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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Decoding the Atomic Structure of Ga 2 Te 5 Pulsed Laser Deposition Films for Memory Applications Using Diffraction and First-Principles Simulations

Neuromorphic computing, reconfigurable optical metamaterials that are operational over a wide spectral range, holographic and nonvolatile displays of extremely high resolution, integrated smart photonics, and many other applications need next-generation phase-change materials (PCMs) with better energy efficiency and wider temperature and spectral ranges to increase reliability compared to current flagship PCMs, such as Ge 2 Sb 2 Te 5 or doped Sb 2 Te. Gallium tellurides are favorable compounds to achieve the necessary requirements because of their higher melting and crystallization temperatures, combined with low switching power and fast switching rate. Ga 2 Te 3 and non-stoichiometric alloys appear to be atypical PCMs; they are characterized by regular tetrahedral structures and the absence of metavalent bonding. The sp 3 gallium hybridization in cubic and amorphous Ga 2 Te 3 is also different from conventional p-bonding in flagship PCMs, raising questions about its phase-change mechanism. Furthermore, gallium tellurides exhibit a number of unexpected and highly unusual phenomena, such as nanotectonic compression and viscosity anomalies just above their melting points. Using high-energy X-ray diffraction, supported by first-principles simulations, we will elucidate the atomic structure of amorphous Ga 2 Te 5 PLD films, compare it with the crystal structure of tetragonal gallium pentatelluride, and investigate the electrical, optical, and thermal properties of these two materials to assess their potential for memory applications, among others.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Stability, Magnetism and Hardness of Iron Carbides from First-Principles Calculations

The stability, electronic structure, magnetism and hardness of iron carbides in eight different space groups were investigated by the first-principles calculations. The electron-ion interactions were modelled using the Vanderbilt ultra-soft pseudopotentials (USPP) and norm-conserving pseudopotentials (NCPP). Generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formulation was used to describe the exchange-correlation energy. The calculated lattice parameters for USPP were in good agreement with experimental results. The formation enthalpy, cohesive energy and elastic constants revealed that with the exception of hex-Fe{sub 2}C, all Fe-C compounds were both thermodynamically and mechanically stable. Meanwhile, the iron carbides’ bulk and shear moduli were also calculated and compared. Furthermore, magnetic properties were studied and it was found that the average magnetic moment of Fe{sub 23}C{sub 6} (2.123 μ{sub B}) was the largest among these compounds, with a value close to that of pure α-Fe (2.24 μ{sub B}μ. As expected, hex-Fe{sub 2}C is a paramagnetic metal based on the obtained densities of states and partial densities of states. Furthermore, the magnetic behaviour of iron carbides with polymorphic transformation was almost identical. Finally, several semi-empirical methods were used for estimating the hardness of the iron carbides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

First-Principle Calculation of Electronic Structures and Magnetic Properties of Heusler Alloys Co{sub 2}ZrPb and Zr{sub 2}MnBa

Using first-principle method of density functional theory (DFT), we have investigated the underlying mechanism related to the electronic and magnetic properties of novel full Heusler alloys Co{sub 2}ZrPb and Zr{sub 2}MnB. Our results show that compound Zr{sub 2}MnB is a nonmagnetic semiconductor whereas Co{sub 2}ZrPb is half metallic a magnetic moment of 2 μ{sub B} per formula unit. It exhibits a large spin gap of 0.334 eV for equilibrium lattice constant and has excellent half-metallic ferrimagnetism with 100% spin polarization at the Fermi level. The half-metallicity of Co{sub 2}ZrPb is stable against lattice change of −2.6 to 0.83%. The Co-based Heusler alloys are expected to be ideal candidates for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic, Mechanical, and Electronic Properties of RhH{sub x} (x = 0, 0.25, 0.75, 1) from First-Principles Calculations

The magnetic, mechanical, and electronic properties of RhH{sub x} (x = 0, 0.25, 0.75, 1) have been investigated by first-principles calculations based on density functional theory. It is shown that RhH{sub x} (x = 0.25, 0.75, 1) in the cubic structure are magnetic except for Rh. The total magnetic moments of RhH{sub x} (x = 0.25, 0.75, 1) decrease with increasing content of H, which mainly results from the change of the Rh moments. The calculated elastic constants indicate that RhH{sub x} (x = 0, 0.25, 0.75, 1) in the cubic structure are mechanically stable. With increasing H content, the bulk modulus, shear modulus, and Young modulus gradually decrease except the bulk modulus of RhH. By means of the values of the B/G ratio, it is concluded that H doping improves the ductile properties of Rh. The strong hybridization between H-s and Rh-d is the origin of magnetism for RhH{sub x} (x = 0.25, 0.75, 1) and decreases with increasing H content.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

A Comprehensive Investigation of Superconductor KBi{sub 2} via First-Principles Calculations

The electronic properties, lattice vibration, and electron-phonon interaction properties of KBi{sub 2} are studied systematically by first-principles calculations. The agreement of calculated Debye temperature, electron-phonon coupling constant, and transition temperature with latest experiments validates the reliability of our work. Our results provide evidence that the superconducting transition of KBi{sub 2} originates from isotropical coupling of all phonon modes according to isotropical Migdal-Eliashberg theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Coupling First Principles Calculations of Electron-Electron and Electron-Phonon Scattering, and Applications to Carbon-based Nanostructures

Here, we report first-principles calculations of electronic gaps, lifetimes, and photoelectron spectra of a series of molecules, performed by efficiently combining the computation of electron-electron and electron-phonon self-energies. The dielectric matrix is represented in terms of dielectric eigenpotentials, utilized for both the calculation of G 0 W 0 quasi-particle energies and the diagonalization of the dynamical matrix; virtual electronic states are never explicitly computed and all self-energies are evaluated over the full frequency spectrum. Our formulation enables electronic structure calculations at the many-body perturbation theory level, inclusive of electron-phonon coupling, for systems with hundreds of electrons.

36 MATERIALS SCIENCE↗
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 OSTI2013

Hydroxylation-induced surface stability of AnO2 (An=U, Np, Pu) from first-principles

The relative stabilities of clean and hydroxylated surfaces of three actinide dioxides (AnO2, An = U, Np, Pu) have been investigated using first-principles methods within the DFT + U framework. In the case of the clean surfaces, the calculated surface energies are consistently the lowest for the (111) surface for all three AnO2 compositions, followed by the (110) and (100) surface energies. In the case of UO2, for instance, the calculated surface energies are 0.78, 1.05, and 1.47 J/m2 for the (111), (110), and (100) surfaces, respectively, reinforcing the well-established surface energy trend for metal-dioxides: (111) < (110) < (100). Dissociated water, at one monolayer coverage, is adsorbed preferentially onto the (100) surface for all three AnO2 systems. In the case of UO2 the water adsorption energy on the (100) surface (- 1.34 J/m2) is almost four times higher than the adsorption energy on the (111) surface (- 0.35 J/m2), and almost twice as large as the adsorption energy on the (110) surface (- 0.77 J/m2). Similar trend in the adsorption energies is observed for both NpO2 and PuO2. As a result, the relative stability of the hydroxylated AnO2 (An = U, Np, Pu) surfaces changes to (100) < (110) < (111). The effects of the geometric relaxations on the clean and hydroxylated surfaces are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗