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33 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa crystallizes in the orthorhombic Amm2 space group. The structure is three-dimensional. there are three inequivalent Mg sites. In the first Mg site, Mg is bonded to two equivalent Mg and eight Ga atoms to form a mixture of distorted edge, corner, and face-sharing MgMg2Ga8 cuboctahedra. Both Mg–Mg bond lengths are 3.06 Å. There are a spread of Mg–Ga bond distances ranging from 2.91–3.06 Å. In the second Mg site, Mg is bonded to two equivalent Mg and eight Ga atoms to form a mixture of distorted edge, corner, and face-sharing MgMg2Ga8 cuboctahedra. Both Mg–Mg bond lengths are 3.07 Å. There are a spread of Mg–Ga bond distances ranging from 2.88–3.06 Å. In the third Mg site, Mg is bonded to six Mg and six Ga atoms to form a mixture of distorted edge, corner, and face-sharing MgMg6Ga6 cuboctahedra. Both Mg–Mg bond lengths are 3.10 Å. There are two shorter (2.98 Å) and four longer (3.06 Å) Mg–Ga bond lengths. There are three inequivalent Ga sites. In the first Ga site, Ga is bonded in a 12-coordinate geometry to eight Mg and four Ga atoms. There are two shorter (2.96 Å) and two longer (3.10 Å) Ga–Ga bond lengths. In the second Ga site, Ga is bonded in a 12-coordinate geometry to six Mg and two equivalent Ga atoms. In the third Ga site, Ga is bonded in a 12-coordinate geometry to eight Mg atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. Mg is bonded to four equivalent Ga atoms to form corner-sharing MgGa4 tetrahedra. All Mg–Ga bond lengths are 2.72 Å. Ga is bonded to four equivalent Mg atoms to form corner-sharing GaMg4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa crystallizes in the hexagonal P-6m2 space group. The structure is three-dimensional. Mg is bonded to six equivalent Ga atoms to form a mixture of distorted corner, edge, and face-sharing MgGa6 cuboctahedra. All Mg–Ga bond lengths are 2.96 Å. Ga is bonded in a 12-coordinate geometry to six equivalent Mg atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa crystallizes in the trigonal P3m1 space group. The structure is three-dimensional. there are three inequivalent Mg sites. In the first Mg site, Mg is bonded to three equivalent Mg and three equivalent Ga atoms to form distorted MgMg3Ga3 cuboctahedra that share corners with six equivalent GaMg3Ga3 cuboctahedra, corners with twelve MgMg9Ga3 cuboctahedra, edges with twelve MgMg3Ga3 cuboctahedra, a faceface with one MgMg6Ga6 cuboctahedra, and a faceface with one GaMg3Ga3 cuboctahedra. All Mg–Mg bond lengths are 3.21 Å. All Mg–Ga bond lengths are 3.00 Å. In the second Mg site, Mg is bonded to nine Mg and three equivalent Ga atoms to form distorted MgMg9Ga3 cuboctahedra that share corners with twelve MgMg3Ga3 cuboctahedra, corners with eighteen GaMg6 cuboctahedra, edges with six equivalent GaMg3Ga3 cuboctahedra, edges with twelve MgMg3Ga3 cuboctahedra, faces with two GaMg6 cuboctahedra, and faces with six equivalent MgMg9Ga3 cuboctahedra. All Mg–Mg bond lengths are 3.06 Å. All Mg–Ga bond lengths are 3.02 Å. In the third Mg site, Mg is bonded to six equivalent Mg and six Ga atoms to form distorted MgMg6Ga6 cuboctahedra that share corners with twelve MgMg3Ga3 cuboctahedra, corners with twelve GaMg6 cuboctahedra, edges with six equivalent MgMg6Ga6 cuboctahedra, edges with twelve GaMg6 cuboctahedra, faces with seven MgMg3Ga3 cuboctahedra, and faces with seven GaMg3Ga3 cuboctahedra. All Mg–Mg bond lengths are 3.06 Å. There are three shorter (3.03 Å) and three longer (3.10 Å) Mg–Ga bond lengths. There are three inequivalent Ga sites. In the first Ga site, Ga is bonded to six Mg atoms to form distorted GaMg6 cuboctahedra that share corners with six equivalent GaMg3Ga9 cuboctahedra, corners with twelve MgMg9Ga3 cuboctahedra, edges with six equivalent MgMg6Ga6 cuboctahedra, edges with six equivalent GaMg6 cuboctahedra, a faceface with one MgMg9Ga3 cuboctahedra, and a faceface with one GaMg3Ga9 cuboctahedra. In the second Ga site, Ga is bonded to three equivalent Mg and three equivalent Ga atoms to form distorted GaMg3Ga3 cuboctahedra that share corners with six equivalent GaMg3Ga9 cuboctahedra, corners with eighteen MgMg3Ga3 cuboctahedra, edges with six equivalent MgMg9Ga3 cuboctahedra, edges with twelve GaMg3Ga3 cuboctahedra, and faces with two MgMg3Ga3 cuboctahedra. All Ga–Ga bond lengths are 3.02 Å. In the third Ga site, Ga is bonded to three equivalent Mg and nine Ga atoms to form distorted GaMg3Ga9 cuboctahedra that share corners with six equivalent MgMg9Ga3 cuboctahedra, corners with eighteen GaMg6 cuboctahedra, edges with six equivalent MgMg6Ga6 cuboctahedra, edges with twelve GaMg3Ga3 cuboctahedra, faces with seven MgMg9Ga3 cuboctahedra, and faces with seven GaMg6 cuboctahedra. All Ga–Ga bond lengths are 3.06 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Mg is bonded to six equivalent Ga atoms to form a mixture of distorted edge and corner-sharing MgGa6 cuboctahedra. All Mg–Ga bond lengths are 2.97 Å. Ga is bonded in a 6-coordinate geometry to six equivalent Mg atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgGa by Materials Project

MgGa is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Mg is bonded to six equivalent Ga atoms to form a mixture of edge and corner-sharing MgGa6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–Ga bond lengths are 2.83 Å. Ga is bonded to six equivalent Mg atoms to form a mixture of edge and corner-sharing GaMg6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Importance of imposing gauge invariance in time-dependent density functional theory calculations with meta-generalized gradient approximations

It has been known for more than a decade that the gauge variance of the kinetic energy density τ leads to additional terms in the magnetic orbital rotation Hessian used in linear-response time-dependent density functional theory (TDDFT), affecting excitation energies obtained with τ-dependent exchange–correlation functionals. While previous investigations found that a correction scheme based on the paramagnetic current density has a small effect on benchmark results, we report more pronounced effects here, in particular, for the popular M06-2X functional and for some other meta-generalized gradient approximations (mGGAs). In the first part of this communication, this is shown by a reassessment of a set of five Ni(II) complexes for which a previous benchmark study that did not impose gauge invariance has found surprisingly large errors for excitation energies obtained with M06-2X. These errors are more than halved by restoring gauge invariance. The variable importance of imposing gauge invariance for different mGGA-based functionals can be rationalized by the derivative of the mGGA exchange energy integrand with respect to τ. In the second part, a large set of valence excitations in small main-group molecules is analyzed. For M06-2X, several selected n → π* and π→π$^{*}_{⊥}$ excitations are heavily gauge-dependent with average changes of –0.17 and –0.28 eV, respectively, while π→π$^{*}_{∥}$ excitations are marginally affected (–0.04 eV). Similar patterns, but of the opposite signs, are found for SCAN0. Here, the results suggest that reevaluation of previous gauge variant TDDFT results based on M06-2X and other mGGA functionals is warranted.

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

Shortcomings of meta-GGA functionals when describing magnetism

Several recent studies have shown that SCAN, a functional belonging to the meta-generalized gradient approximation (MGGA) family, leads to significantly overestimated magnetic moments in itinerant ferromagnetic metals. However, this behavior is not inherent to the MGGA level of approximation since TPSS, for instance, does not lead to such severe overestimations. In order to provide a broader view of the accuracy of MGGAfunctionals for magnetism, we extend the assessment to more functionals but also to antiferromagnetic solids. The results show that to describe magnetism there is overall no real advantage in using a MGGA functional compared to GGAs. For both types of approximation, an improvement in ferromagnetic metals is necessarily accompanied by a deterioration (underestimation) in antiferromagnetic insulators, and vice versa. Furthermore, we also provide some analysis in order to understand in more detail the relation between the mathematical form of the functionals and the results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Laplacian-level meta-generalized gradient approximation for solid and liquid metals

In this work, we derive and motivate a Laplacian-level, orbital-free meta-generalized-gradient approximation (LL-MGGA) for the exchange-correlation energy, targeting accurate ground-state properties of sp and sd metallic condensed matter, in which the density functional for the exchange-correlation energy is only weakly nonlocal due to perfect long-range screening. Our model for the orbital-free kinetic energy density restores the fourth-order gradient expansion for exchange to the r 2 SCAN meta-GGA [Furness et al., J. Phys. Chem. Lett. 11, 8208 (2020)], yielding a LL-MGGA we call OFR2. OFR2 matches the accuracy of SCAN for prediction of common lattice constants and improves the equilibrium properties of alkali metals, transition metals, and intermetallics that were degraded relative to the PBE GGA values by both SCAN and r 2 SCAN. We compare OFR2 to the r 2 SCAN -L LL-MGGA [Mejia-Rodriguez and Trickey, Phys. Rev. B 102, 121109(R) (2020)] and show that OFR2 tends to outperform r 2 SCAN -L for the equilibrium properties of solids, but r 2 SCAN -L much better describes the atomization energies of molecules than OFR2 does. For best accuracy in molecules and nonmetallic condensed matter, we continue to recommend SCAN and r 2 SCAN. Numerical performance is discussed in detail, and our paper provides an outlook to machine learning.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Predicting Bond Dissociation Energies and Bond Lengths of Coordinatively Unsaturated Vanadium–Ligand Bonds

Understanding the electronic structure of coordinatively unsaturated transition-metal compounds and predicting their physical properties are of great importance for catalyst design. Bond dissociation energy D e and bond length r e are two of the fundamental quantities for which good predictions are important for a successful design strategy. In the present work, recent experimentally measured bond energies and bond lengths of VX diatomic molecules (X = C, N, S) are used as a gauge to consider the utility of a number of electronic structure methods. Single-reference methods are one focus because of their efficiency and utility in practical calculations, and multireference configuration interaction (MRCISD) methods and a composite coupled cluster (CCC) method are a second focus because of their potential high accuracy. The comparison is especially challenging because of the large multireference M diagnostics of these molecules, in the range 0.15–0.19. For the single-reference methods, Kohn–Sham density functional theory (KS-DFT) has been tested with a variety of approximate exchange-correlation functionals. Of these, MOHLYP provides the bond dissociation energies in best agreement with experiments, and BLYP provides the bond lengths that are in best agreement with experiments; but by requiring good performance for both the D e and r e of the vanadium compounds, MOHLYP, MN12-L, MGGA_MS1, MGGA_MS0, O3LYP, and M06-L are the most highly recommended functionals. The CCC calculations include up to connected pentuple excitations for the valence electrons and up to connected quadruple excitations for the core–valence terms; this results in highly accurate dissociation energies and good bond lengths. In conclusion, averaged over the three molecules, the mean unsigned deviation of CCC bond energies from experimental ones is only 0.4 kcal/mol, demonstrating excellent convergence of theory and experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Analytical harmonic vibrational frequencies with VV10-containing density functionals: Theory, efficient implementation, and benchmark assessments

VV10 is a powerful nonlocal density functional for long-range correlation that is used to include dispersion effects in many modern density functionals, such as the meta-generalized gradient approximation (mGGA), B97M-V, the hybrid GGA, ωB97X-V, and the hybrid mGGA, ωB97M-V. While energies and analytical gradients for VV10 are already widely available, this study reports the first derivation and efficient implementation of the analytical second derivatives of the VV10 energy. The additional compute cost of the VV10 contributions to analytical frequencies is shown to be small in all but the smallest basis sets for recommended grid sizes. Here, this study also reports the assessment of VV10-containing functionals for predicting harmonic frequencies using the analytical second derivative code. The contribution of VV10 to simulating harmonic frequencies is shown to be small for small molecules but important for systems where weak interactions are important, such as water clusters. In the latter cases, B97M-V, ωB97M-V, and ωB97X-V perform very well. The convergence of frequencies with respect to the grid size and atomic orbital basis set size is studied, and recommendations are reported. Finally, scaling factors to allow comparison of scaled harmonic frequencies with experimental fundamental frequencies and to predict zero-point vibrational energy are presented for some recently developed functionals (including r2SCAN, B97M-V, ωB97X-V, M06-SX, and ωB97M-V).

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

Inversion, chemical complexity, and interstitial transport in spinels

Abstract Spinels with the generic chemical formula AB 2 O 4 have potential applications in nuclear energy and batteries. In both cases, their functionality is related to mass transport through the crystal. Here, using long‐time atomistic simulations, we examine the impact of the cation structure on interstitial transport in two spinel chemistries, inverse MgGa 2 O 4 and double MgAlGaO 4 . We emphasize two aspects of the transport properties: the unit mechanisms that are described by individual barriers, for which we introduce pole‐figure‐like plots, and the aggregate behavior of those unit mechanisms. Compared to previous work on normal spinels, we find that inversion significantly reduces the rate of interstitial transport in these structures and has an impact on the stability of defects as they move through the lattice. In particular, B cation interstitials are found to be kinetically stable only in the inverse MgGa 2 O 4 . These results provide new insight into relationship between structure, chemistry, and transport in spinels.

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

Closing the Gap Between Experiment and Theory: Reactive Scattering of HCl from Au(111)

Accurate simulation of molecules reacting on metal surfaces, which can help in improving heterogeneous catalysts, remains out of reach for several reactions. For example, a large disagreement between theory and experiment for HCl reacting on Au(111) still remains, despite many efforts. In this work, the dissociative chemisorption of HCl on Au(111) is investigated with a recently developed MGGA density functional (MS-RPBEl) and a high-dimensional neural network potential. Additionally, previous experimental sticking probabilities are re-examined. A considerably improved agreement between experiment and theory is obtained, although theory still overestimates experimental sticking probabilities by a factor of 2-7 at the highest incidence energy. Computed and measured vibrational transition probabilities are also in improved agreement. Several dynamical effects such as angular steering and energy transfer from the molecule to the surface are found to play an important role.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Ultra-thin lithium aluminate spinel ferrite films with perpendicular magnetic anisotropy and low damping

Ultra-thin films of low damping ferromagnetic insulators with perpendicular magnetic anisotropy have been identified as critical to advancing spin-based electronics by significantly reducing the threshold for current-induced magnetization switching while enabling new types of hybrid structures or devices. Here, we have developed a new class of ultra-thin spinel structure Li 0.5 Al 1.0 Fe 1.5 O 4 (LAFO) films on MgGa 2 O 4 (MGO) substrates with: 1) perpendicular magnetic anisotropy; 2) low magnetic damping and 3) the absence of degraded or magnetic dead layers. These films have been integrated with epitaxial Pt spin source layers to demonstrate record low magnetization switching currents and high spin-orbit torque efficiencies. These LAFO films on MGO thus combine all of the desirable properties of ferromagnetic insulators with perpendicular magnetic anisotropy, opening new possibilities for spin based electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Prediction of structure and cation ordering in an ordered normal-inverse double spinel

Abstract Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials. While solid solutions combining two “single spinels” have been explored in a number of past studies, no ordered “double” spinels have been reported. Based on our first principles computations, here we predict the existence of such a double spinel compound MgAlGaO 4 , formed by an equimolar mixing of MgAl 2 O 4 normal and MgGa 2 O 4 inverse spinels. After studying the details of its atomic and electronic structure, we use a cluster expansion based effective Hamiltonian approach with Monte Carlo simulations to study the thermodynamic behavior and cation distribution as a function of temperature. Our simulations provide strong evidence for short-ranged cation order in the double spinel structure, even at significantly elevated temperatures. Finally, an attempt was made to synthesize the predicted double spinel compound. Energy Dispersive X-ray Spectrometry and X-ray diffraction Rietveld refinements were performed to characterize the single-phase chemical composition and local configurational environments, which showed a favorable agreement with the theoretical predictions. These findings suggest that a much larger number of compounds can potentially be realized within this chemical space, opening new avenues for the design of spinel-structured materials with tailored functionality.

36 MATERIALS SCIENCE↗