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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Understanding initial zeolite oligomerization steps with first principles calculations

Zeolites are porous aluminosilicate materials that find various applications in the chemical industry in separations, catalysis, ion exchange, and so forth. However, despite their widespread use, the reaction mechanisms occurring during zeolite growth are still unclear. In the study, we use density functional theory calculations to gain insights into the thermodynamics of oligomerization, which constitute the initial steps of zeolite growth. By taking into consideration solvent and temperature effects, our results demonstrate that the growth of aluminosilicate systems is significantly more exothermic than their pure silicate counterparts. Under pH neutral conditions, water prefers to dissociate on the early-growth-stage aluminosilicate complexes rather than desorb, thus generating potential Brønsted acid sites on the oligomers. Additionally, (alumino)silicate growth pathways are evaluated in the presence of Na + cation, as well as the Ca 2+ cation for the pure silicate pathway. The presence of cations increases the exothermicity of growth, with Ca 2+ exhibiting the most energetically favorable growth environment for the silicate systems. Importantly, we demonstrate through reaction extent analysis that the presence of cations modulates the speciation of the formed oligomers, with Na + favoring linear species in addition to the generally preferred cyclic ones. Overall, this work provides a fundamental understanding of the thermodynamics of complex reaction paths that occur during early stages of zeolite growth and suggests that the initial growth steps can have significant impact on the final zeolite structure.

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

Pd-vacancy complex in Ge: TDPAC and ab initio study

Low temperature metal-induced-crystallized germanium is a promising alternative for silicon in Complementary Metal-Oxide-Semiconductor (CMOS) technology. Palladium (Pd) is one of the metals suitable for inducing the low temperature crystallization. It is not certain, how residual Pd atoms are integrated into the Ge lattice. Therefore, time-different γ-γ perturbed angular correlation (TDPAC) technique using the {sup 100}Pd(→{sup 100}Rh) nuclear probe has been applied to study the hyperfine interactions of this probe in single crystalline undoped Ge. A Pd-vacancy (Pd-V) complex with a unique interaction frequency of 8.4(2) Mrad/s has been identified. The Pd-V complex has been measured to have a maximum fraction after annealing at 350 °C. Density functional theory calculations have confirmed that the Pd-V complex may have the split-vacancy configuration in Ge, in contrast to the full-vacancy configuration observed in Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effects of the local structure dependence of evaporation fields on field evaporation behavior

Accurate three dimensional reconstructions of atomic positions and full quantification of the information contained in atom probe microscopy data rely on understanding the physical processes taking place during field evaporation of atoms from needle-shaped specimens. However, the modeling framework for atom probe microscopy has only limited quantitative justification. Building on the continuum field models previously developed, we introduce a more physical approach with the selection of evaporation events based on density functional theory calculations. This model reproduces key features observed experimentally in terms of sequence of evaporation, evaporation maps, and depth resolution, and provides insights into the physical limit for spatial resolution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electronic structure, optical and thermal/concentration quenching properties of Lu{sub 2−2x}Eu{sub 2x}WO{sub 6} (0 ≤ x ≤0.2)

Highlights: • The band gap of Lu{sub 2}WO{sub 6} is calculated to be 3.13 eV using the CASTEP mode. • Valent state and occupation site of Eu are clarified by X-ray absorption fine structure (XAFS) spectra. • The thermal/concentration quenching mechanisms of Eu in Lu{sub 2}WO{sub 6} have been investigated in detail. - Abstract: Density functional theory calculations on monoclinic Lu{sub 2}WO{sub 6} is carried out using the Cambridge Sequential Total Energy Package code. The result indicates that Lu{sub 2}WO{sub 6} is a broad band gap semiconductor with an indirect band gap of 3.13 eV. Eu ions are trivalency and the average coordination number is 7.6(5), indicating that the site of Lu is occupied by Eu. The activation energy ΔE is calculated as 0.314 eV. In addiation, the thermal quenching mechnism of Eu-activated Lu{sub 2}WO{sub 6} and the different concentration quenching mechanisms for {sup 5}D{sub 0} and {sup 5}D{sub 1} emissions of Eu ions have been proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Targeting One- and Two-Dimensional Ta–Te Structures via Nanotube Encapsulation

Fine control over material synthesis on the nanoscale can facilitate the stabilization of competing crystalline structures. In this report we demonstrate how carbon nanotube reaction vessels can be used to selectively create one-dimensional TaTe 3 chains or two-dimensional TaTe 2 nanoribbons with exquisite control of the chain number or nanoribbon thickness and width. Transmission electron microscopy and scanning transmission electron microscopy reveal the detailed atomic structure of the encapsulated materials. Complex superstructures such as multichain spiraling and apparent multilayer moirés are observed. The rare 2H phase of TaTe 2 (1H in monolayer) is found to be abundant as an encapsulated nanoribbon inside carbon nanotubes. The experimental results are complemented by density functional theory calculations for the atomic and electronic structure, which uncovers the prevalence of 2H-TaTe 2 due to nanotube-to-nanoribbon charge transfer and size confinement. Calculations also reveal new 1T' type charge density wave phases in TaTe 2 that could be observed in experimental studies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Fast and stable deep-learning predictions of material properties for solid solution alloys

We present a novel deep learning (DL) approach to produce highly accurate predictions of macroscopic physical properties of solid solution binary alloys and magnetic systems. The major idea is to make use of the correlations between different physical properties in alloy systems to improve the prediction accuracy of neural network (NN) models. We use multitasking NN models to simultaneously predict the total energy, charge density and magnetic moment. These physical properties mutually serve as constraints during the training of the multitasking NN, resulting in more reliable DL models because multiple physics properties are correctly learned by a single model. Two binary alloys, copper–gold (CuAu) and iron–platinum (FePt), were studied. Our results show that once the multitasking NN's are trained, they can estimate the material properties for a specific configuration hundreds of times faster than first-principles density functional theory calculations while retaining comparable accuracy. We used a simple measure based on the root-mean-squared errors to quantify the quality of the NN models, and found that the inclusion of charge density and magnetic moment as physical constraints leads to more stable models that exhibit improved accuracy and reduced uncertainty for the energy predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium

The amorphous iron-germanium system ( a - Fe x Ge 1 - x ) lacks long-range structural order and hence lacks a significant Brillouin zone. The magnetization of a - Fe x Ge 1 - x is well explained by the Stoner model for Fe concentrations x above the onset of magnetic order around x = 0.4 , indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe- 3 d states sufficiently to polarize the electronic structure despite k being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity ρ x y AH relative to crystalline FeGe; this ρ x y AH is compared to density-functional theory calculations of the anomalous Hall conductivity to resolve its underlying mechanisms. The intrinsic mechanism, typically understood as the Berry curvature integrated over occupied k states but shown here to be equivalent to the density of curvature integrated over occupied energies in aperiodic materials, dominates the anomalous Hall conductivity of a - Fe x Ge 1 - x ( 0.38 ≤ x ≤ 0.61 ). The density of curvature is the sum of spin-orbit correlations of local orbital states and can hence be calculated with no reference to k space. This effect and the accompanying Stoner-like model for the intrinsic anomalous Hall conductivity establish a unified understanding of the underlying physics of the anomalous Hall effect in both crystalline and disordered systems.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Giant Rashba splitting in 2D organic-inorganic halide perovskites measured by transient spectroscopies

Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural “multiple quantum wells” that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to “Rashba splitting” close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C 6 H 5 C 2 H 4 NH 3 ) 2 PbI 4 . The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. In conclusion, this finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with –Li–N–B–N– repeating units in α-Li{sub x}BN{sub 2} (1 ⩽ x ⩽ 3)

Ultrahigh energy density batteries based on α-Li{sub x}BN{sub 2} (1 ⩽ x ⩽ 3) positive electrode materials are predicted using density functional theory calculations. The utilization of the reversible LiBN{sub 2} + 2 Li{sup +} + 2 e{sup −} ⇌ Li{sub 3}BN{sub 2} electrochemical cell reaction leads to a voltage of 3.62 V (vs Li/Li{sup +}), theoretical energy densities of 3251 Wh/kg and 5927 Wh/l, with capacities of 899 mAh/g and 1638 mAh/cm{sup 3}, while the cell volume of α-Li{sub 3}BN{sub 2} shrinks only 2.8% per two-electron transfer on charge. These values are far superior to the best existing or theoretically designed intercalation or conversion-based positive electrode materials. For comparison, the theoretical energy density of a Li–O{sub 2}/peroxide battery is 3450 Wh/kg (including the weight of O{sub 2}), that of a Li–S battery is 2600 Wh/kg, that of Li{sub 3}Cr(BO{sub 3})(PO{sub 4}) (one of the best designer intercalation materials) is 1700 Wh/kg, while already commercialized LiCoO{sub 2} allows for 568 Wh/kg. α-Li{sub 3}BN{sub 2} is also known as a good Li-ion conductor with experimentally observed 3 mS/cm ionic conductivity and 78 kJ/mol (≈0.8 eV) activation energy of conduction. The attractive features of α-Li{sub x}BN{sub 2} (1 ⩽ x ⩽ 3) are based on a crystal lattice of 1D conjugated polymers with –Li–N–B–N– repeating units. When some of the Li is deintercalated from α-Li{sub 3}BN{sub 2} the crystal becomes a metallic electron conductor, based on the underlying 1D conjugated π electron system. Thus, α-Li{sub x}BN{sub 2} (1 ⩽ x ⩽ 3) represents a new type of 1D conjugated polymers with significant potential for energy storage and other applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Stability and Band-Gap Tuning of the Chalcogenide Perovskite BaZrS 3 in Raman and Optical Investigations at High Pressures

Here, we report experiments and calculations investigating the pressure and temperature dependences of the optical phonons in BaZrS 3 , and the pressure dependence of its absorption edge. BaZrS 3 is a chalcogenide perovskite in a novel class of materials being considered for photovoltaics. It is studied by Raman spectroscopy as functions of temperature (at 1 atm) and pressure (at 120 and 295 K), and by pressure-transmission spectroscopy at 295 K. Density-functional-theory calculations predict the allowed Raman lines, their intensities, their pressure shifts, and the band-gap pressure shift. Cooling shifts all but one of the phonon peaks to higher frequencies; the temperature coefficients are typical of semiconductors. A strong low-temperature peak at 392.3 cm -1 is attributed to resonant forbidden LO scattering; its shift with temperature has the opposite sign. The pressure coefficients of the phonon frequencies for all observed Raman peaks are positive, indicating no mode softening. The rates of pressure shift also are typical, and show the customary scaling with phonon frequency. Experiment and theory show good agreement on the pressure-induced frequency shifts. The BaZrS 3 absorption edge moves to lower energy with pressure, reflecting a reduction of the band gap. The measured shift is ~-0.015 eV / GPa, slightly less than the density-functional-theory result of -0.025 eV / GPa. We find no evidence that the perovskite structure of BaZrS 3 undergoes any phase changes under hydrostatic pressure to at least 8.9 GPa. Our results indicate the robust structural stability of BaZrS 3 , and suggest cation alloying as a viable approach for band-gap engineering for photovoltaic and other applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

ReaxFF Parameter Set for Boron Clusters and Icosahedral Boron Crystals: Comparison with Density Functional Theory and Machine-Learning Potentials

Icosahedral boron materials, which include regular icosahedra of 12 boron atoms have gained increasing attention due to their potential applications as superhard materials, semiconductors, and energy storage media. However, the synthesis of high quality crystals of these materials has been a major barrier to the development of these applications. To enable computational prediction of synthesis conditions yielding high-quality icosahedral boron crystals, herein we tested and refined a set of ReaxFF parameters for the nucleation and growth of such crystals. We focused on matching the relative energies of small boron clusters obtained by density functional theory since such small clusters and similar motifs are likely present in crystal nuclei and at the interface of growing crystals. Using a training set of B 80 clusters, including a low-energy core–shell structure containing a B 12 icosahedron core and a high-energy single-shell structure produced in preliminary ReaxFF simulations, the ReaxFF parameter set was refined to better reproduce energies calculated by density functional theory (DFT). Among existing ReaxFF parameter sets and the machine-learning interatomic potentials MACE-MP-0, MACE-MP-0b3, MACE-MPA-0, PFP v7.0.0, and SevenNet-MF-ompa, only our new parameter set and PFP v7.0.0 correctly ranked these B 80 clusters. This refinement led to improved agreement with DFT for a test set of 58 clusters consisting of 8–103 boron atoms. Furthermore, our refined parameter set yielded greater local icosahedral structure than the previously existing ReaxFF parameter set for larger scale simulations of crystallization from supercooled liquid boron. Additionally, simulations of solid boron in contact with molten nickel using our refined ReaxFF parameters yielded a boron solubility value that agrees moderately well with experimental expectations, while the previous boron parameters gave a value that was much too low.

boron↗
DOE OSTI2025

Size-dependent attraction of Cu solutes to clusters formed at Ag grain boundaries

We report a size-dependent solute clustering mechanism at grain boundaries in a sputtered ultrafine-grained Ag-Cu alloy, where large Cu clusters form despite weak individual solute-solute interactions. X-ray diffraction confirms limited Cu solubility in the Ag matrix, while scanning transmission electron microscopy reveals Cu clustering at both ordinary GBs and GB junctions. Density functional theory calculations show that 12-atom Cu clusters are energetically preferred, while smaller three-atom clusters are significantly less stable. Additional calculations demonstrate a marked increase in solute-cluster attraction energy with cluster size. As a result, these findings point to a previously unrecognized pathway for grain-boundary solute clustering in immiscible systems, driven by collective solute-cluster interactions, with implications for segregation behavior and stability in nanocrystalline and ultrafine-grained alloys.

Alloys↗
DOE OSTI2025

Computational Investigation of a CO 2 Conversion Strategy via Diels–Alder Reaction in a Carbon Capture Solvent

Molecular-level insights into reactive separations are crucial for the design of new conversion pathways of carbon dioxide (CO 2 ). This work explores a postulated pathway that directs CO 2 to undergo inverse-electron-demand Diels–Alder reactions to produce heterocycles using the CO 2 chemically fixed on water-lean solvent molecules. Density functional theory calculations are applied to evaluate the lowest unoccupied molecular orbital (LUMO) energies of three types of reactants (1,3-butadiene, 1,3-cyclohexadiene, and 1,2,4,5-tetrazine) with various functional substituents. These calculations also provide a data set (5.8k data) for developing a machine learning model to efficiently predict LUMO energies. A computational screening of LUMO energies for an additional 47k diene and tetrazine candidates is performed, and a list of candidates with lowered LUMO energies by electron-withdrawing substituents is provided. These candidates are further examined by their reaction energy barriers computed from the interatomic potential or density functional theory. Two major energy barriers are identified, one for the proton transfer within the water-lean solvent and the other for the CO 2 transfer from the solvent molecule to the reactant candidate (diene or tetrazine). The functional substituents have a more significant impact on the second barrier but a very slight one on the first barrier. This exploratory work demonstrates a new possibility for guiding experimental efforts toward the chemical conversion of fixated CO 2 to value-added compounds.

Chemical reactions↗
DOE OSTI2013

Residual stress dependant anisotropic band gap of various (hkl) oriented BaI{sub 2} films

The thermally evaporated layer structured BaI{sub 2} grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass m{sub e}{sup *}=0.66m{sub 0} and the hole effective mass m{sub h}{sup *}=0.53m{sub 0} have been determined from the calculated band structure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Rapid generation of optimal generalized Monkhorst-Pack grids

In this report computational modeling of the properties of crystalline materials has become an increasingly important aspect of materials research, consuming hundreds of millions of CPU-hours at scientific computing centers around the world each year, if not more. A routine operation in such calculations is the evaluation of integrals over the Brillouin zone. We have previously demonstrated that performing such integrals using generalized Monkhorst-Pack k-point grids can roughly double the speed of these calculations relative to the widely-used traditional Monkhorst-Pack grids. However the generation of optimal generalized Monkhorst-Pack grids is not implemented in most software packages due to the computational cost and difficulty of identifying the best grids. To address this problem, we present new algorithms that allow rapid generation of optimal generalized Monkhorst-Pack grids on the fly. We demonstrate that the grids generated by these algorithms are on average significantly more efficient than those generated using existing algorithms across a range of grid densities. For grids that correspond to a real-space supercell with at least 50 Å between lattice points, which is sufficient to converge density functional theory calculations within 1 meV/atom for nearly all materials, our algorithm finds optimized grids in an average of 0.19 s on a single processing core. To facilitate the widespread adoption of this approach, we present new open-source tools including a library designed for integration with third-party software packages.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Rational Design of Nickel Hydroxide-Based Nanocrystals on Graphene for Ultrafast Energy Storage

Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one-pot synthesis, and characterization of a series of Ni hydroxide-based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co-/Mn-substituted Ni hydroxides grown on reduced graphene oxide (rGO) is designed and prepared, demonstrating capacities of 665 and 427 C g -1 at current densities of 2 and 20 A g -1 , respectively. The superior performance is attributed mainly to the low deprotonation energy and the facile electron transport, as elaborated by theoretical calculations. When coupled with an electrode based on organic molecular-modified rGO, the resulting hybrid device demonstrates an energy density of 74.7 W h kg -1 at a power density of 1.68 kW kg -1 while maintaining capacity retention of 91% after 10,000 cycles (20 A g -1 ). In conclusion, the findings not only provide a promising electrode material for high-performance hybrid capacitors but also open a new avenue toward knowledge-based design of efficient electrode materials for other energy storage applications.

25 ENERGY STORAGE↗
DOE OSTI2022

Modeling the thermodynamics of the FeTi hydrogenation under para-equilibrium: An ab-initio and experimental study

We report FeTi-based hydrides have recently re-attracted attention as stationary hydrogen storage materials due to favorable reversibility, good sorption kinetics and relatively low costs compared to alternative intermetallic hydrides. Employing the OpenCalphad software, the thermodynamics of the (FeTi) 1-x H x (0 ≤ x ≤ 1) system were assessed as a key basis for modeling hydrogenation of FeTi-based alloys. New thermodynamic data were acquired from our experimental pressure-composition-isotherm (PCI) curves, as well as first-principles calculations utilizing density functional theory (DFT). The thermodynamic phase models were carefully selected based on critical analysis of literature information and ab-initio investigations. Key thermodynamic properties such as dissociation pressure, formation enthalpies and phase diagrams were calculated in good agreement to our performed experiments and literature-reported data. This work provides an initial perspective, which can be extended to account for higher-order thermodynamic assessments and subsequently enables the design of novel FeTi-based hydrides. In addition, the assessed thermodynamic data can serve as key inputs for kinetic models and hydride microstructure simulations.

08 HYDROGEN↗
DOE OSTI2026

A Robust Methodology to Elucidate Kinetics of Room Temperature Electrochemical Propane Adsorption on Platinum

Electrocatalytic activation of alkanes can further decarbonize chemical manufacturing by leveraging affordable renewable electricity and readily available shale gas reserves in the United States. Earlier works have identified the unique role of Pt in adsorbing and activating alkanes, like propane, at room temperature in acidic, aqueous electrolytes, revealing spontaneous formation of deeply dehydrogenated propane-derived surface species with an intact C 3 - backbone. Although an adsorption mechanism was hypothesized, it has not been explicitly investigated to date, preventing the quantification of kinetic rate parameters. A robust methodology to investigate and benchmark propane adsorption kinetics on Pt is critical for the rational design of electrocatalysts that exhibit higher selectivity toward desired partially oxidized products. Herein, we analyze an oxidative current transience that appears during the adsorption of propane on Pt in aqueous electrochemical conditions and develop a methodology that elucidates the adsorption mechanism and enables quantification of rate parameters such as order dependences and apparent activation barriers. This method yields an expected first-order dependence with respect to propane concentration at low coverage and reveals a second-order dependence with respect to the concentration of surface active sites. Additionally, the apparent activation barrier for propane adsorption was calculated using an Arrhenius analysis of the current transience under temperature control. The experimentally measured activation barrier of 35 kJ mol –1 is in excellent agreement with the theoretical barrier calculated by density functional theory (DFT). The kinetic analysis was extended, via the use of transition state theory, to extract entropy and enthalpy of activation, yielding consistent results with the proposed two-step adsorption mechanism and DFT calculations. These results demonstrate reliable quantification of kinetic parameters for electrocatalytic activation of C–H bonds in alkanes that can be employed for rational catalyst development for a versatile range of electrocatalytic conditions.

alkane activation↗