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Results for “density functional calculation”

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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Constituents of magnetic anisotropy and a screening of spin-orbit coupling in solids

Using quantum mechanical perturbation theory (PT) we analyze how the energy of perturbation of different orders is renormalized in solids. We test the validity of PT analysis by considering a specific case of spin-orbit coupling as a perturbation. We further compare the relativistic energy and the magnetic anisotropy from the PT approach with direct density functional calculations in FePt, CoPt, FePd, MnAl, MnGa, FeNi, and tetragonally strained FeCo. In addition using decomposition of anisotropy into contributions from individual sites and different spin components we explain the microscopic origin of high anisotropy in FePt and CoPt magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Spectroscopic and Computational Investigation of Room-Temperature Decomposition of a Chemical Warfare Agent Simulant on Polycrystalline Cupric Oxide

Certain organophosphorus molecules are infamous due to their use as highly toxic nerve agents. The filtration materials currently in common use for protection against chemical warfare agents were designed before organophosphorus compounds were used as chemical weapons. A better understanding of the surface chemistry between simulant molecules and the individual filtration-material components is a critical precursor to the development of more effective materials for filtration, destruction, decontamination, and/or sensing of nerve agents. Here in this paper, we report on the surface adsorption and reactions of a sarin simulant molecule, dimethyl methylphosphonate (DMMP), with cupric oxide surfaces. In situ ambient pressure X-ray photoelectron and infrared spectroscopies are coupled with density functional calculations to propose mechanisms for DMMP decomposition on CuO. We find extensive room temperature decomposition of DMMP on CuO, with the majority of decomposition fragments bound to the CuO surface. We observe breaking of PO-CH 3 , P-OCH 3 , and P-CH 3 bonds at room temperature. On the basis of these results, we identify specific DMMP decomposition mechanisms not seen on other metal oxides. Participation of lattice oxygen in the decomposition mechanism leads to significant changes in chemical and electronic surface environment, which are manifest in the spectroscopic and computational data. This study establishes a computational baseline for the study of highly toxic organophosphorous compounds on metal oxide surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2017

First-Principles Simulation of the (Li–Ni–Vacancy)O Phase Diagram and Its Relevance for the Surface Phases in Ni-Rich Li-Ion Cathode Materials

Despite several reports on the surface phase transformations from a layered to a disordered spinel and a rock-salt structure at the surface of the Ni-rich cathodes, the precise structures and compositions of these surface phases are unknown. The phenomenon, in itself, is complex and involves the participation of several contributing factors. Of these factors, transition metal (TM) ion migration toward the interior of the particle and hence formation of TM-densified surface layers, triggered by oxygen loss, is thermodynamically probable. Here in this study, we simulate the thermodynamic phase equilibria as a function of TM ion content in the cathode material in the context of lithium nickel oxides, using a combined approach of first-principles density functional calculations, the cluster expansion method, and grand canonical Monte Carlo simulations. We developed a unified lattice Hamiltonian that accommodates not only rock-salt like structures but also topologically different spinel-like structures. Also, our model provides a foundation to investigate metastable cation compositions and kinetics of the phase transformations. Our investigations predict the existence of several Ni-rich phases that were, to date, unknown in the scientific literature. Our simulated phase diagrams at finite temperature show a very low solubility range of the prototype spinel phase. Finally, we find a partially disordered spinel-like phase with far greater solubility that is expected to show very different Li diffusivity compared to that of the prototype spinel structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

MnSb 2 S 4 Monolayer as an Anode Material for Metal-Ion Batteries

Here, we present density functional calculations showing that monolayer MnSb 2 S 4 is promising as an anode material for Li-, Na-, and Mg-ion batteries, and that the adsorption of Zn or Al atoms on the surface of MnSb 2 S 4 monolayer is not energetically favorable. The calculations show electron transfer from Li, Na, or Mg to the empty orbitals of nearby Sb and S atoms. The calculations indicate that an adsorption mechanism is followed by a conversion mechanism during charging, and the storage capacities can reach as high as 879 mA h/g for Li, Na, and Mg. The most favorable diffusion path for Li, Na, and Mg on the surface of MnSb2S4 monolayer is along the b direction; the lowest diffusion barriers for one Li, Na, and Mg are 0.18, 0.10, 0.32 eV, respectively. Good charge–discharge rates can be expected for the MnSb 2 S 4 monolayer when it is used as an electrode for Li-, Na-, and Mg-ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2019

Formation and Composition-Dependent Properties of Alloys of Cubic Halide Perovskites

Distinct shortcomings of individual halide perovskites for solar applications, such as restricted range of band gaps, propensity of $ABX_3$ to decompose into $AX + BX_2$, or oxidation of 2$ABX_3$ into $A_2BX_6$, have led to the need to consider alloys of individual perovskites such as [FA,Cs][Pb,Sn][Br,I] 3 . This proposition creates a nontrivial material selection problem associated with a six-component structure, spanning a continuum of three sets of compositions (one for each sublattice) and requiring control of phase separation or ordering in each alloyed subfield. Not surprisingly, material and structure choices were made thus far mostly via trial-and-error explorations among a large number of arrangements. In this work, we use ideas from solid-state theory of semiconductor alloys to analyze the behaviors of the canonical [FA,Cs][Pb,Sn]I 3 alloy system, where FA is formamidinium. Density functional calculations utilizing specially constructed supercells are used to calculate the composition dependence of band gaps, energy of decomposition, and alloy mixing enthalpies. A number of clear trends are observed for A-site alloys [Cs,FA]SnI 3 and [Cs,FA]PbI 3 , as well as for B-site alloys Cs[Sn,Pb]I 3 and FA[Sn,Pb]I 3 . To understand the physical reasons that control these trends, we decompose the alloy properties into distinct physical terms: (i) the energies associated with removing the octahedral deformations (tilting, rotations, B-site displacements) of the individual components, (ii) the energies for compressing the larger component and expanding the smaller one to the alloy volume V(x), (iii) the charge-transfer energies associated with placing the alloyed units onto a common lattice, and finally, (iv) structural relaxation of all bonds within the cells. This analysis clarifies the origin of the observed trends in band gaps, decomposition energies, and mixing enthalpies. Unlike a number of previous calculations, we find that the proper description of alloy physics requires that even the pure, nonalloyed, end-point compounds need to be allowed to develop local environment-dependent octahedral deformation that lowers significantly the total energy and raises their band gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Phase Stability and Ordering in Rock Salt-Based Thermoelectrics: NaSbX 2 , AgSbX 2 , and Their Alloys with PbX and SnX (X = S, Se, Te)

Using first-principles density functional calculations, we study the phase stability and cation ordering behavior in ABX 2 , PbX-ABX 2 , and SnTe-ABTe 2 (A = Na, Ag; B = Sb; X = S, Se, Te) thermoelectrics that crystallize in rock salt-based lattices. We construct, separately for each ABX 2 system, cluster expansions fitted to T = o K energies of cation-ordered arrangements in order to identify the respective ground-state structures. We calculate the mixing energetics of A/B and {Pb, Sn}/AB cation-disordered solid solutions in ABX 2 and {Pb, Sn}X-ABX 2 systems, respectively, using special quasirandom structures. We find that (i) L11, a rock salt-based superlattice with A 1 B 1 stacking of cations along [111], is often the most favored cation ordering type across ABX 2 systems due to the ability of this superlattice structure to accommodate large size mismatches between A and B; (ii) A/B cation ordering is only weakly preferred over disorder in all ABX 2 systems, by <34 meV/cation, consistent with experimental observations of disordered rock salt phases at high temperatures; (iii) Na-containing ABX 2 systems strongly favor the formation of the ternary compound over the corresponding ground-state mixture of A 2 X + B 2 X 3 binary compounds, while Ag-containing ABX 2 systems do not; (iv) the experimentally reported noncubic phases of NaSbS 2 and AgSbS 2 are in fact distorted cation-ordered rock salt structures; and (v) all {Pb, Sn}X-ABX 2 systems show miscibility gaps in their phase diagrams, and this is partly due to the unfavorable electrostatics of ternary cation mixing relative to Pb or Sn + AB cation phase separation; however, estimation of the miscibility gap temperature using mean-field configurational entropy indicates that all systems except PbTe-AgSbTe 2 will readily mix at relatively low temperatures. Furthermore, our study of the structural properties, alloying behavior, and solubility limits in these promising materials is a crucial step forward in improving their thermoelectric performance.

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

Highly Active Rhenium-, Ruthenium-, and Iridium-Based Dichalcogenide Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions in Aprotic Media

Transition metal dichalcogenides (TMDCs) have garnered much attention recently due to their remarkable performance for different electrochemical systems. In this study, we report on the synthesis and catalysis of less studied TMDC nanoflakes (NFs) with a design space comprised of three transition metals (rhenium, ruthenium, and iridium) and three chalcogens (sulfur, selenium, and tellurium) for the oxygen reduction and evolution reactions (ORR and OER) in an aprotic hybrid electrolyte containing 0.1 M lithium bis(trifluoromethanesulfonypimide salt in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid and dimethyl sulfoxide. Our results indicate that among the tested catalysts, ReS2 exhibits the highest current density for both ORR and OER, beyond those of the state-of-the-art catalysts used in aprotic media with Li salts. We performed density functional calculations to provide a mechanistic understanding of the reactions in the ReS2 NFs/ionic liquid system. These novel bifunctional catalyst results could open a way for exploiting the unique properties of these materials in Li-O-2 batteries as well as other important electrochemical systems.

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

Homoleptic Perchlorophenyl “Ate” Complexes of Thorium(IV) and Uranium(IV)

The reaction of AnCl 4 (DME) n (An = Th, n = 2; U, n = 0) with 5 equiv of LiC 6 Cl 5 in Et 2 O resulted in the formation of homoleptic actinide-aryl “ate” complexes [Li(DME) 2 (Et 2 O)] 2 [Li(DME) 2 ][Th(C 6 Cl 5 ) 5 ] 3 ([Li][1]) and [Li(Et 2 O) 4 ][U(C 6 Cl 5 ) 5 ] ([Li][2]). Similarly, the reaction of AnCl 4 (DME)n (An = Th, n = 2; U, n = 0) with 3 equiv of LiC 6 Cl 5 in Et 2 O resulted in the formation of heteroleptic actinide-aryl “ate” complexes [Li(DME) 2 (Et 2 O)][Li(Et 2 O) 2 ][ThCl 3 (C 6 Cl 5 ) 3 ] ([Li][3]) and [Li(Et 2 O) 3 ][UCl 2 (C 6 Cl 5 ) 3 ] ([Li][4]). Density functional calculations show that the An–C ipso σ-bonds are considerably more covalent for the uranium complexes vs the thorium analogues, in line with past results. Additionally, good agreement between experiment and calculations is obtained for the 13 C ipso NMR chemical shifts in [Li][1] and [Li][3]. Here, the calculations demonstrate a deshielding by ca. 29 ppm from spin–orbit coupling effects originating at Th, which is a direct consequence of 5f orbital participation in the Th–C bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb 1.5 Nb 2 O 6.5

Lone pair-driven distortions are a hallmark of many technologically important lead (Pb)-based materials. Here, the role of Pb 2+ in polar perovskites is well understood and easily manipulated for applications in piezo- and ferroelectricity, but the control of ordered lone pair behavior in Pb-based pyrochlores is less clear. Crystallographically and geometrically more complex than the perovskite structure, the pyrochlore structure is prone to geometric frustration of local dipoles due to a triangular arrangement of cations on a diamond lattice. The role of vacancies on the O' site of the pyrochlore network has been implicated as an important driver for the expression and correlation of stereochemically active lone pairs in pyrochlores such as Pb 2 Ru 2 O 6.5 and Pb 2 Sn 2 O 6 . In this work we report on the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb 1.5 Nb 2 O 6.5 upon cooling. We find that local distortions are present at all temperatures that can be described by cristobalite-type cation ordering, and this ordering persists to longer length scales upon cooling. From a crystallographic perspective, the material remains disordered and does not undergo an observable phase transition. In combination with density function calculations, we propose that the stereochemical activity of the Pb 2+ lone pairs is driven by proximity to O' vacancies, and the crystallographic site disorder of the O' vacancies prohibits long range correlation of lone pair-driven distortions. This in turn prevents a low-temperature phase transition and results in an elevated dielectric permittivity across a broad temperature range.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-Throughput Screening for Boride Superconductors

A high-throughput screening using density functional calculations is performed to search for stable boride superconductors from the existing materials database. The workflow employs the fast frozen-phonon method as the descriptor to evaluate the superconducting properties quickly. Twenty-three stable candidates were identified during the screening. The superconductivity was obtained earlier experimentally or computationally for almost all found binary compounds. Previous studies on ternary borides are very limited. Here our extensive search among ternary systems confirmed superconductivity in known systems and found several new compounds. Among these discovered superconducting ternary borides, TaMo 2 B 2 shows the highest superconducting temperature of ∼12 K. Most predicted compounds were synthesized previously; therefore, our predictions can be examined experimentally. Our work also demonstrates that the boride systems can have diverse structural motifs that lead to superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

High-Pressure Behavior of Silver Fluorides up to 40 GPa

A combined experimental–theoretical study of silver(I) and silver(II) fluorides under high pressure is reported. For Ag I , the CsCl-type structure is stable to at least 39 GPa; the overtone of the IR-active mode is seen in the Raman spectrum. Its Ag II F 2 sibling is a unique compound in many ways: it is more covalent than other known difluorides, crystallizes in a layered structure, and is enormously reactive. Using X-ray diffraction and guided by theoretical calculations (density functional theory), we have been able to elucidate crystal structures of high-pressure polymorphs of AgF 2 . The transition from ambient pressure to an unprecedented nanotubular structure takes place via an intermediate orthorhombic layered structure, which lacks an inversion center. Here, the observed phase transitions are discussed within the broader framework of the fluorite → cotunnite → Ni 2 In series, which has been seen for other metal difluorides.

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

Validation of Density Functionals for Adsorption Energies on Transition Metal Surfaces

The quantitative prediction of adsorption energies of radicals and molecules on surfaces is essential for the design and understanding of heterogeneous catalytic processes. Here, a recent paper by Wellendorff et al. collected an experimental database of 39 reaction energies involving adsorption energies on transition metal surfaces that can be used as benchmarks for testing quantum mechanical electronic structure methods, and we compared the experimental data to Kohn–Sham density functional calculations with six exchange–correlation functionals. In this paper, we rearranged the data into two categories: open-shell radical adsorption reactions and closed-shell molecular adsorption reactions. We recalculated the adsorption energies with PBE, and we also calculated them with three functionals, M06-L, GAM, and MN 15 -L, that were not studied in the Wellendorff et al. paper; then we compared our results to the benchmark data. Of the nine functionals that have been compared to the databases, we find that BEEF-vdW, GAM, and RPBE perform best for the open-shell radical adsorption reactions, and MN15-L performs best for the closed-shell molecular adsorption, followed by BEEF-vdW and M06-L.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Magic Numbers and Stabilities of Photoionized Water Clusters: Computational and Experimental Characterization of the Nanosolvated Hydronium Ion

The stability and distributions of small water clusters generated in a supersonic beam expansion are interrogated by tunable vacuum ultraviolet (VUV) radiation generated at a synchrotron. Time-of-flight mass spectrometry reveals enhanced population of various protonated water clusters (H + (H 2 O) $n$ ) based upon ionization energy and photoionization distance from source, suggesting there are "magic" numbers below the traditional $n$ = 21 that predominates in the literature. These intensity distributions suggest that VUV threshold photoionization (11.0-11.5 eV) of neutral water clusters close to the nozzle exit leads to a different nonequilibrium state compared to a skimmed molecular beam. This results in the appearance of a new magic number at 14. Metadynamics conformer searches coupled with modern density functional calculations are used to identify the global minimum energy structures of protonated water clusters between $n$ = 2 and 21, as well as the manifold of low-lying metastable minima. New lowest energy structures are reported for the cases of $n$ = 5, 6, 11, 12, 16, and 18, and special stability is identified by several measures. These theoretical results are in agreement with the experiments performed in this work in that $n$ = 14 is shown to exhibit additional stability, based on the computed second-order stabilization energy relative to most cluster sizes, though not to the extent of the well-known $n$ = 21 cluster. Other cluster sizes that show some additional energetic stability are $n$ = 7, 9, 12, 17, and 19. To gain insight into the balance between ion-water and water-water interactions as a function of the cluster size, an analysis of the effective two-body interactions (which sum exactly to the total interaction energy) was performed. In conclusion, this analysis reveals a crossover as a function of cluster size between a water-hydronium-dominated regime for small clusters and a water-water-dominated regime for larger clusters around $n$ = 17.

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

Sulfur-Donor Solvents Strongly Coordinate Pb 2+ in Hybrid Organic–Inorganic Perovskite Precursor Solutions

Strong coordination between Lewis-basic processing additives and the Lewis-acidic lead halide in hybrid organic–inorganic perovskite (HOIP) precursor solutions is required to solubilize the lead halide, and subsequently access the appropriate crystallization kinetics and attain the desired morphology of perovskite active layers. While oxygen-donor solvents and additives, such as dimethylformamide and dimethyl sulfoxide, are widely used for perovskite processing, we demonstrate here that “soft” sulfur-donor solvents exhibit stronger coordination to the “borderline soft” Lewis acid Pb 2+ center of PbI 2 relative to “hard” O-donor solvents in the precursor solution. The stronger coordination of S-donor solvents compared to O-donor solvents to Pb 2+ implies that such compounds can be useful additives to HOIP precursor solutions. Density-functional calculations of the enthalpy change resulting from the coordination of solvents to Pb 2+ provide direct numerical comparison of the strength of O-donor and S-donor coordination with Pb 2+ and expands the library of candidate S-donor compounds. Our results provide a roadmap for processing additive selection and expand the previously limited choice of perovskite processing additives to include strongly coordinating S-donor compounds.

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

Plasmon-Induced CO 2 Conversion on Al@Cu 2 O: A DFT Study

In plasmonic catalysis, localized surface plasmon was leveraged to drive chemical reactions. This approach is promising for catalyzing many challenging reactions at relatively low temperatures and pressures. We apply density functional calculations to provide the insight for the mechanism of plasmon-induced direct CO 2 dissociation on an Al@Cu 2 O core-shell structure, which was observed in a previous experimental study. We show that the interaction of Cu 2 O with CO 2 results in the accessible antibonding states of the adsorbed CO 2 in the range of visible light. Though the intrinsic activation barrier for direct CO 2 dissociation is as high as 3.6 eV, we find that an effective reaction barrier of CO 2 dissociation under photon excitation could be reduced by 2 eV. We also show that the charge transfer is more pronounced in the final state of the dissociation due to the strong hybridization of the dissociated species with Cu 2 O. These results thus provide an explanation for the visible-light-induced direct CO 2 dissociation at relatively low temperature and under ambient pressure. This finding also shows the promising impact of plasmonic catalysis in mitigating carbon emission.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Defect Stability in CdTe Based on Formation Energies and Migration Barriers

Native point defects are thought to play a key role in CdTe, either as compensation centers in intentionally doped material, as a source of conductivity in nominally undoped material, or as electron–hole recombination centers. Here, the discussion of their concentration and impact has often centered only on formation energies and transition levels. Using hybrid density functional calculations, including the effects of spin–orbit coupling (SOC), we discuss the stability of native point defects in CdTe based on their formation energies and migration barriers. We show that although Cd interstitials are the lowest energy donor defects, they are unstable at room temperature due to a low migration barrier. They are important for maintaining charge neutrality during growth or annealing at high temperatures, but once the material is brought to room temperature, they are not frozen in as often assumed and are expected to anneal out, leaving the other more stable defects to determine the conductivity. Taking this into account in the solution of the charge neutrality equation, we are able to predict the conductivity type and carrier concentrations that are in good agreement with experimental observations.

14 SOLAR ENERGY↗
DOE OSTI2025

Selective Adsorption of Thiol-Containing Molecules on Copper Sulfide Surfaces via Molecule–Surface Disulfide Bridges

Recent results in the fields of nanoenhanced agriculture and expanding interest in prebiotic chemistry have placed increased emphasis on understanding the chemically selective interaction of small molecules with the surfaces of metal sulfides. Here, we present an integrated experimental and computational study of the interaction of thiol-containing molecules with copper sulfide (covellite) surfaces in aqueous media. In situ Fourier-transform infrared (FTIR) measurements and ex situ X-ray photoelectron spectroscopy (XPS) measurements show that molecules bearing free thiol groups, including glutathione and cysteine, bind strongly to CuS (covellite) nanoparticles and to CuS (001) single crystals, while control studies show that similar molecules lacking the free thiol group exhibit much less binding. Additional experiments show that these thiol-containing molecules interact transiently with CuO nanoparticle surfaces but are readily removed by rinsing. The FTIR and XPS experiments demonstrate that adsorption of molecular thiols to CuS surfaces occurs in a chemically selective manner. Further experimental studies and density functional calculations show that the preferred mode of binding is through the surface S atoms, forming a Solid–S–S–Molecule disulfide linkage. While the role of disulfide linkages in controlling structure and function of proteins and other biomolecules is widely known, the formation of surface disulfide linkages as a motif for covalent molecular binding at surfaces has not been established previously.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetic MoS 2 Interface Monolayer on a CdS Nanowire by Cation Exchange

MoS 2 atomic layers have recently attracted much interest because of their two-dimensional structure as well as tunable optical, electrical, and mechanical properties for next-generation electronic and electro-optical devices. We have achieved facile fabrication of MoS 2 thin films on CdS nanowires by cation exchange in solution at room temperature and importantly observed their extraordinary magnetic properties. We establish the atomic structure of the MoS 2 /CdS heterostructure by taking atomic images of the MoS 2 /CdS interface as well as performing first-principles density functional geometry optimizations and scanning transmission electron microscopy annular dark field image simulations. Furthermore, our first-principles density functional calculations for the MoS 2 /CdS heterostructure reveal that the magnetism in the MoS 2 /CdS heterostructure stems from the ferromagnetic MoS 2 monolayer next to the MoS 2 /CdS interface. The ferromagnetism is attributed to the partial occupation of the Mo d x 2 –y 2 /d xy conduction band in the interfacial MoS 2 monolayer caused by the mixed covalent–ionic bonding among the MoS 2 and CdS monolayers near the MoS 2 /CdS interface. Finally, these findings of the ferromagnetic MoS 2 monolayer with large spin polarization at the MoS 2 /semiconductor interface suggest a new route for fabrication of the transition metal dichalcogenide-based magnetic semiconductor multilayers for applications in spintronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗