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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Elasticity and phase transformation at high pressure in coesite from experiments and first-principles calculations

Here, the crystal structure and equation of state of coesite (space group C2/ c ) and its high pressure polymorph coesite-II (space group P2 1 / n ) under pressure have been studied using X-ray powder diffraction in a diamond anvil cell (DAC) up to 31 GPa at room temperature and first-principles calculations at 0 K up to 45 GPa. New diffraction peaks appear above 20 GPa, indicating the formation of coesite-II structure. The calculated enthalpies provide theoretical support for the pressure-induced phase transformation from coesite to coesite-II at ~21.4 GPa. Compared with coesite, the coesite-II structure is characterized by a ‘doubled’ b -axis and the breakdown of the linear Si1-O1-Si1 angle in coesite into two distinct angles - one is ~176°, close to linear, whereas the other decreases by 22° to 158°. Coesite is very anisotropic with the a -axis the shortest and twice more compressible than the b- and c-axis. By comparison, coesite-II is not so anisotropic with similar compressibilities in its a-, b-, and c -axis. As analyzed by a third-order Eulerian finite strain equation of state, the bulk modulus of coesite at 21.4 GPa is 182.3GPa, and that of coesite-II is 140.8 GPa, indicating that coesite-II is much more compressible than coesite. Furthermore, the existence of coesite-II in the coldest subduction zone will change the elasticity and anisotropic properties of the subducting materials dramatically.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermodynamic modeling of the La-Te system aided by first-principles calculations

A complete thermodynamic description of the La-Te binary system is developed by means of CALculation of PHAse Diagram (CALPHAD) method in combination with available experimental data in the literature and the present first-principles calculations based on density functional theory. The intermetallic phases with homogeneity ranges, La3-xTe4 and LaTe2-x, are modeled using a two-sublattice (La,Va)3(Te)4 model and a three-sublattice (La)1(Te)1(Te,Va)1 model based on their structure features, respectively. The intermetallic phases, LaTe and LaTe3, are treated as stoichiometric compounds. The thermodynamic properties of the intermetallic compounds and their corresponding end members at finite temperatures are predicted using first-principles quasi-harmonic approach. The associate solution model is used to describe the short-range ordering behavior of the liquid phase. The calculated phase diagram agrees well with the available phase equilibrium data in the literature.

Chemistry↗
DOE OSTI2016

First-principles calculations of thermoelectric properties of TiN/MgO superlattices: The route for an enhancement of thermoelectric effects in artificial nanostructures

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic structure and magnetic anisotropy of L1{sub 0}-FePt thin film studied by hard x-ray photoemission spectroscopy and first-principles calculations

We have studied the electronic structure of the L1{sub 0} ordered FePt thin film by hard x-ray photoemission spectroscopy (HAXPES), cluster model, and first-principles calculations to investigate the relationship between the electronic structure and perpendicular magneto-crystalline anisotropy (MCA). The Fe 2p core-level HAXPES spectrum of the ordered film revealed the strong electron correlation in the Fe 3d states and the hybridization between the Fe 3d and Pt 5d states. By comparing the experimental valence band structure with the theoretical density of states, the strong electron correlation in the Fe 3d states modifies the valence band electronic structure of the L1{sub 0} ordered FePt thin film through the Fe 3d-Pt 5d hybridization. These results strongly suggest that the strong electron correlation effect in the Fe 3d states and the Fe 3d-Pt 5d hybridization as well as the spin-orbit interaction in the Pt 5d states play important roles in the perpendicular MCA for L1{sub 0}-FePt.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Thermodynamic re-modeling of the Yb-Sb system aided by first-principles calculations

Here, the thermodynamic description of the Yb-Sb binary system is developed by means of the CALculations of PHAse Diagrams (CALPHAD) method by combining experimental data in the literature and predictions from first-principles calculations based on density functional theory (DFT) in the literature and the present work. Two pseudopotentials of Yb are compared in the present DFT-based calculations with 14 and 13 f-electrons frozen in the core, i.e., 5p 6 6s 2 and 5p 6 6s 2 5 d 1 electrons as valence electrons, termed Yb_2 and Yb_3, respectively. It is shown that the phonon spectrum of the YbSb phase calculated using the Yb_3 pseudopotential does not have imaginary phonon modes and is subsequently used to predict its temperature dependent thermodynamic properties by the DFT-based quasiharmonic phonon calculations. The present thermodynamic database includes the Yb 16 Sb 11 phase in addition to five intermetallic phases that were considered in previous modeling studies, i.e., YbSb 2 , YbSb, Yb 11 Sb 10 , Yb 4 Sb 3 , and Yb 5 Sb 3 . The high temperature orthorhombic structure of the Yb 5 Sb 3 phase is not considered in the present work as it was stabilized by hydrogen. The associate solution model is used to describe the short-range ordering behavior in the liquid phase. The calculations from the present thermodynamic model show good agreement with thermochemical and phase equilibrium data from both the present work and the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic and ferroelectric orders in strained Gd{sub 1/2}Na{sub 1/2}TiO{sub 3}: First-principles calculations

The emergent magnetic and ferroelectric orders in A-site ordered Gd{sub 1/2}Na{sub 1/2}TiO{sub 3} under lattice strain are investigated using the first-principles calculations. It is revealed that the lattice prefers the perovskite structure with alternatively stacked Ga-O and Na-O layers along the b-axis and the ground spin state favors the G-type antiferromagnetic (G-AFM) order. The ac-plane biaxial strain can remarkably tune the amplitude of ferroelectric polarization, while the G-AFM spin structure is robustly unaffected. The ±4% strains can trigger the change of polarization up to ±50% relative to the polarization value of unstrained structure. The present work suggests a possible scenario to control emergent multiferroic behaviors in Gd{sub 1/2}Na{sub 1/2}TiO{sub 3} via lattice strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

First principle calculation in FeCo overlayer on GaAs substrate

In this work the first principle electronic structure calculation is reported for FeCo/GaAs thin film system to investigate the effect of orientation on the electronic structural properties. A unit cell describing FeCo layers and GaAs layers is constructed for (100), (110), (111) orientation with vacuum of 30Å to reduce dimensions. It is found that although the (110) orientation is energetically more favorable than others, the magnetic moment is quite large in (100) and (111) system compared to the (110) and is due to the total DOS variation with orientation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Insight into ideal shear strength of Ni-based dilute alloys using first-principles calculations and correlational analysis

Here the present work examines the effect of alloying elements (denoted X) on the ideal shear strength for 26 dilute Ni-based alloys, Ni11X, as determined by first-principles calculations of pure alias shear deformations. The variations in ideal shear strength are quantitatively explored with correlational analysis techniques, showing the importance of atomic properties such as size and electronegativity. The shear moduli of the alloys are affirmed to show a strong linear relationship with their ideal shear strengths, while the shear moduli of the individual alloying elements were not indicative of alloy shear strength. Through combination with available ideal shear strength data on Mg alloys, a potential application of the Ni alloy data is demonstrated in the search for a set of atomic features suitable for machine learning applications to mechanical properties. As another illustration, the calculated Ni ideal shear strengths play a key role in a predictive multiscale framework for deformation behavior of single crystal alloys at large strains, as shown by simulated stress–strain curves.

36 MATERIALS SCIENCE↗
DOE OSTI2022

First-Principles Calculations of the Structural, Electronic, Optical, and Mechanical Properties of 21 Pyrophosphate Crystals

Pyrophosphate crystals have a wide array of applications in industrial and biomedical fields. However, fundamental understanding of their electronic structure, optical, and mechanical properties is still scattered and incomplete. In the present research, we report a comprehensive theoretical investigation of 21 pyrophosphates A 2 M (H 2 P 2 O 7 ) 2 •2H 2 O with either triclinic or orthorhombic crystal structure. The molecule H 2 P 2 O 7 is the dominant molecular unit, whereas A = (K, Rb, NH 4 , Tl), M = (Zn, Cu, Mg, Ni, Co, Mn), and H 2 O stand for the cation elements, transition metals, and the water molecules, respectively. The electronic structure, interatomic bonding, partial charge distribution, optical properties, and mechanical properties are investigated by first-principles calculations based on density functional theory (DFT). Most of these 21 crystals are theoretically investigated for the first time. The calculated results show a complex interplay between A, M, H 2 P 2 O 7 , and H 2 O, resulting in either metallic, half-metallic, or semi-conducting characteristics. The novel concept of total bond order density (TBOD) is used as a single quantum mechanical metric to characterize the internal cohesion of these crystals to correlate with the calculated properties, especially the mechanical properties. This work provides a large database for pyrophosphate crystals and a road map for potential applications of a wider variety of phosphates.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Structural, Electronic, and Magnetic Properties of Binary Heusler Alloys ZCl{sub 3} (Z = Li, Na, K, Rb) with DO{sub 3}-Type Structure from First Principle Calculations

The structural, electronic, and magnetic properties of binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) with DO{sub 3}-type structure have been studied by means of the first principle calculations. The calculated results reveal that all four alloys are half-metallic (HM) materials at the stable ferromagnetic ground state. The total magnetic moments (M{sub t}) of LiCl{sub 3}, NaCl{sub 3}, KCl{sub 3,} and RbCl{sub 3} alloys are all 2.000 μ{sub B}/f.u., following the Slater-Pauling rule M{sub t} = 24 − Z{sub t} for the former two alloys, while satisfying the Slater-Pauling rule M{sub t} = 30 − Z{sub t} for the latter two alloys. The spin-up band gaps of the binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) are 5.598, 5.687, 6.744, and 6.745 eV at equilibrium lattice constants 7.076, 7.237, 7.588, and 7.819 Å, respectively. The positive cohesion energy and negative formation energy indicate that they are thermodynamically stable. Moreover, wide HM regions for binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) show their HM characters are robust when the lattices are expanded or compressed under uniform and tetragonal strains. Finally, the total charge density, magnetic charge density, and charge density difference of the (110) plane for LiCl{sub 3} alloy are analyzed in detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Fe-vacancy ordering in superconducting K 1– x Fe 2– y Se 2 : First-principles calculations and Monte Carlo simulations

Fe vacancies in the 33 K superconductor K 1–x Fe 2–y Se 2 show ordering schemes that may be correlated with its superconducting properties. First-principles calculations and kinetic Monte Carlo simulations lead to a very simple model for vacancy ordering. Repulsive dipolar interactions between Fe vacancies show three ground states: a $\sqrt{8}\times \sqrt{10}$ rhombus-ordered structure for 12.5% vacancies, a $\sqrt{5}\times \sqrt{5}$ squared lattice for 20% vacancies, and a $\sqrt{5}\times \sqrt{5}$ rhombus-ordered structure for 25% vacancies. Other structural states are derived from these three ground states and may contain additional disordered spatial regions. As a result, the repulsive interaction between Fe vacancies arises from enhanced Fe–Se covalent bonds, which differs from the well-known attractive interaction of Fe vacancies in body-centered cubic Fe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Optical properties of group-3 metal hexaboride nanoparticles by first-principles calculations

LaB{sub 6} nanoparticles are widely used as solar control materials for strong near-infrared absorption and high visible transparency. In order to elucidate the origin of this unique optical property, first-principles calculations have been made for the energy-band structure and dielectric functions of R{sup III}B{sub 6} (R{sup III} = Sc, Y, La, Ac). On account of the precise assessment of the energy eigenvalues of vacant states in conduction band by employing the screened exchange method, as well as to the incorporation of the Drude term, dielectric functions and various physical properties of LaB{sub 6} have been reproduced in excellent agreement with experimental values. Systematic examinations of dielectric functions and electronic structures of the trivalent metal hexaborides have clarified the origin of the visible transparency and the near-infrared plasmon absorption of R{sup III}B{sub 6} nanoparticles.

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

Strongly tunable anisotropic thermal transport in MoS 2 by strain and lithium intercalation: first-principles calculations

The possibility of tuning the vibrational properties and the thermal conductivity of layered van der Waals materials either chemically or mechanically paves the way to significant advances in nanoscale heat management. Using first-principles calculations we investigate the modulation of heat transport in MoS 2 by lithium intercalation and cross-plane strain. Here, we find that both the in-plane and cross-plane thermal conductivity (κ τ ,κ z ) of MoS 2 are extremely sensitive to both strain and electrochemical intercalation. Combining lithium intercalation and strain, the in-plane and cross-plane thermal conductivity can be tuned over one and two orders of magnitude, respectively. Furthermore, since and respond in different ways to intercalation and strain, the thermal conductivity anisotropy can be modulated by two orders of magnitude. The underlying mechanisms for such large tunability of the anisotropic thermal conductivity of MoS 2 are explored by computing and analyzing the dispersion relations, group velocities, relaxation times and mean free paths of phonons. Since both intercalation and strain can be applied reversibly, their stark effect on thermal conductivity can be exploited to design novel phononic devices, as well as for thermal management in MoS 2 -based electronic and optoelectronic systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Assessing the roles of Cu- and Ag-deficient layers in chalcopyrite-based solar cells through first principles calculations

Chalcopyrites are a demonstrated material platform for realizing efficient thin-film photovoltaics, with the most well known Cu(In,Ga)Se 2 (CIGS)-based solar cells exceeding 23%. Several factors, including flexibility in tuning the absorber bandgap, enhanced surface treatments, and the electrically benign nature of common defects are responsible for the existing high performance and future promise in chalcopyrite-based photovoltaic devices. The introduction of Cu-poor phases (also known as ordered-vacancy compounds or OVCs) between the absorber and buffer layers in CIGS solar cells is known to enhance device performance; however, the overall properties and role of OVCs remain poorly understood. Using first principles calculations based on the density functional theory with screened hybrid functionals, here we explore the electronic structure and stability of OVCs and their band offsets with defect-free chalcopyrite layers in Cu- and Ag-based compounds (ABX 2 where A=Cu, Ag; B=In, Ga, Al; and X=S, Se). Using AB 3 X 5 and AB 5 X 8 stoichiometries as model OVC systems, we report on the variation of the bandgap with the A/B ratio and discuss the trends in other Cu- and Ag-based chalcopyrites beyond CuInSe 2 . We find that the valence and conduction bands are lower in energy in OVCs with respect to the parent ABX 2 chalcopyrite owing to a reduced p–d interaction between X and A atoms. We additionally perform device-level simulations to assess the implications of the results, finding that the valence band offsets of OVCs are favorable, while the conduction band offsets of chalcopyrites beyond CuInSe 2 -based absorbers may be detrimental in conventional solar cell device designs.

14 SOLAR ENERGY↗
DOE OSTI2017

Stability of Cd 1– x Zn x O y S 1– y Quaternary Alloys Assessed with First-Principles Calculations

One route to decreasing the absorption in CdS buffer layers in Cu(In,Ga)Se 2 and Cu 2 ZnSn(S,Se) 4 thin-film photovoltaics is by alloying. Here we use first-principles calculations based on hybrid functionals to assess the energetics and stability of quaternary Cd, Zn, O, and S (Cd 1–x Zn x O y S 1–y ) alloys within a regular solution model. Our results identify that full miscibility of most Cd 1–x Zn x O y S 1–y compositions and even binaries like Zn(O,S) is outside typical photovoltaic processing conditions. Finally, the results suggest that the tendency for phase separation of the oxysulfides may drive the nucleation of other phases such as sulfates that have been increasingly observed in oxygenated CdS and ZnS.

14 SOLAR ENERGY↗
DOE OSTI2015

First-principles calculations of phonons and Raman spectra in monoclinic CsSnCl 3

Density functional perturbation theory is used here to calculate the phonons at the zone center for monoclinic CsSnCl 3 . Furthermore, we report the calculated normal mode frequencies classified according to irreducible representations for both infrared-active and Raman-active modes. We also report the dielectric constants and Born-effective charges and investigate the anisotropy of the nonanalyticities near the zone center. The first-principles calculated Raman-active phonon frequencies and simulated Raman spectra are compared with experimental results from literature. We find that aside from the first-order allowed Raman modes of A g and B g symmetries, forbidden LO Raman modes appear prominently in the measured spectra. Several of these modes are close to or coincide with allowed modes and alter their intensity, while others appear as separate otherwise unexplained peaks. With this interpretation, good agreement is obtained with experimental mode frequencies to within a few cm -1 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

First-principles calculation of principal Hugoniot and K-shell X-ray absorption spectra for warm dense KCl

Principal Hugoniot and K-shell X-ray absorption spectra of warm dense KCl are calculated using the first-principles molecular dynamics (FPMD) method. Evolution of electronic structures as well as the influence of the approximate description of ionization on pressure (caused by the underestimation of the energy gap between conduction bands and valence bands) in the first-principles method are illustrated by the calculation. It is shown that approximate description of ionization in FPMD has small influence on Hugoniot pressure due to mutual compensation of electronic kinetic pressure and virial pressure. The calculation of X-ray absorption spectra shows that the band gap of KCl persists after the pressure ionization of the 3p electrons of Cl and K taking place at lower energy, which provides a detailed understanding to the evolution of electronic structures of warm dense matter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
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↗