Search NASA⌕ Search

SEARCH · Search NASA

Results for “first principles”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

First-principles study of migration and diffusion mechanisms of helium in α-Be

The behavior of interstitial helium in α-Be has been studied with first-principles method. It is found that the most favored position for helium is the basal octahedral (BO) site, closely followed by the basal tetrahedral (BT) site, in agreement with previous predictions. The interaction energy between the helium and the neighborhood Be atoms and the deformation energy of α-Be matrix are calculated. The feasible minimum-energy pathways (MEP) of interstitial helium atoms in α-Be matrix and the corresponding atomic structures of the saddle points associated with the each MEP are investigated. The temperature-dependent diffusion coefficients have also been predicted. It is confirmed that the interstitial helium diffuses two-dimensionally at low temperatures; however, it can diffuse three-dimensionally at higher temperatures. Besides, the microscopic parameters in the pre-factor and activation energy of the diffusion coefficients are obtained. Both diffusion coefficients are higher than the available experiment data, which may attribute to the fact that under real condition the diffusion is not free, i.e. the actual α-Be matric has various defects and impurities which heavily affect the diffusion of helium. Therefore, our theoretical prediction is the upper bound for helium diffusion in α-Be matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Lessons learned from first-principles calculations of transition metal oxides

Transition metal oxides (TMOs) are an important class of materials with diverse applications, ranging from memristors to photoelectrochemical cells. First-principles calculations are critical for understanding these complex materials at an atomic level and establishing relationships between atomic and electronic structures, particularly for probing quantities difficult or inaccessible to experiment. Here, we discuss computational strategies used to understand TMOs by focusing on two examples, a photoanode material, BiVO4, and an oxide for low-power electronics, La1−xSrxCoO3. We highlight key aspects required for the modeling of TMOs, namely, the descriptions of how oxygen vacancies, extrinsic doping, the magnetic state, and polaron formation impact their electronic and atomic structures and, consequently, many of the observed properties.

Vo, Hien (ORCID:0000000158315599)↗
DOE OSTI2022

Single-molecule magnet Mn 12 on GaAs-supported graphene: Gate field effects from first principles

In this work, we study gate field effects on the Mn 12 O 12 (COOH) 16 (H 2 O) 4 | graphene | GaAs heterostructure via first-principles calculations. We find that under moderate doping levels electrons can be added to but not taken from the single-molecule magnet Mn 12 O 12 (COOH) 16 (H 2 O) 4 (Mn 12 ). The magnetic anisotropy energy (MAE) of Mn 12 decreases as the electron doping level increases, due to electron transfer from graphene to Mn 12 and change in the band alignment between Mn 12 and graphene. At an electron doping level of -5.00×10 13 cm -2 , the MAE decreases by about 18% compared with zero doping. The band alignment between graphene and GaAs is more sensitive to electron doping than to hole doping, since the valence band of GaAs is close to the Fermi level. The GaAs substrate induces a small band gap in the supported graphene under zero gate field and a nearly strain-free configuration. Finally, we propose a vertical tunnel junction for probing the gate dependence of MAE via electron transport measurements.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

First Principles Investigation of the Magnetic, Magnetoelectric, and Optical Properties of Double Perovskites Containing Ions of Transition Metals LaPbTSbO{sub 6} (T = Fe, Co, Ni)

Within the first principles approach implemented in the VASP package, a correlation between magnetic, electronic, polarization, and optical properties, on the one hand, and the structural ordering of cations, on the other hand, is investigated in double perovskites LaPbTSbO{sub 6} (T = Fe, Co, Ni). Two types of cation ordering are considered: simultaneous layered (LL) and checkerboard (RR) ordering of both cations. These two types of ordering are chosen due to their significance; namely, the ordering RR is one of the most implementable types of cation ordering in double perovskites, and compounds with layered ordering can be considered as a heterostructure consisting of periodically alternating metal–nonmagnetic metal layers, which is of interest for experimental synthesis and investigation. It is found that the type of cation ordering in compounds with T = Fe and Ni radically changes the magnetic and/or electronic properties of the compound. Moreover, it is found that low-symmetry stable phases are polar for both types of cation ordering, and the values of spontaneous polarization are evaluated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Ionic ASi{sub 2}N{sub 3} (A=Li, Na, K and Rb) stabilized by the covalent Si–N bonding: First-principles calculations

The structural, elastic and electronic properties of LiSi{sub 2}N{sub 3} and its substitutions by Na, K and Rb were investigated through first-principles computations. The expansion of lattice parameters of ASi{sub 2}N{sub 3} from Li, Na, K to Rb is found to be determined by the bond angle of Si–N1–Si, which suggests a possible way to improve the lithium ionic conductivity by substitutions. ASi{sub 2}N{sub 3} (A=Li, Na, K and Rb) shows the similar elastic behaviors, while the electronic band gap gradually decreases from 5.1 to 3.4 eV from LiSi{sub 2}N{sub 3} to RbSi{sub 2}N{sub 3}. Interestingly, the analysis of electronic structure, crystal orbital Hamiltonian populations and Bader charges shows that the covalence of Si–N bonding is critical for the stability of ASi{sub 2}N{sub 3} phase. Among ASi{sub 2}N{sub 3} phases, there is a relatively high ionicity in NaSi{sub 2}N{sub 3}; the Si–N bond strength in [Si{sub 2}N{sub 3}]{sup −} net for KSi{sub 2}N{sub 3} and RbSi{sub 2}N{sub 3} is comparable to LiSi{sub 2}N{sub 3}, but stronger than NaSi{sub 2}N{sub 3}. - Graphic abstract: Universal trend of structural and electronic properties in alkaline metal silicon nitrides, ASi{sub 2}N{sub 3}, A=Li, Na, K and Rb. - Highlights: • Trend in structure, electronic and mechanical properties of ASi{sub 2}N{sub 3} (A=Li-Rb) were predicted. • Lattice expansion of ASi{sub 2}N{sub 3} induced by the bond angle of Si–N1–Si was found. • Calculated band gap decreases from 5.1 to 3.4 eV from LiSi{sub 2}N{sub 3} to RbSi{sub 2}N{sub 3}. • Covalent Si–N bonding is critical for the stability of ASi{sub 2}N{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Photoelectron Spectra of Aqueous Solutions from First Principles

We present a combined computational and experimental study of the photoelectron spectrum of a simple aqueous solution of NaCl. Measurements were conducted on microjets, and first-principles calculations were performed using hybrid functionals and many-body perturbation theory at the G0W0 level, starting with wave functions computed in ab initio molecular dynamics simulations. We show excellent agreement between theory and experiments for the positions of both the solute and solvent excitation energies on an absolute energy scale and for peak intensities. The best comparison was obtained using wave functions obtained with dielectric-dependent self-consistent and range-separated hybrid functionals. Our computational protocol opens the way to accurate, predictive calculations of the electronic properties of electrolytes, of interest to a variety of energy problems.

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

Influence of defects and dopants on the photovoltaic performance of Bi 2 S 3 : First-principles insights

Bi 2 S 3 has attracted extensive attention recently as a light-absorber, sensitizer or electron acceptor material in various solar cells. Using first-principles calculations, we find that the photovoltaic efficiency of Bi 2 S 3 solar cells is limited by its intrinsic point defects, i.e., both S vacancy and S interstitial can have high concentration and produce deep defect levels in the bandgap, leading to non-radiative recombination of electron–hole carriers and reduced minority carrier lifetime. Unexpectedly most of the intrinsic defects in Bi 2 S 3 , including even the S interstitial, act as donor defects, explaining the observed n-type conductivity and also causing the high p-type conductivity impossible thermodynamically. Doping in Bi 2 S 3 by a series of extrinsic elements is studied, showing that most of the dopant elements such as Cu, Br and Cl make the material even more n-type and only Pb doping makes it weakly p-type. Based on this, we propose that the surface region of n-type Bi 2 S 3 nanocrystals in p-PbS/n-Bi 2 S 3 nano-heterojunction solar cells may be type-inverted into p-type due to Pb doping, with a buried p–n junction formed in the Bi 2 S 3 nanocrystals, which provides a new explanation to the longer carrier lifetime and higher efficiency. Lastly, considering the relatively low conduction band and high n-type conductivity, we predict that Cu, Br and Cl doped Bi 2 S 3 may be an ideal n-type electron acceptor or counter electrode material, while the performance of Bi 2 S 3 as a light-absorber or sensitizer material is intrinsically limited.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetocrystalline anisotropy of 4d/5d transition metals on a Co(0001) surface: A first-principles study

Magnetism and magnetocrystalline anisotropy (MCA) of 4d and 5d transition metal monolayers have been investigated in the presence of a Co(0001) substrate using first-principles electronic structure calculations. Magnetization of Co-group elements undergoes a transition from an in-plane to perpendicular MCA on Co(0001), whose energies (E{sub MCA}) are +0.75 meV/cell and +3.67 meV/cell for Rh/Co(0001) and Ir/Co(0001), respectively. On the other hand, the Fe-group Ru/Co(0001) and Os/Co(0001) exhibit the in-plane MCA with antiparallel spin moments to that of the Co substrate. From band analysis, enhancement of MCA in the Ir/Co(0001) is mainly due to the Ir atom by 〈m=0| l{sub x} |m=±1〉 matrix in the ↑↓-channel, where negative MCA found in Os/Co(0001) is due to Co with dominant contribution from 〈m=0| l{sub x} |m=±1〉 and 〈m=±2| l{sub x} |m=±1〉 matrices in the ↓↓- and ↑↓-channel, respectively. The significant enhancement of E{sub MCA} in Rh/ and Ir/ Co(0001) is ascribed to larger spin-orbit coupling of 4d and 5d orbitals, mainly by coupling between m = 0 and m = ±1 states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Submonolayer Ag films on Fe(100): A first-principles analysis of energetics controlling adlayer thermodynamics and kinetics

Epitaxial growth of Ag on Fe(100) and postdeposition relaxation have been studied in several experiments. We provide a first-principles density functional theory analysis of key adatom interaction energies and diffusion barriers controlling growth and relaxation kinetics for the submonolayer regime, as these have not been assessed previously. A cluster expansion approach is used to obtain an extensive set of conventional lateral interactions between adatoms on fourfold hollow adsorption sites. We find robust oscillatory decay of pair interactions with increasing separation, and of trio interactions with increasing perimeter length. First- and second-nearest-neighbor pair interactions, as well as compact linear and bent trio interactions, dominate. The adatom terrace diffusion barrier is estimated to be E d ≈ 0.39 eV. We also provide a limited analysis of unconventional interactions for which one adatom is at the bridge-site transition state for hopping and one or more others are at fourfold hollow sites. Furthermore, energy barriers for diffusion along island edges can be determined with the aid of both conventional and unconventional interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic Behavior of Superconductor 2H-NbSe{sub 2} Intercalated with Iron: First Principle Study

We investigated the structural, electronic and magnetic properties of iron-intercalated niobium selinium, Fe{sub x}NbSe{sub 2} for x=0 and 0.25, from first principle calculations. The tendency towards the localization in superconductors criterion which has been extracted from our results placed the NbSe{sub 2} compound in vicinity of Fe-based superconductors. The DFT plus on-site coulomb repulsion U (DFT+U) and the exact exchange for correlated electrons (DFT+EECE)-based hybrid functional were used including the spin-orbit coupling (SOC) to understand the role of correlation and SOC effects in this type of systems. The calculation predicts the correct antiferromagnetic (AF) ground state, from both the generalized gradient approximation (GGA) and GGA+U. A shift up in the Fermi energy was observed after the intercalation of 2H-NbSe{sub 2} with iron, indicating a charge transfer from Fe to the host compound. The obtained magnetic moment of Fe is enhanced by a correlation effect in both DFT+U and DFT+EECE over the small value from GGA-only. Moreover, a large unquenched orbital magnetic moment is saturated to m{sub orb}∼0.6μ{sub B} under a moderate correlation effect of U{sub eff}∼2.0eV. We also show that the correlation effect is important along side with SOC in order to get a true picture for the band filling consistent with that of the crystal field splitting and a large unquenched m{sub orb} which has been found experimentally in such systems. Therefore, the combination of correlation and SOC effects is a decisive choice for further studies of such systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

First principles study of structural, electronic and mechanical properties of alkali nitride-KN

The structural, electronic and elastic properties of alkali- metal nitride (KN) is investigated by the first principles calculations based on density functional theory as implemented in Vienna ab-initio simulation package. At ambient pressure KN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that the KN is half metallic ferromagnet at normal pressure. A pressure-induced structural phase transition from NaCl to ZB phase is observed in KN. Half metallicity and ferromagnetism is maintained at all pressures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure and dynamics of aqueous solutions from PBE-based first-principles molecular dynamics simulations

Establishing an accurate and predictive computational framework for the description of complex aqueous solutions is an ongoing challenge for density functional theory based first-principles molecular dynamics (FPMD) simulations. In this context, important advances have been made in recent years, including the development of sophisticated exchange-correlation functionals. On the other hand, simulations based on simple generalized gradient approximation (GGA) functionals remain an active field, particularly in the study of complex aqueous solutions due to a good balance between the accuracy, computational expense, and the applicability to a wide range of systems. In such simulations we often perform them at elevated temperatures to artificially “correct” for GGA inaccuracies in the description of liquid water; however, a detailed understanding of how the choice of temperature affects the structure and dynamics of other components, such as solvated ions, is largely unknown. In order to address this question, we carried out a series of FPMD simulations at temperatures ranging from 300 to 460 K for liquid water and three representative aqueous solutions containing solvated Na + , K + , and Cl - ions. We show that simulations at 390–400 K with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional yield water structure and dynamics in good agreement with experiments at ambient conditions. Simultaneously, this computational setup provides ion solvation structures and ion effects on water dynamics consistent with experiments. These results suggest that an elevated temperature around 390–400 K with the PBE functional can be used for the description of structural and dynamical properties of liquid water and complex solutions with solvated ions at ambient conditions.

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

First-principles studies of the unconventional spin-phonon coupling mediated by spin-orbit coupling in pyrochlore Cd 2 Os 2 O 7

Here, we perform systematic first-principles calculations to investigate the spin-phonon coupling (SPC) mediated by spin-orbit coupling in pyrochlore Cd 2 Os 2 O 7 which has the peculiar all-in-all-out noncollinear magnetic ground state. A large SPC coefficient up to 34 cm –1 is found, showing the potential of using this material in thermal spin devices.We further split the SPC coefficients to contributions from symmetric exchange interaction, Dzyaloshinskii-Moriya interaction (DMI), and the single ion anisotropy. Unlike what was assumed in previous studies, the DMI contributions are rather important for most modes. These results provide insights for the understanding of the SPC in 5d transition metal oxides and useful guidance for the search of excellent spin caloritronic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Optical and electronic properties of doped p -type CuI: Explanation of transparent conductivity from first principles

In this paper, we report the properties of the reported transparent conductor CuI, including the effect of heavy p-type doping. The results, based on first-principles calculations, include an analysis of the electronic structure and calculations of optical and dielectric properties. We find that the origin of the favorable transparent conducting behavior lies in the absence in the visible of strong interband transitions between deeper valence bands and states at the valence-band maximum that become empty with p-type doping. Instead, strong interband transitions to the valence-band maximum are concentrated in the infrared with energies below 1.3 eV. This is contrast to the valence bands of many wide-band-gapmaterials. Turning to the mobility,we find that the states at the valence-band maximum are relatively dispersive. This originates from their antibonding Cu d–I p character. We find a modest enhancement of the Born effective charges relative to nominal values, leading to a dielectric constant ε(0) = 6.3. This is sufficiently large to reduce ionized impurity scattering, leading to the expectation that the properties of CuI can still be significantly improved through sample quality.

14 SOLAR ENERGY↗
DOE OSTI2022

First-principles insight into all-optical spin switching in the half-metallic Heusler ferrimagnet Mn 2 RuGa

All-optical spin switching (AOS) represents a new frontier in magnetic storage technology—spin manipulation without a magnetic field—but its underlying working principle is not well understood. Many AOS ferrimagnets such as GdFeCo are amorphous and renders the high-level first-principles study unfeasible. The crystalline half-metallic Heusler Mn 2 RuGa presents an opportunity. Here we carry out hitherto the comprehensive density functional investigation into the material properties of Mn 2 RuGa, and introduce two concepts—the spin anchor site and the optical active site—as two pillars for AOS in ferrimagnets. In Mn 2 RuGa, Mn(4a) serves as the spin anchor site, whose band is below the Fermi level and has a strong spin moment, while Mn(4c) is the optical active site whose band crosses the Fermi level. Our magneto-optical Kerr spectrum and band structure calculation jointly reveal that the delicate competition between the Ru-4d and Ga-4p states is responsible for the creation of these two sites. Furthermore these two sites found here not only present a unified picture for both Mn 2 RuGa and GdFeCo, but also open the door for future applications. Specifically, we propose a Mn 2 Ru x Ga-based magnetic tunnel junction where a single laser pulse can control magnetoresistance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure of amorphous GeSe{sub 9} by neutron diffraction and first-principles molecular dynamics: Impact of trajectory sampling and size effects

The structure of glassy GeSe{sub 9} was investigated by combining neutron diffraction with density-functional-theory-based first-principles molecular dynamics. In the simulations, three different models of N = 260 atoms were prepared by sampling three independent temporal trajectories, and the glass structures were found to be substantially different from those obtained for models in which smaller numbers of atoms or more rapid quench rates were employed. In particular, the overall network structure is based on Se{sub n} chains that are cross-linked by Ge(Se{sub 4}){sub 1/2} tetrahedra, where the latter are predominantly corner as opposed to edge sharing. The occurrence of a substantial proportion of Ge–Se–Se connections does not support a model in which the material is phase separated into Se-rich and GeSe{sub 2}-rich domains. The appearance of a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor does, however, indicate a non-uniform distribution of the Ge-centered structural motifs on an intermediate length scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

First principles based screen for identification of transparent conductors

We present methods for screening semiconductors as transparent conductors leading to a simple application dependent fitness function. We report a simple first principles based screen amenable to high throughput calculations for transparent conductors. These include the effects of doping on transparency, which can be important. In particular, doping leads to conduction accompanied by losses in transparency, both of which are materials dependent, but rarely considered in computational materials selection. We consider both the Drude contribution to optical conductivity, as well as new interband transitions that arise as a material is doped. This leads to a simple application dependent optical and electrical fitness function (OEF) that can be applied to semiconductors to identify materials that may be useful transparent conductors. The OEF goes beyond the frequently discussed criteria of suitable band gap and low effective mass. Finally, we illustrate this by application to a number of proposed p-type transparent conducting materials.

14 SOLAR ENERGY↗