bromide
The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI 3 , FA + = CH[NH 2 ] 2 + ) and formamidinium lead bromide (FAPbBr 3 ), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic Pm$\bar{3}$m (No. 221) to tetragonal P 4 /mbm (No. 127) for both materials upon cooling; this occurs for FAPbBr 3 between 275 and 250 K, and for FASnI 3 between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic Pnma (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI 3 and FAPbBr 3 are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K –1 for FASnI 3 at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the inorganic network and dynamic lone pair stereochemical activity on the B-site. Unusual pseudocubic behavior is displayed in the tetragonal phase of the FAPbBr 3 , similar to that previously observed in FAPbI 3 .
Vanadium-Substituted Tunnel Structured Silver Hollandite (Ag 1.2 V x Mn 8-x O 16 ) Impact on Morphology and Electrochemistry
Abstract not provided
Photophysical Properties and Electronic Structure of Porphyrins Bearing Zero to Four meso -Phenyl Substituents: New Insights into Seemingly Well Understood Tetrapyrroles
Six free base porphyrins bearing 0–4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S 1 ) decreases with an increase in the number of meso-phenyl groups; the values in toluene are H 2 P-0 (15.5 ns) > H 2 P-1 (14.9 ns) > H 2 P-2c (14.4 ns) > H 2 P-2t (13.8 ns) ~ H 2 P-3 (13.8 ns) > H 2 P-4 (12.8 ns), where “H 2 P” refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and “c” and “t” refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are H 2 P-0 (0.049) < H 2 P-1 (0.063) ~ H 2 P-2c (0.063) < H 2 P-2t (0.071) < H 2 P-3 (0.073) < H 2 P-4 (0.090). Similar trends occur in DMF. All radiative and non-radiative (internal conversion and intersystem crossing) rate constants for S 1 decay increase with increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S o → S 1 absorption manifold. Here, the trend is reproduced by time-dependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S o → S 1 radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S o → S 1 absorption wavelength. Polarizations of the S 1 and S 2 excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy and life-sciences research.
Effect of Block Length and Side Chain Length Ratios on Determining a Multicompartment Micelle Structure
Previous work has identified the importance of the lipophilic-fluorophilic block length ratio R l in predicting the morphology of linear lipophilic-hydrophilic-fluorophilic (hereafter referred to as BAC) micelle systems. Here, a generalized form R of this structural parameter is developed that makes no assumption of BAC triblock copolymer linearity, while still providing accurate predictions of micelle morphology. Here, the morphologies of BAC micelles formed by triblock copolymers with R <<1 or R >>1 have similar features, with the only notable difference being an inversion of the lipophilic and fluorophilic regions. A destabilization of the single-core micelle structure occurs as R approaches unity from either direction. Finally, the extent to which micelle morphology depends on polymer architecture instead of composition alone is examined, with decreased patchiness observed in BAC systems with very long block lengths. Through modification of both the R-value and the polymer architecture, the micelle morphology can be effectively tuned for use in immobilized catalysis and nanoreactor applications.
Structure Evolution of Chemically Degraded ZIF-8
Exposure of the metal–organic zeolitic imidazolate framework-8 (ZIF-8) to humid SO x gas cleaves Zn–N bonds, leading to framework disorder and a corresponding decrease in pore accessibility. This work shows that framework disorder exhibits a critical behavior with respect to water concentration in the gas stream. While the quantity of sulfate adsorbed at Zn nodes is at or below the detection threshold for the X-ray pair distribution function technique, precluding direct observation of the adsorbate, powder diffraction reveals clear structural alterations. Here, the onset of linker cleavage correlates to a large microstrain that diminishes as framework disorder increases. Energy-dispersive X-ray spectroscopy shows that the sulfur distribution throughout representative grains is uniform, implying the structure transformation is homogeneously nucleated and not dependent upon gas diffusion kinetics.
Molecular Dynamics Simulations of Water Structure and Diffusion in a 1 nm Diameter Silica Nanopore as a Function of Surface Charge and Alkali Metal Counterion Identity
Water confined in nanopores-particularly in pores narrower than 2 nm-displays distinct physicochemical properties that remain incompletely examined despite their importance in nanofluidics, molecular biology, geology, and materials sciences. In this study, we use molecular dynamics simulations to investigate the coordination structure and mobility of water and alkali metals (Li, Na, K, Cs) inside a 1 nm diameter cylindrical silica nanopore as a function of surface charge density, a model system particularly relevant to the alteration kinetics of silicate glasses and minerals in geologic formations. We find that the presence of a negative surface charge and adsorbed counterions within the pore strongly impacts water structure and dynamics. In particular, it significantly orients water O-H bonds toward the surface and slows water diffusion by almost 1 order of magnitude. Ion crowding in the charged nanopore enhances the tendency of counterions to coordinate closely with the silica surface, which moderates the impact of ions on water dynamics. Co-ions are strongly excluded from the nanopore at all surface charges, suggesting that the 1 nm diameter cylindrical silica nanopores likely exhibit nearly ideal semipermeable membrane transport properties.
Mineral–Water Interface Structure of Xenotime (YPO 4 ) {100}
Crystal truncation rod (CTR) measurements and density functional theory (DFT) calculations were performed to determine the atomic structure of the mineral–water interface of the {100} surface of xenotime (nominally YPO 4 ). This mineral is important, because it incorporates a variety of rare earth elements (REEs) in its crystal structure. REEs are critical materials necessary for a variety of renewable and energy efficient technologies. Current beneficiation techniques are not highly selective for REE ore minerals, and large amounts go to waste; this is a first step toward designing more efficient beneficiation. Evidence is found for minor relaxation of the surface within the topmost monolayer with little or no relaxation in subsurface layers. Justification for ordered water at the interface is found, where water binds to surface cations and donates hydrogen bonds to surface phosphates. The average bond lengths between cations and oxygens on water are 228 pm in the best fit to the CTR data, versus 243 and 251 pm for the DFT. No agreement on water positions bound to surface phosphates is obtained. Altogether, the findings suggest that ligands used in beneficiation with a single anionic headgroup, such as fatty acids, will have limited selectivity for xenotime relative to undesirable minerals.
Anomalies and Local Structure of Liquid Water from Boiling to the Supercooled Regime as Predicted by the Many-Body MB-pol Model
For the past 50 years, researchers have sought molecular models that can accurately reproduce water’s microscopic structure and thermophysical properties across broad ranges of its complex phase diagram. Herein, molecular dynamics simulations with the many-body MB-pol model are performed to monitor the thermodynamic response functions and local structure of liquid water from the boiling point down to deeply supercooled temperatures at ambient pressure. The isothermal compressibility and isobaric heat capacity show maxima near 223 K, in excellent agreement with recent experiments, and the liquid density exhibits a minimum at ~208 K. A local tetrahedral arrangement, where each water molecule accepts and donates two hydrogen bonds, is found to be the most probable hydrogen-bonding topology at all temperatures. Further, this work suggests that MB-pol may provide predictive capability for studies of liquid water’s physical properties across broad ranges of thermodynamic states, including the so-called water’s “no man’s land” which is difficult to probe experimentally.
The Relationship between Ionic Conductivity and Solvation Structures of Localized High-Concentration Fluorinated Electrolytes for Lithium-Ion Batteries
Localized high concentration electrolyte (LHCE) combines a diluent with high concentration electrolyte (HCE), offering promising properties. The ions, solvent, and diluent interact to form a complex heterogenous liquid structure, where high salt concentration clusters are embedded in diluent. Optimizing LHCE for desired electrolyte properties like high ionic conductivity, low viscosity, effective solid electrolyte interphase (SEI) formability, within the vast chemical and compositional design space requires deeper understanding and theoretical guidance. For this work, we investigated the structures and conductivity of LHCEs based on a fluorinated solvent with two different diluents at varying concentrations. The 2,2,3,3-tetrafluoropropyl trifluoroacetate (TFPTFA) enters the solvation cluster due to its stronger Li-ion interactions, whereas 1,1,2,2-tetrafuoroethyl 2,2,2-trifuoroethyl ether (TFETFE) enters only at extremely high diluent concentrations. The ionic conductivity increases with decreasing diluent concentrations, with a slope change during cluster percolation. Overall, TFETFE demonstrates higher effectiveness than TFPTFA, forming higher local salt concentration clusters, and resulting in higher ionic conductivity.
Effects of Nonhydrostatic Stress on Structural and Optoelectronic Properties of Methylammonium Lead Bromide Perovskite
We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr3) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr3 show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr3 leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to a third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressure-induced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N–H···Br hydrogen-bonding interactions, affect the Pb–Br distance and the bandgap evolution under pressure.