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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Computational Screening of Cationic Zeolites for n -Butane/Methane Separations Using Quantitatively Accurate First-Principles-Derived Force Fields

We developed force fields for linear alkanes in Na and Ca-exchanged zeolites based on periodic first-principles calculations with coupled cluster corrections. These force fields were validated by comparing the simulated adsorption properties with extensive experimental data. As an example of using these force fields, we screened silica and aluminosilicate zeolites for separation of n-butane/methane mixtures in several different separation processes. A key step in this screening was the use of an interpolation method to create accurate zeolite structures that take account of the lattice constant changes for different Si/Al ratios for a given zeolite topology. We focused on zeolite structures that have known synthesis routes, so the materials selected from our calculations can be tested experimentally. Several promising materials that show good separation performance were selected for more detailed simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

First-principles study of electronic structure and photocatalytic properties of MnNiO 3 as an alkaline oxygen-evolution photocatalyst

We present a first-principles study of MnNiO 3 , a promising oxygen-evolution photocatalyst. Using density functional theory with the PBE + U functional and the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), we compute and analyze the ground-state geometry and electronic structure. We find that MnNiO 3 is a ferrimagnetic semiconductor with an indirect band gap, consistent with experimental observations. We also predict that MnNiO3 has promising band edge positions relative to the vacuum, with potential to straddle the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) redox potentials in aqueous solution. A detailed analysis of the band structure and density of states provides a clear explanation for why MnNiO 3 has appropriate electronic properties for OER. Furthermore, comprehensive calculations of its Pourbaix diagram suggest that MnNiO 3 is stable in alkaline solution at potentials relevant for oxygen evolution.

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

First-principles studies of strongly correlated states in defect spin qubits in diamond

Using a recently developed quantum embedding theory, we present first-principles calculations of strongly correlated states of spin defects in diamond. Using this theory, effective Hamiltonians are constructed, which can be solved by classical and quantum computers; the latter promise a much more favorable scaling as a function of system size than the former. In particular, we report a study on the neutral group-IV vacancy complexes in diamond, and we discuss their strongly correlated spin-singlet and spin-triplet excited states. Finally, our results provide valuable predictions for experiments aimed at optical manipulation of these defects for quantum information technology applications.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Lithium-decorated oxidized graphyne for hydrogen storage by first principles study

The geometric stability and hydrogen storage capacity of Li decorated oxidized γ-graphyne are studied based on the first-principles calculations. It is found that oxygen atoms trend to bond with acetylenic carbons and form C=O double bonds on both sides of graphyne. The binding energy of single Li atom on oxidized graphyne is 3.29 eV, owning to the strong interaction between Li atom and O atom. Meanwhile, the dispersion of Li is stable even under a relatively high density. One attached Li atom can at least adsorb six hydrogen molecules around. Benefitting from the porous structure of graphyne and the high attached Li density, a maximum hydrogen storage density 12.03 wt. % is achieved with four Li atoms in graphyne cell. The corresponding average binding energy is 0.24 eV/H{sub 2}, which is suitable for reversible storage. These results indicate that Li decorated graphyne can serve as a promising hydrogen storage material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

First-principles study of carbon impurities in CuInSe{sub 2} and CuGaSe{sub 2}, present in non-vacuum synthesis methods

A first-principles study of the structural and electronic properties of carbon impurities in CuInSe{sub 2} and CuGaSe{sub 2} is presented. Carbon is present in organic molecules in the precursor solutions used in non-vacuum growth methods for CuInSe{sub 2} and CuGaSe{sub 2} based photovoltaic cells. These growth methods make more efficient use of material, time, and energy than traditional vacuum methods. The formation energies of several carbon impurities are calculated using the hybrid HSE06 functional. C{sub Cu} acts as a shallow donor, C{sub In} and interstitial C yield deep donor levels in CuInSe{sub 2}, while in CuGaSe{sub 2} C{sub Ga} and interstitial C act as deep amphoteric defects. So, these defects reduce the majority carrier (hole) concentration in p-type CuInSe{sub 2} and CuGaSe{sub 2} by compensating the acceptor levels. The deep defects are likely to act as recombination centers for the photogenerated charge carriers and are thus detrimental for the performance of the photovoltaic cells. On the other hand, the formation energies of the carbon impurities are high, even under C-rich growth conditions. Thus, few C impurities will form in CuInSe{sub 2} and CuGaSe{sub 2} in thermodynamic equilibrium. However, the deposition of the precursor solution in non-vacuum growth methods presents conditions far from thermodynamic equilibrium. In this case, our calculations show that C impurities formed in non-equilibrium tend to segregate from CuInSe{sub 2} and CuGaSe{sub 2} by approaching thermodynamic equilibrium, e.g., via thorough annealing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

A first principles study of half-metallic ferromagnetism in In{sub 1-x}Ti{sub x}P (x = 0.06) diluted magnetic semiconductor

A first principles approach has been used to calculate the electronic and magnetic properties of In{sub 1-x}Ti{sub x}P (x = 0.06) diluted magnetic semiconductor (DMS) compound. The calculations have been carried out using the highly precise all electron full potential Linear Augmented Plane Wave (FPLAPW) method within generalized gradient approximation (GGA) as exchange-correlation (XC) potentials. The estimated results show that the Ti-doping generate robust half metallic ferromagnetism with the 100% spin polarization at Fermi level (E{sub F}) in InP. Due to this peculiar property, the resultant compound behaves as true half-metallic ferromagnet which is best suited for spintronic applications. The total magnetic moments of this compound are mainly due to Ti-d states present at E{sub F} with almost negligible contribution from other atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

First-principles study of tantalum-arsenic binary compounds

The electronic properties and phase stability of binary tantalum arsenides are investigated using first-principles calculations. We have found strong covalent bonding between Ta and As in TaAs 2 . In the lower As content compounds, the Ta is less covalent and donates charge to the As backbone. The covalency of other compounds becomes smaller with the increase of Ta composition. Compounds on the convex hull are TaAs 2 , TaAs, Ta 5 As 4 , and Ta 3 As, while Ta 2 As is slightly above the hull. Trends in the electronic and other properties are discussed. We identify the space group of TaAs 2 to be centrosymmetric C2/m and find semimetallic behavior in TaAs and TaAs 2 . The other binary compounds are metallic systems with finite density of states at the Fermi level. The plasma energies show moderate conductivity anisotropy in Ta 5 As 4 and Ta 2 As, which are also the most conductive compounds among the binary phases.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic and optical properties of RESn{sub 3} (RE=Pr & Nd) intermetallics: A first principles study

A theoretical study of structural, electronic and optical properties of RESn{sub 3} (RE = Pr & Nd) intermetallics have been investigated systematically using first principles density functional theory. The calculations are carried out within the PBE-GGA and LSDA for the exchange correlation potential. The ground state properties such as lattice parameter (a{sub 0}), bulk modulus (B) and its pressure derivative (B′) are calculated and the calculated lattice parameters show well agreement with the experimental results. We first time predict elastic constants for these compounds. From energy dispersion curves, it is found that these compounds are metallic in nature. The linear optical response of these compounds are also studied and the higher value of static dielectric constant shows the possibility to use them as good dielectric materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

First Principles Evaluation of Phase Stability in the In-Sn Binary System

Abstract The In-Sn binary alloy system exhibits several unusual features that challenge crystallographic and thermodynamic expectations. We combine first principles total energy calculation with simple thermodynamic modeling to address two key points. First, we evaluate energies along the Bain path to interpret the discontinuous transition between the phases α-In (Pearson type tI2) and β-In 3 Sn (also Pearson type tI2) that are identical in symmetry. Second, we demonstrate that the solid solution phases β-In 3 Sn and γ-InSn 4 (Pearson type hP1) exist at high temperatures only, and they exhibit eutectoid decompositions at low temperatures.

Chemistry↗
DOE OSTI2016

The investigation of Ag/ZnO interface system by first principle: The structural, electronic and optical properties

Ag/ZnO interfaces have been investigated for both of Zn-termination and O-termination by the first principle based on density functional theory. Our calculations demonstrate that the Ag atoms go inward from the Ag/ZnO interface, and the Zn and O atoms are all move outward bulk in the Zn-termination interface, and the changes are just opposite for O-termination. These behaviors are in agreement with the other studies in literatures. Furthermore, an expansion situation is observed in the first two Zn-O bilayer and first three Ag monolayers for both of Zn-termination and O-termination interfaces by comparing with the pure ZnO(0001) and Ag(111) surfaces. Moreover, the valence-band both of O-2p and Zn-3d states of Ag/ZnO interface gradual close to Femi level as the Zn, O atoms locate at the deeper layer for Zn-termination, but it is the other way round for O-termination. Calculated absorption spectrum indicates that the absorption intensity of Zn-termination interface is stronger than that of O-termination in the lower energy range (visible light region). These properties of ZnO surfaces are also evaluated for comparison with interfaces. - Graphical abstract: The structures of Ag/ZnO interface: Zn-termination (left) and O-termination (right). In this Ag/ZnO interface system, the ZnO (0001) surface is rotated 30°(R30), and Ag (111) surface is built (2×2) supercell, then a (2×√3) R30 Ag/ZnO interface is constructed using the supercell method (i.e. periodically repeated slabs). The lattice mismatch of (2×√3) R30 Ag/ZnO (2.6% mismatch) is smaller than that of (1×1) Ag/ZnO (11% mismatch).

36 MATERIALS SCIENCE↗
DOE OSTI2019

Unusual pressure-induced electronic structure evolution in organometal halide perovskite predicted from first-principles

Pressure has been demonstrated to be an effective parameter to alter the atomic and electronic structures of materials. By using the first-principles calculations based on density functional theory (DFT), we systematically investigated the changes in the atomic and electronic structures of the cubic MAPbI 3 phase under pressures. It is found that the band gap of the compressed cubic MAPbI 3 structure exhibits a remarkable redshift to 1.114/1.380 eV in DFT/HSE-SOC calculation under a mild pressure of 2.772 GPa, and subsequently shows a widening at higher pressures until ~20 GPa. As the pressure further increases, the band gap closes at ~80 GPa. Detailed structural and electronic characteristic analyses indicate that the band gap of the cubic MAPbI 3 structure is determined by two competing effects: the lattice contraction decreases its band gap while the PbI 6 octahedral tilting increases it. Given that, pressure can be a powerful tool to help understanding the optoelectronic properties of perovskite materials.

36 MATERIALS SCIENCE↗
DOE OSTI2014

First-principles investigation of the electronic and Li-ion diffusion properties of LiFePO{sub 4} by sulfur surface modification

We present a first-principles calculation for the electronic and Li-ion diffusion properties of the LiFePO{sub 4} (010) surface modified by sulfur. The calculated formation energy indicates that the sulfur adsorption on the (010) surface of the LiFePO{sub 4} is energetically favored. Sulfur is found to form Fe-S bond with iron. A much narrower band gap (0.67 eV) of the sulfur surface-modified LiFePO{sub 4} [S-LiFePO{sub 4} (010)] is obtained, indicating the better electronic conductive properties. By the nudged elastic band method, our calculations show that the activation energy of Li ions diffusion along the one-dimensional channel on the surface can be effectively reduced by sulfur surface modification. In addition, the surface diffusion coefficient of S-LiFePO{sub 4} (010) is estimated to be about 10{sup −11} (cm{sup 2}/s) at room temperature, which implies that sulfur modification will give rise to a higher Li ion carrier mobility and enhanced electrochemical performance.

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

First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn₂GaC

In this paper, we report the by first-principles predicted properties of the recently discovered magnetic MAX phase Mn₂GaC. The electronic band structure and vibrational dispersion relation, as well as the electronic and vibrational density of states, have been calculated. The band structure close to the Fermi level indicates anisotropy with respect to electrical conductivity, while the distribution of the electronic and vibrational states for both Mn and Ga depend on the chosen relative orientation of the Mn spins across the Ga sheets in the Mn–Ga–Mn trilayers. In addition, the elastic properties have been calculated, and from the five elastic constants, the Voigt bulk modulus is determined to be 157 GPa, the Voigt shear modulus 93 GPa, and the Young's modulus 233 GPa. Furthermore, Mn₂GaC is found relatively elastically isotropic, with a compression anisotropy factor of 0.97, and shear anisotropy factors of 0.9 and 1, respectively. The Poisson's ratio is 0.25. Evaluated elastic properties are compared to theoretical and experimental results for M₂AC phases where M = Ti, V, Cr, Zr, Nb, Ta, and A = Al, S, Ge, In, Sn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effects of surface bond relaxation on electronic structure of Sb{sub 2}Te{sub 3} nano-films by first-principles calculation

The effects of vertical compressive stress on Sb{sub 2}Te{sub 3} nano-films have been investigated by the first principles calculation, including stability, electronic structure, crystal structure, and bond order. It is found that the band gap of nano-film is sensitive to the stress in Sb{sub 2}Te{sub 3} nano-film and the critical thickness increases under compressive stress. The band gap and band order of Sb{sub 2}Te{sub 3} film has been affected collectively by the surface and internal crystal structures, the contraction ratio between surface bond length of nano-film and the corresponding bond length of bulk decides the band order of Sb{sub 2}Te{sub 3} film.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phonon transport properties of two-dimensional electride Ca 2 N—A first-principles study

We investigate phonon transport in dicalcium nitride (Ca 2 N), an electride with two-dimensional confined electron layers, using first-principles density functional theory and the phonon Boltzmann transport equation. The in-plane (κ[100]) and out-of-plane (κ[001]) lattice thermal conductivities at 300 K are found to be 11.72 W m -1 K -1 and 2.50 W m -1 K -1 , respectively. Spectral analysis of lattice thermal conductivity shows that ~85% of $κ_{[100]}$ and $κ_{[001]}$ is accumulated by phonons with frequencies less than 5.5 THz and 2.5 THz, respectively. Modal decomposition of lattice thermal conductivity further reveals that the optical phonons contribute to ~68% and ~55% of overall $κ_{[100]}$ and $κ_{[001]}$, respectively. Phonon dispersion suggests that the large optical phonon contribution is a result of low frequency optical phonons with high group velocities and the lack of phonon bandgap between the acoustic and optical phonon branches. We find that the optical phonons with frequencies below ~5.5 THz have similar three-phonon phase space and scattering rates as acoustic phonons. Comparison of the contributions from emission and absorption processes reveals that the three-phonon phase space and scattering rates of phonons—optical or acoustic—with frequencies below 5.5 THz are largely dominated by absorption processes. We conclude that the large contribution to lattice thermal conductivity by optical phonons is due to the presence of multiple low frequency optical phonon modes with high group velocities and similar phase space and scattering rates as the acoustic phonons. This study provides the frequency and temperature dependent lattice thermal conductivity and insights into phonon transport in Ca 2 N, both of which have important implications for the development of Ca 2 N based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

First-principles calculation of spin and orbital contributions to magnetically ordered moments in Sr 2 IrO 4

Here, we show how an accurate first-principles treatment of the canted-anti-ferromagnetic ground state of Sr 2 IrO 4 , a prototypical 5d correlated spin-orbit coupled material, can be obtained without invoking any free parameters, such as the Hubbard $\textit{U}$ or tuning the spin-orbit coupling strength. Our theoretically predicted iridium magnetic moment of 0.250$μ_B$, canted by 12.6° off the $\textit{a}$ axis, is in accord with experimental results. By resolving the magnetic moments into their spin and orbital components, we show that our theoretically obtained variation of the magnetic scattering amplitude $\langle M_m \rangle$ as a function of the polarization angle is consistent with recent nonresonant magnetic x-ray scattering measurements. The computed value of the band gap (55 meV) is also in line with the corresponding experimental values. A comparison of the band structure to that of the cuprates suggests the presence of incommensurate charge-density wave phases in Sr 2 IrO 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

First-principles mode-by-mode analysis for electron-phonon scattering channels and mean free path spectra in GaAs

We present a first-principles framework to investigate the electron scattering channels and transport properties for polar materials by combining the exact solution of the linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from the polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature, and it makes a significant contribution to mobility. Furthermore, we achieved good agreement with experimental data for the mobility, and we identified that electrons with mean free paths between 130 and 210 nm provide the dominant contribution to the electron transport at 300 K. As a result, such information provides a deeper understanding of the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

36 MATERIALS SCIENCE↗