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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

First-Principles Analysis of Phase Stability in Layered–Layered Composite Cathodes for Lithium-Ion Batteries

The atomic order in layered-layered composites with composition xLi 2 MnO 3 ·(1 - x)LiCoO 2 is investigated with first-principles calculations at the GGA+U level. This material, and others in its class, are often regarded as solid solutions; however, only a minute solubility of Li 2 MnO 3 in a LiCoO 2 host is predicted. Calculations of Co vacancy formation and migration energies in LiCoO 2 are presented to elucidate the rate of vacancymediated ordering in the transition-metal-layer. The neutral Co vacancy formation energy is predicted to be in a range centered slightly above 1 eV but varies widely with the oxygen chemical potential. The calculated migration energy for the vacancy with charge q = -3e is approximately 1 eV. These values are small enough to be consistent with rapid ordering in the transition metal layer at typical synthesis temperatures and, therefore, separated Li 2 MnO 3 and LiCoO 2 phases. The relatively small (of the order of a few nm) Li 2 MnO 3 domain sizes observed with TEM in some xLi 2 MnO 3 ·(1 - x)LiMO 2 composites may result from other factors, such as coherency strain, which perhaps block further domain coarsening in these materials.

25 ENERGY STORAGE↗
DOE OSTI2021

Spin–spin interactions in defects in solids from mixed all-electron and pseudopotential first-principles calculations

Abstract Understanding the quantum dynamics of spin defects and their coherence properties requires an accurate modeling of spin-spin interaction in solids and molecules, for example by using spin Hamiltonians with parameters obtained from first principles calculations. We present a real-space approach based on density functional theory for the calculation of spin-Hamiltonian parameters, where only selected atoms are treated at the all-electron level, while the rest of the system is described with the pseudopotential approximation. Our approach permits calculations for systems containing more than 1000 atoms, as demonstrated for defects in diamond and silicon carbide. We show that only a small number of atoms surrounding the defect needs to be treated at the all-electron level, in order to obtain an overall all-electron accuracy for hyperfine and zero-field splitting tensors. We also present results for coherence times, computed with the cluster correlation expansion method, highlighting the importance of accurate spin-Hamiltonian parameters for quantitative predictions of spin dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2013

First principle study of elastic and thermodynamic properties of FeB{sub 4} under high pressure

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB{sub 4} at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB{sub 4} are also determined in the pressure range of 0–100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB{sub 4} is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB{sub 4} possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB{sub 4} is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB{sub 4} are successfully obtained in the present work.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

First-principles study on the magnetic properties in Mg doped BiFeO{sub 3} with and without oxygen vacancies

The magnetic properties of Mg-doped BiFeO{sub 3} (BFO) with and without oxygen vacancies are studied through first-principles calculations. The Mg-doping prefers to occupy the ferromagnetic planes and produces an obvious improved magnetization, and the magnetization is linearly enhanced with increasing Mg-doped content, which is consistent with the trend reported in experiment. However, our calculated result is significantly larger than the experimental one, and the reason is revealed that the relative energy differences of various spin-ordering configurations are small. Furthermore, oxygen vacancy in Mg-doped BFO can further enhance the magnetization, while keeping the insulating band gap character. The calculated results imply that the oxygen vacancy in Mg-doped BFO would be an effective way to improve the multiferroicity of BFO.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electronic and magnetic properties of armchair MoS{sub 2} nanoribbons under both external strain and electric field, studied by first principles calculations

The electronic and magnetic properties of armchair edge MoS{sub 2} nanoribbons (MoS{sub 2}-ANRs) underboth the external strain and transverse electric field (E{sub t}) have been systematically investigated by using the first-principles calculations. It is found that: (1) If no electric field is applied, an interesting structural phase transition would appear under a large tensile strain, leading to a new phase MoS{sub 2}-A'NR, and inducing a big jump peak of the band gap in the transition region. But, the band gap response to compressive strains is much different from that to tensile strain, showing no the structural phase transition. (2) Under the small tensile strains (<10%), the combined E{sub t} and tensile strain give rise to a positive superposition (resonant) effect on the band gap reduction at low E{sub t} (<3 V/nm), and oppositely a negative superposition (antiresonant) one at high E{sub t} (>4 V/nm). On the other hand, the external compressive strains have always presented the resonant effect on the band gap reduction, induced by the electric field. (3) After the structural phase transition, an external large tensile strain could greatly reduce the critical field E{sub tc} causing the band gap closure, and make the system become a ferromagnetic (FM) metal at a relative low E{sub t} (e.g., <4 V/nm), which is very helpful for its promising applications in nano-mechanical spintronics devices. (4) At high E{sub t} (>10 V/nm), the magnetic moments of both the MoS{sub 2}-ANR and MoS{sub 2}-A'NR in their FM states could be enhanced greatly by a tensile strain. Our numerical results of effectively tuning physical properties of MoS{sub 2}-ANRs by combined external strain and electric field may open their new potential applications in nanoelectronics and spintronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

First principle study of structural, electronic and magnetic properties of zigzag boron nitride nanoribbon: Role of vacancies

We study the effect of vacancies on the structural, electronic and magnetic properties of zigzag boron nitride nanoribbon (ZBNNR) by using first principle calculations. We find that the shift of the vacancies with respect to the ribbon edges causes change in the structural geometry, electronic structure and magnetization of ZBNNR. These vacancies also produce band gap modulation and consequently results the magnetization of ZBNNR.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The temperature-dependent diffusion coefficient of helium in zirconium carbide studied with first-principles calculations

The temperature-dependent diffusion coefficient of interstitial helium in zirconium carbide (ZrC) matrix is calculated based on the transition state theory. The microscopic parameters in the activation energy and prefactor are obtained from first-principles total energy and phonon frequency calculations including the all atoms. The obtained activation energy is 0.78 eV, consistent with experimental value. Besides, we evaluated the influence of C and Zr vacancies as the perturbation on helium diffusion, and found the C vacancy seems to confine the mobility of helium and the Zr vacancy promotes helium diffusion in some extent. These results provide a good reference to understand the behavior of helium in ZrC matrix.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

First principles calculation of two dimensional antimony and antimony arsenide

This work focuses on the strain dependence of the electronic properties of two dimensional antimony (Sb) material and its alloy with As (SbAs) using density functional theory based first principles calculations. Both systems show indirect bandgap semiconducting character which can be transformed into a direct bandgap material with the application of relatively small strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

First-principles study of hydrogen behavior near W/WC interfaces

Hydrogen (H) solution, migration, and binding to vacancies near the tungsten/tungsten carbide (W/WC) interfaces based on hcp WC or bcc W (W/WCb and Wb/WC) have been studied by first-principles simulations to provide insights into the effect of C on H behavior in W exposed to C deposition in fusion reactors. The H behavior near the W/WC interfaces is related to the tensile or compressive strain due to the lattice match and interfacial effects. Near the W/WCb interface, the H solution energy in hcp WC increases approaching the interface, while it decreases from the W side to the interface. The configuration change of the W slab significantly decreases the H solution energy but has only a modest effect on the H migration energy barrier in comparison with the values in bcc W. The interface enhances the H binding to vacancies in WC layers. Both the interface and configuration adjustment of the W slab reduce the H binding to vacancies on the W side of the interface. Near the Wb/WC interface, H solution energies in both WC and W slabs weakly decrease approaching the interface. The configuration change of the WC slab significantly increases the H solution energy and binding energy to vacancies but decreases the H migration energy barrier in comparison with the values in hcp WC. The H energetics near the W/WC interfaces indicate the likelihood of H localization and diffusion in the W side of the interface, rather than diffusing through the interface into WC layers.

Yang, L. (ORCID:0000000322166071)↗
DOE OSTI2024

First-principles investigation of near-field energy transfer between localized quantum emitters in solids

We present a predictive and general approach to investigate near-field energy transfer processes between localized defects in semiconductors, which couples first-principles electronic structure calculations and a nonrelativistic quantum electrodynamics description of photons in the weak-coupling regime. The approach is general and can be readily applied to investigate broad classes of defects in solids. We apply our approach to investigate an exemplar point defect in an oxide, the F center in MgO, and we show that the energy transfer from a magnetic source, e.g., a rare-earth impurity, to the vacancy can lead to spin nonconserving long-lived excitations that are dominant processes in the near field, at distances relevant to the design of photonic devices and ultrahigh dense memories. We also define a descriptor for coherent energy transfer to predict geometrical configurations of emitters to enable long-lived excitations, that are useful to design optical memories in semiconductor and insulators. Published by the American Physical Society 2024

Chattaraj, Swarnabha (ORCID:0000000329333581)↗
DOE OSTI2019

Solute-dislocation interaction in Mg from first-principles: and twinning dislocations with flexible boundary conditions

We relax the cores of $\langle c+a \rangle$ edge and screw dislocations, and $(10\overline{1}2)$ and $(10\overline{1}1)$ twinning dislocations in Mg using first-principles calculations with flexible boundary conditions (FBC). FBC couple highly distorted cores to a harmonic lattice through lattice Green functions (LGF), allowing dislocations to relax as isolated defects. Unlike previous FBC calculations using bulk LGFs, we use LGFs specifically for dislocated geometries. The optimized geometries provide sites for computing solute-dislocation interactions via direct substitution, as well as local strain and slip for input to computationally efficient geometric solute interaction models. The interactions are inputs for solid-solution strengthening models. We compute interactions between Al, Ca, Mn, Sn, and Zn and the dislocations, and predict corresponding changes in stress for dislocation motion. We compare the strengthening of each of these non-basal deformation modes to basal strengthening and examine implications for ductility in Mg alloys. Funding from DOE-USAMP Cooperative Agreement Number DE-EE0007756.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural Polymorphism in “Kesterite” Cu 2 ZnSnS 4 : Raman Spectroscopy and First-Principles Calculations Analysis

This work presents detailed structural and vibrational characterization of different Cu 2 ZnSnS 4 (CZTS) polymorphs (space groups: I$\bar{4}$, P$\bar{4}$2c, and P$\bar{4}$2m), using Raman spectroscopy and first-principles calculations. Multiwavelength Raman measurements on bulk crystalline CZTS samples permitted determination of the vibrational modes of each polymorph structure, with frequencies matching well with the results obtained from simulations. Lastly, the results present Raman spectra fingerprints as well as experimental references for the different polymorph modifications.

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

Understanding the Solid-State Electrode–Electrolyte Interface of a Model System Using First-Principles Statistical Mechanics and Thin-Film X-ray Characterization

Intermixing of atomic species at the electrode-electrolyte boundaries can impact the properties of the interfaces in solid-state batteries. Herein, this work uses first-principles statistical mechanics along with experimental characterization to understand intermixing at the electrode-electrolyte interface. For the model presented in this work, lithium manganese oxide (LiMn 2 O 4 , LMO) and lithium lanthanum titanate (Li 3x La 2/3-x TiO 3 , LLTO) are employed as the cathode and electrolyte, respectively. The results of the computational work show that Ti-Mn intermixing at the interface is significant at synthesis temperatures. The experimental results in this work find that, at some critical temperatures between 600 and 700 degrees C for material preparation, the interface of LLTO-LMO becomes blurred. Calculations predict that the interface is unstable with regard to Ti-Mn intermixing starting at 0 K, suggesting that the critical temperature found in the experiment is related to kinetics. The work overall suggests that, in designing a solid-state battery, the fundamental reactions such as intermixing need to be considered.

25 ENERGY STORAGE↗
DOE OSTI2014

Communication: Charge-transfer rate constants in zinc-porphyrin-porphyrin-derived dyads: A Fermi golden rule first-principles-based study

We investigate photoinduced charge transfer (CT) processes within dyads consisting of porphyrin derivatives in which one ring ligates a Zn metal center and where the rings vary by their degree of conjugation. Using a first-principles approach, we show that molecular-scale means can tune CT rates through stabilization affected by the polar environment. Such means of CT tuning are important for achieving high efficiency optoelectronic applications using organic semiconducting materials. Our fully quantum mechanical scheme is necessary for reliably modeling the CT process across different regimes, in contrast to the pervading semi-classical Marcus picture that grossly underestimates transfer in the far-inverted regime.

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

Study of Pb-doped Ge{sub 2}Sb{sub 2}Te{sub 5} in crystalline phase using first principle calculations

To improve the phase change characteristics of Ge{sub 2}Sb{sub 2}Te{sub 5} (GST), doping is used as one of the effective methods. 4.4 atomic % of Pb doped GST has been studied using first principle calculations. No effect of doping on Te-Ge and Te-Sb bond length has been observed, but the Te-Te bond gets shrink with Pb doping. Due to which the Sb{sub 2}Te{sub 3} segregates as a second phase, with increased doping concentration of Pb in GST alloy. Using such type of calculation, we can calculate the desirable concentration of dopant atoms to prepare the desired material. We can control any segregation in required material with pre-theoretical calculations. The metallic nature of Pd doped GST has been discussed with band structure plots. The metallic character of alloys calculated as in this paper will be helpful to understand the tuning of conductivity of phase change materials, which helps to enhance the phase change properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles study of graphene under c-HfO{sub 2}(111) layers: Electronic structures and transport properties

We investigated the electronic properties, stability, and transport of graphene under c-HfO{sub 2}(111) layers by performing first-principles calculations with special attention to the chemical bonding between graphene and HfO{sub 2} surfaces. When the interface of HfO{sub 2}/graphene is terminated by an O layer, the linear dispersion of graphene is preserved and the degradation of transport is suppressed. For other interface structures, HfO{sub 2} is tightly adsorbed on graphene and the transport is strictly limited. In terms of the stability of the interface structures, an O-terminated interface is preferable, which is achieved under an O-deficient condition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Energy barriers for Pb adatom diffusion on stepped ultrathin Pb(111) quantum nanofilms: First-principles calculations

Various properties of Pb(111) nanofilms, prototypical quantum films, have been studied extensively. However, key ab initio -level energy barriers for Pb adatom diffusion on stepped Pb(111) nanofilm surfaces are still not available. Using first-principles density functional theory, we calculate these barriers for films with thicknesses of few monolayers (ML). We find that two-atom exchange is always much more favorable than direct hopping to cross A- or B-type steps. Ehrlich-Schwoebel (ES) barriers for downward transport to a higher-coordination step-edge site depend strongly on the film thickness. For such transport from 2- to 1-ML terraces, or from 4- to 3-ML terraces, there is no an ES barrier, but large ES barriers of more than 100 meV are found from 3- to 2-ML terraces. We also obtain the barriers for diffusion along the step edges and find that these step-edge barriers are significantly larger than terrace diffusion barriers. In addition, we analyze energetics for diffusion on the top flat surface of a nanofilm supported on a vicinal surface, and thus having underlying buried or ghost steps. We quantify the tilted potential energy surface in both ghost A- and B-step regions separating 2- and 3-ML (as well as 3- and 4-ML) terraces. Finally, consequences are discussed for the growth kinetics of supported Pb nanofilms, where the support does not strongly affect electronic states within the film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

A First Principle Study of Half-Heusler Compounds CrTiZ (Z = P, As)

Half-Heusler compounds CrTiZ (Z = P, As) are studied in the framework of Density Functional Theory (DFT). Structural, mechanical, elastic, electronic, magnetic, and optical properties are derived using first principles. The full-potential linearized augmented plane wave (FP-LAPW) method is used for these investigations as implemented in the WIEN2k simulation package. We have found that the equilibrium lattice parameters are in the range 5.5–5.9 Å. Small band gaps of 0.5–0.6 eV in the minority spin-down states are observed. On the other hand, in the majority spin-up states, there is no band gap. Hence, the compounds are of half-metallic nature, having 100 % polarization at Fermi level. Comparatively, a higher total magnetic moment of ∼ 3μ{sub B} is found for both the compounds. Furthermore, optical properties are derived using dielectric function. Reflectivity, refractive index, conductivity and absorption coefficient are calculated. It is revealed from the imaginary part of the dielectric function that the compounds are optically metallic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗