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All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Defect identification based on first-principles calculations for deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid-functional calculations for two important defects in GaN, which have similar thermodynamic transition levels and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and, in some cases, large in comparison to the ionization energy. By calculating quantities that can be directly compared with the experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS.

Activation energies↗
DOE OSTI2021

Structural, Electrical, and Electromechanical Properties of Inverse Hybrid Perovskites from First-Principles: The Case of (CH 3 NH 3 ) 3 OI

Inverse-hybrid perovskites (IHPs) with large polarization have recently been predicted from first-principles computations. Here, we use one representative from the IHP class of materials, (CH 3 NH 3 ) 3 OI (MA3OI), to propose a route to the first-principles prediction of structural and electrical properties, such as polarization, polarization reversibility, and the associated coercive field for hybrid organic–inorganic perovskites. The route relies on the construction of the polarization reversal path that models experimental measurements. Such a path was found to play an important role in the ground-state search as well as in the identification of competing structural variations. The latter is believed to be the origin of the structural disorder that is characteristic of hybrid organic–inorganic perovskites. The application of such an approach to MA3OI leads to the prediction of several structural variations that are expected to result in a structurally disordered phase above 766 K and of the polar ground state with a polarization of 25.3 μC/cm 2 that is reversible with the application of an electric field. The upper estimate for the coercive field associated with homogeneous polarization reversal is 6.9 GV/m. The piezoelectric constants of MA3OI are predicted to be an order of magnitude smaller in comparison with a prototypical inorganic ferroelectric PbTiO 3 ; however, the low symmetry of the MA3OI structure yields finite values for all components of the piezoelectric tensor. The polarization in MA3OI is tunable by the epitaxial strain (11.5% change under 5% epitaxial strain), although less so as compared with PbTiO 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Phase diagram of magnetic shape memory alloy Ni 50 Mn $50–x$ In $x$ , 0 < $x$ , 25 from first principles, via spin cluster expansion and phonon vibrational entropies

The metamagnetic shape memory Heusler alloy Ni 50 Mn $50–x$ In $x$ exhibits a rich phase diagram featuring competing magnetic states, coupled magnetic–structural phase transitions, and strong compositional sensitivity. Existing first-principles approaches struggletocapturetheintertwinedchemical, magnetic, andvibrationaleffectsinthesealloys, necessitating a more integrated modeling framework. We develop a spin cluster expansion (spin-CE) framework augmented by a quasi-harmonic phonon model to capture both configurational (chemical and magnetic) and vibrational contributions to the free energy of Ni 50 Mn $50–x$ In $x$ over the full compositional range 0 ≤ x ≤25. The spin-CE includes both chemical clusters and composition-dependent Ising spin interactions, with parameters fit to a first-principles density functional theory (DFT) dataset. Using this approach, we predict the complete magnetostructural phase diagram and transformation temperatures of Ni 50 Mn $50–x$ In $x$ across the composition space. We find that vibrational entropy alone is insufficient to reproduce the martensitic transformation in the magnetic shape memory alloy regime, highlighting the essential role of magnetism. Incorporating both magnetic and vibrational contributions allows us to reproduce all experimentally known phases, including the disappearance of the stable martensite phase at a critical In concentration and the Curie temperature of the austenite phase. The method also captures the transition with increasing In in martensite from antiferromagnetic to ferromagnetic order and predicts re-entrant ferromagnetism, though the latter occurs at higher In content than reported experimentally. We discuss possible sources of this discrepancy and highlight the broader applicability of the method to other magnetostructurally complex materials, where it may offer mechanistic insight and predictive design capabilities.

Cluster expansion↗
DOE OSTI2018

The Electronic Structures and Ferromagnetism of Cu-Doped ZnO: the First-Principle Calculation Study

We have carried out first-principle spin-polarized calculations to investigate the electronic structures and the magnetic properties of Cu-doped wurtzite ZnO. It was found that the Cu{sub Zn} substitution can introduce 1 μ{sub B} local magnetic moment into the doped system, whereas the ferromagnetic coupling between Cu{sub Zn}s is so weak that the system cannot keep ferromagnetism at higher temperature. In Zn vacancy (V{sub Zn}) and Cu{sub Zn} coexisted system, Cu is in the Cu {sup 2+}/Cu {sup 3+} mixed valence under the influence of acceptor defect V{sub Zn}. Cu-3d electrons couple with O(around V{sub Zn})-sp electrons and then form Cu{sup 2+} −V{sub Zn} −Cu{sup 2+} or Cu{sup 2+} −VZn−−Cu{sup 3+} bound magnetic polaron (BMP) in the range of 1.27 ∼ 1.90 nm. The overlapping between BMPs is the main ferromagnetism origin of ZnO/Cu. According to our analysis, we thought that ZnO/Cu system will display stronger ferromagnetism when the Cu concentration attains 3% in atomic percentage. The calculated results and analysis will shed light on the origin of ferromagnetism in ZnO/Cu.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Structural, electronic and magnetic properties of the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions: Spin polarized first principles total energy calculations

In this work, using first principles total energy calculations within the periodic spin polarized density functional theory, we have investigated the structural, electronic, and magnetic properties of manganese gallium (MnGa) alloys. Specifically, we explore the MnGa(111)-1 $\times$ 2 and 2 $\times$ 2 reconstructions. The surface formation energies reveal that selected substitutions occur under Mn (Ga) rich growth conditions. Structures with top layers missing all Mn (Ga) atoms and two layers deep substitutions are also investigated. However, the formation energy shows that these structures are less favorable. For the stable structures, the magnetic properties per layer are proportional to the Mn:Ga ratio. Also, the density of states shows that the MnGa surfaces are metallic. The projected density of states shows that the electronic states in the vicinity of the Fermi level are due mainly to the manganese 3d orbitals. However charge density plots reveal that Mn 3d electrons are closer to the nucleus than Ga sp electrons. Consequently, experimental scanning tunneling microscopy images reveal periodically-arranged bright features, corresponding to the Ga atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Probing Solid–Solid Interfacial Reactions in All-Solid-State Sodium-Ion Batteries with First-Principles Calculations

We present an exposition of first-principles approaches to elucidating interfacial reactions in all-solid-state sodium-ion batteries. We will demonstrate how thermodynamic approximations based on assumptions of fast alkali diffusion and multispecies equilibrium can be used to effectively screen combinations of Na-ion electrodes, solid electrolytes, and buffer oxides for electrochemical and chemical compatibility. We find that exchange reactions, especially between simple oxides and thiophosphate groups to form PO 4 3– , are the main cause of large driving forces for cathode/solid electrolyte interfacial reactions. A high reactivity with large volume changes is also predicted at the Na anode/solid electrolyte interface, while the Na 2 Ti 3 O 7 anode is predicted to be much more stable against a broad range of solid electrolytes. We identify several promising binary oxides, Sc 2 O 3 , SiO 2 , TiO 2 , ZrO 2 , and HfO 2 , that are similarly or more chemically compatible with most electrodes and solid electrolytes than the commonly used Al 2 O 3 is. Finally, we show that ab initio molecular dynamics simulations of the NaCoO 2 /Na 3 PS 4 interface model predict that the formation of SO 4 2– -containing compounds and Na 3 P is kinetically favored over the formation of PO 4 3– -containing compounds, in contrast to the predictions of the thermodynamic models. As a result, this work provides useful insights into materials selection strategies for enabling stable electrode/solid electrolyte interfaces, a critical bottleneck in designing all-solid-state sodium-ion batteries, and outlines several testable predictions for future experimental validation.

25 ENERGY STORAGE↗
DOE OSTI2018

First-principles investigation of native interstitial diffusion in Cr2O3

First principles density functional theory (DFT) investigation of native interstitials and the associated self-diffusion mechanisms in a-Cr2O3 reveals that interstitials are more mobile than vacancies of corresponding species. Cr interstitials occupy the unoccupied Cr sublattice sites that are octahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell configuration orientated along the [221] direction (diagonal) of the corundum lattice. Calculations predict that neutral O interstitials are predominant in O-rich conditions and Cr interstitials in +2 and +1 charge states are the dominant interstitial defects in Cr-rich conditions. Similar to that of the vacancies, the charge transition levels of both O and Cr interstitials are located deep within the bandgap. Transport calculations reveal a rich variety of interstitial diffusion mechanisms that are species, charge, and orientation dependent. Cr interstitials diffuse preferably along the diagonal of corundum lattice in a two step process via an intermediate defect complex comprising a Cr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr bilayer. This mechanism is similar to that of the vacancy mediated Cr diffusion along the c-axis with intermediate Cr vacancy and Cr Frenkel defect combination. In contrast, O interstitials diffuse via bond switching mechanism. O interstitials in -1 and -2 charge states have very high mobility compared to neutral O interstitials.

Medasani, Bharat K.↗
DOE OSTI2016

A perfect wetting of Mg monolayer on Ag(111) under atomic scale investigation: First principles calculations, scanning tunneling microscopy, and Auger spectroscopy

First principles calculations, scanning tunneling microscopy, and Auger spectroscopy experiments of the adsorption of Mg on Ag(111) substrate are conducted. This detailed study reveals that an atomic scale controlled deposition of a metallic Mg monolayer perfectly wets the silver substrate without any alloy formation at the interface at room temperature. A liquid-like behavior of the Mg species on the Ag substrate is highlighted as no dot formation is observed when coverage increases. Finally a layer-by-layer growth mode of Mg on Ag(111) can be predicted, thanks to density functional theory calculations as observed experimentally.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

A Scalable O(N) Algorithm for Large-Scale Parallel First-Principles Molecular Dynamics Simulations

Traditional algorithms for first-principles molecular dynamics (FPMD) simulations only gain a modest capability increase from current petascale computers, due to their O(N 3 ) complexity and their heavy use of global communications. To address this issue, we are developing a truly scalable O(N) complexity FPMD algorithm, based on density functional theory (DFT), which avoids global communications. The computational model uses a general nonorthogonal orbital formulation for the DFT energy functional, which requires knowledge of selected elements of the inverse of the associated overlap matrix. We present a scalable algorithm for approximately computing selected entries of the inverse of the overlap matrix, based on an approximate inverse technique, by inverting local blocks corresponding to principal submatrices of the global overlap matrix. The new FPMD algorithm exploits sparsity and uses nearest neighbor communication to provide a computational scheme capable of extreme scalability. Accuracy is controlled by the mesh spacing of the finite difference discretization, the size of the localization regions in which the electronic orbitals are confined, and a cutoff beyond which the entries of the overlap matrix can be omitted when computing selected entries of its inverse. We demonstrate the algorithm's excellent parallel scaling for up to O(100K) atoms on O(100K) processors, with a wall-clock time of O(1) minute per molecular dynamics time step.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Unveiling X-ray absorption signatures of boron nitride via first-principles simulation and machine learning

Boron nitride (BN) allotropes hold great promise in many advanced applications ranging from optical and photonic devices to energy storage and battery systems to tribological components. The diverse functionalities of this material stem from BN’s highly tunable structural and electronic properties, which are governed by the versatile boron–nitrogen bonding configurations. Exploring the structural landscape of BN can unveil novel structures possessing unique properties suited for specific applications, therefore accelerating the design of next-generation advanced functional materials. In this work, we leverage boron K-edge X-ray absorption spectroscopy (XAS) as an effective probe for local structural features and chemical environments. A total of 210 BN crystal structures are generated via analogies to the extensive array of carbon allotropes, and XAS is simulated for each unique local motif within the resulting collection of structures. A mapping between structural features and spectral signatures was established by synergizing first-principle simulations with data-driven based post-analysis approaches. Specifically, we developed a neural network model that can satisfactorily predict spectra line shapes from local structural descriptors. Toward automatic spectroscopic interpretation of any new BN structures, supervised machine learning models, trained on this structure–spectrum dataset, can accurately infer local coordination environments from simulated XAS, highlighting the strength of this unique approach of combining high-fidelity first-principles simulation and machine-learning to accelerate target design of novel BN materials via rational understanding of local structure-spectrum correlations.

36 MATERIALS SCIENCE↗
NASA NTRS1987

Diagnosis: Reasoning from first principles and experiential knowledge

Completeness, efficiency and autonomy are requirements for suture diagnostic reasoning systems. Methods for automating diagnostic reasoning systems include diagnosis from first principles (i.e., reasoning from a thorough description of structure and behavior) and diagnosis from experiential knowledge (i.e., reasoning from a set of examples obtained from experts). However, implementation of either as a single reasoning method fails to meet these requirements. The approach of combining reasoning from first principles and reasoning from experiential knowledge does address the requirements discussed above and can possibly ease some of the difficulties associated with knowledge acquisition by allowing developers to systematically enumerate a portion of the knowledge necessary to build the diagnosis program. The ability to enumerate knowledge systematically facilitates defining the program's scope, completeness, and competence and assists in bounding, controlling, and guiding the knowledge acquisition process.

Williams, Linda J. F.↗
DOE OSTI2025

First-principles effective Hamiltonian for finite-temperature modeling of nonperovskite ferroelectrics

First-principles-based effective Hamiltonian techniques have been widely employed for over three decades to investigate ferroelectricity and related phenomena in perovskite materials. These techniques offer high accuracy, transferability, compatibility with various finite-temperature algorithms, computational efficiency, and ease in incorporating interactions with external fields. They have been adapted to study diverse phenomena, ranging from topological dipole patterns in ferroelectric nanostructures to multicaloric effects. In this work, we develop an effective Hamiltonian for the nonperovskite ferroelectric HfO 2 (hafnia). Applying this methodology to explore the finite-temperature and finite-electric-field properties of ferroelectric hafnia revealed (1) exceptionally large intrinsic coercive fields, an order of magnitude higher than those observed in perovskite ferroelectrics; (2) their atomistic origin; and (3) the existence of a regime where the relationship between the coercive field and the energy barrier for polarization reversal is counterintuitive. Here, these developments could accelerate progress both in methodological advancements for simulating ferroics and in the atomistic understanding of a broad range of ferroelectrics.

Electric polarization↗
DOE OSTI2017

First-principles study lone-pair effects of Sb (III)-S chromophore influence on SHG response in quaternary potassium containing silver antimony sulfides

First-principles studies of the geometric structures, electronic structures and optical properties of non-centrosymmetrical (NCS) KAg{sub 2}SbS{sub 4}, K{sub 2}AgSbS{sub 4} and K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7} and centrosymmetrical (CS) KAg{sub 2}SbS{sub 3} and K{sub 3}Ag{sub 9}Sb{sub 4}S{sub 12} have been performed by means of density functional theory. It is indicated that although they have similarity in their anionic groups and, the electronic structures, prominent features of the optical anisotropy or second-order nonlinear optical (NLO) susceptibilities are apparently different. The calculated birefringences are 0.0537, 0.0343, 0.1324, 0.2217 and 0.0604 which are attributed to the different anionic groups [SbS{sub 3}] and triangles [AgS{sub 3}]. Also, the calculated NLO responses are about 0.5×, 1.0× and 2.0× times than that commercial AgGaS{sub 2} (AGS, d{sub 36}=11 pm/V) for K{sub 2}AgSbS{sub 4}, KAg{sub 2}SbS{sub 4} and K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7}, respectively. In comparison with the absolute magnitude of second harmonic generation (SHG) coefficients, the order K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7}>KAg{sub 2}SbS{sub 4}>K{sub 2}AgSbS{sub 4} is clearly established in the SHG response. Further analysis based on the real-space atom-cutting method reveals that the main sources of the SHG properties of these compounds are from the Sb-S group, especially K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7} is mainly attributed to the lone pair stereochemical activity of Sb (III)-S group. - Graphical abstract: Compounds K{sub 2}AgSbS{sub 4}, KAg{sub 2}SbS{sub 4} and K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7} exhibit second-order NLO response: 15×, 30× and 62× times than that KDP (KH{sub 2}PO{sub 4}, d{sub 36}=0.39 pm/V), respectively. First-principles methods reveal that the large SHG response of K{sub 2}Ag{sub 3}Sb{sub 3}S{sub 7} is dominated by the Sb-S chromophore with lone pairs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

First-principle-based full-dispersion Monte Carlo simulation of the anisotropic phonon transport in the wurtzite GaN thin film

In this study, we developed a first-principle-based full-dispersion Monte Carlo simulation method to study the anisotropic phonon transport in wurtzite GaN thin film. The input data of thermal properties in MC simulations were calculated based on the first-principle method. The anisotropy of thermal conductivity in bulk wurtzite GaN is found to be strengthened by isotopic scatterings and reduced temperature, and the anisotropy reaches 40.08% for natural bulk GaN at 100 K. With the GaN thin film thickness decreasing, the anisotropy of the out-of-plane thermal conductivity is heavily reduced due to both the ballistic transport and the less importance of the low-frequency phonons with anisotropic group velocities. On the contrary, it is observed that the in-plane thermal conductivity anisotropy of the GaN thin film is strengthened by reducing the film thickness. And the anisotropy reaches 35.63% when the natural GaN thin film thickness reduces to 50 nm at 300 K with the degree of specularity being zero. The anisotropy is also improved by increasing the surface roughness of the GaN thin film.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A comparative first-principles study of martensitic phase transformations in TiPd2 and TiPd

Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2 B19) and subsequent monoclinic transformations (B19 B19 ) as the system is cooled. To evaluate the transition temperature for TiPd2 we employ a theoretical approach based on a phenomenological Landau theory of the structural phase transition and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b oI6 transition temperature is in good agreement with reported experimental results. To investigate the TiPd B2 B19 transformation, we employ both the Cauchy-Born rule and a soft-mode- based approach, and elucidate on the importance of coupling of lattice distortion and atomic displacements (i.e., shuffling) in the formation of the final structure. The estimated B2 B19 transition temperature for TiPd system agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19 B19 structural phase transformation.

Krcmar, Maja↗
DOE OSTI2017

Phase Equilibria and Condensed Phase Properties of Fluorinated Alkanes via First Principles Simulations

Vapor liquid equilibria (VLE) of organic compounds is governed by weak non-covalent interactions. This presents a significant challenge in predicting VLE using a first principles approach where density functional theory is used to model system potential. However, a dispersion corrected approach has significantly improved the performance of traditional density functionals. Here, we predict the VLE for a nonpolar carbon tetrafluoride (R14) and polar 1,1,1,2-tetrafluoroethane (R134a)via first principles Gibbs ensemble Monte Carlo simulations using PBED3 density functional. We find that the saturated liquid density for R14 is slightly under predicted for subcritical temperatures, while for R134a it is under predicted at lower reduced temperatures and over predicted at higher reduced temperatures. In the liquid phase, angle-resolved pair correlation functions indicate slight preference for R143a to align in parallel and antiparallel orientation in the first solvation shell.The vibrational spectra reveal peak broadening in the condensed phase due to solvothermal effects. Altogether, we find thePBED3 functional performance is better for the nonpolar molecule as compared to the polar one.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

First-principles design of a half-filled flat band of the kagome lattice in two-dimensional metal-organic frameworks

Here, we design from first principles a type of two-dimensional metal-organic framework (MOF) using phenalenyl-based ligands to exhibit a half-filled flat band of the kagome lattice, which is one of a family of lattices that show Lieb-Mielke-Tasaki’s flat-band ferromagnetism. Among various MOFs, we find that trans-Au-THTAP (THTAP=trihydroxytriaminophenalenyl) has such an ideal band structure, where the Fermi energy is adjusted right at the flat band due to unpaired electrons of radical phenalenyl. The spin-orbit coupling opens a band gap giving a nonzero Chern number to the nearly flat band, as confirmed by the presence of the edge states in first-principles calculations and by fitting to the tight-binding model. This is a novel and realistic example of a system in which a nearly flat band is both ferromagnetic and topologically nontrivial.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗