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At least 1,711 records · Page 95
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DOE OSTI2014

Electrical, thermal, and species transport properties of liquid eutectic Ga-In and Ga-In-Sn from first principles

Using ab initio molecular dynamics, the atomic structure and transport properties of eutectic Ga-In and Ga-In-Sn are investigated. The Kubo-Greenwood (K-G) and the Ziman-Faber (Z-F) formulations and the Wiedemann-Franz (W-F) law are used for the electrical and electronic thermal conductivity. The species diffusivity and the viscosity are also predicted using the mean square displacement and the Stokes-Einstein (S-E) relation. Alloying Ga causes more disordered structure, i.e., broadening the atomic distance near the In and Sn atoms, which reduces the transport properties and the melting temperature. The K-G treatment shows excellent agreement with the experimental results while Z-F treatment formula slightly overestimates the electrical conductivity. The predicted thermal conductivity also shows good agreement with the experiments. The species diffusivity and the viscosity are slightly reduced by the alloying of Ga with In and Sn atoms. Good agreements are found with available experimental results and new predicted transport-property results are provided.

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

Structural, electronic, vibrational, and dielectric properties of LaBGeO{sub 5} from first principles

Structural, electronic, vibrational, and dielectric properties of LaBGeO{sub 5} with the stillwellite structure are determined based on ab initio density functional theory. The theoretically relaxed structure is found to agree well with the existing experimental data with a deviation of less than 0.2%. Both the density of states and the electronic band structure are calculated, showing five distinct groups of valence bands. Furthermore, the Born effective charge, the dielectric permittivity tensors, and the vibrational frequencies at the center of the Brillouin zone are all obtained. Compared to existing model calculations, the vibrational frequencies are found in much better agreement with the published experimental infrared and Raman data, with absolute and relative rms values of 6.04 cm{sup −1}, and 1.81%, respectively. Consequently, numerical values for both the parallel and perpendicular components of the permittivity tensor are established as 3.55 and 3.71 (10.34 and 12.28), respectively, for the high-(low-)frequency limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

First principle study of elastic and thermodynamic properties of ZrZn{sub 2} and HfZn{sub 2} under high pressure

A comprehensive investigation of the structural, elastic, and thermodynamic properties for Laves-phases ZrZn{sub 2} and HfZn{sub 2} are conducted using density functional total energy calculations combined with the quasi-harmonic Debye model. The optimized lattice parameters of ZrZn{sub 2} and HfZn{sub 2} compare well with available experimental values. We estimated the mechanical behaviors of both compounds under compression, including mechanical stability, Young's modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the thermodynamic properties as a function of pressure and temperature are analyzed and found to be in good agreement with the corresponding experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

First principles study of the magnetic properties of LaOMnAs

Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here, a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the ab plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Crystal structure and physical properties of Mo{sub 2}B: First-principle calculations

Several decades ago, Mo{sub 2}B was assumed to have an Al{sub 2}Cu-type structure with I4/mcm space group. Using ab initio phonon calculations, we identify the earlier proposed Al{sub 2}Cu-type structure is dynamically unstable at ambient pressure. An energetically more favorable phase with the tetragonal I4/m structure was then predicted by employing frozen-phonon technique. The currently predicted I4/m phase is mechanically and dynamically stable and energetically more favorable than that of the earlier proposed Al{sub 2}Cu-type structure. The electronic structures calculations indicate that Mo{sub 2}B is a metal with several bands crossing the Fermi level. Our analysis indicates that the three-dimensional network of the covalent Mo-B bond is responsible for the ultra-incompressible property of Mo{sub 2}B.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

First-principles predicted low-energy structures of NaSc(BH{sub 4}){sub 4}

According to previous interpretations of experimental data, sodium-scandium double-cation borohydride NaSc(BH{sub 4}){sub 4} crystallizes in the crystallographic space group Cmcm where each sodium (scandium) atom is surrounded by six scandium (sodium) atoms. A careful investigation of this phase based on ab initio calculations indicates that the structure is dynamically unstable and gives rise to an energetically and dynamically more favorable phase with C222{sub 1} symmetry and nearly identical x-ray diffraction pattern. By additionally performing extensive structural searches with the minima-hopping method we discover a class of new low-energy structures exhibiting a novel structural motif in which each sodium (scandium) atom is surrounded by four scandium (sodium) atoms arranged at the corners of either a rectangle with nearly equal sides or a tetrahedron. These new phases are all predicted to be insulators with band gaps of 7.9–8.2 eV. Finally, we estimate the influence of these structures on the hydrogen-storage performance of NaSc(BH{sub 4}){sub 4}.

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

First principles calculation of a large variation in dielectric tensor through the spin crossover in the CsFe[Cr(CN){sub 6}] Prussian blue analogue

The dielectric response of spin-crossover (SCO) materials is a key property facilitating their use in next-generation information processing technologies. Solid state hybrid density functional theory calculations show that the temperature-induced and strongly hysteretic SCO transition in the Cs{sup +}Fe{sup 2+}[Cr{sup 3+}(CN{sup −}){sub 6}] Prussian blue analogue (PBA) is associated with a large change (Δ) in both the static, Δε{sup 0}(HS − LS), and high frequency, Δε{sup ∞}(HS − LS) dielectric constants. The SCO-induced variation in CsFe[Cr(CN){sub 6}] is significantly greater than the experimental Δε values observed previously in other SCO materials. The phonon contribution, Δε{sup phon}(HS − LS), determined within a lattice dynamics approach, dominates over the clamped nuclei term, Δε{sup ∞}(HS − LS), and is in turn dominated by the low-frequency translational motions of Cs{sup +} cations within the cubic voids of the Fe[Cr(CN){sub 6}]{sup −} framework. The Cs{sup +} translational modes couple strongly to the large unit cell volume change occurring through the SCO transition. PBAs and associated metal-organic frameworks emerge as a potentially fruitful class of materials in which to search for SCO transitions associated with large changes in dielectric response and other macroscopic properties.

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

Initial and secondary oxidation products on the Si(111)-(7 × 7) surface identified by atomic force microscopy and first principles calculations

We investigate the initial and secondary oxidation products on the Si(111)-(7 × 7) surface at room-temperature using atomic force microscopy (AFM) and density functional theory calculations. At the initial oxidation stages, we find that there are two types of bright spots in AFM images. One of them is identified as a Si adatom with one O atom inserted into one of the backbonds, while the other is ascribed to a Si adatom with two inserted O atoms. We observe that the latter one turns into the secondary oxidation product by a further coming O{sub 2} molecule, which appears as a more protruded bright spot. The atomic configuration of this product is identified as Si adatom whose top and all three backbonds make bonds with O atoms. The appearances of initial and secondary oxidation products are imaged as bright and dark sites by scanning tunneling microscopy, respectively. It is revealed that AFM gives us the topographic information close to the real atomic corrugation of adsorbed structures on the semiconductor surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Trapping helium in Y{sub 2}Ti{sub 2}O{sub 7} compared to in matrix iron: A first principles study

Pyrochlore Y{sub 2}Ti{sub 2}O{sub 7} is a primary precipitate phase in nano-structured ferritic alloys (NFAs) for fission and fusion energy applications. We report a theoretical study for assessing the relative stability of trapping helium in Y{sub 2}Ti{sub 2}O{sub 7} versus in matrix iron. Various defect structures and the associated energies are examined and compared. Results reveal that helium can be deeply trapped in Y{sub 2}Ti{sub 2}O{sub 7} and that the corresponding self-interaction is essentially repulsive. Transmutant helium in NFAs prefers to occupy individual octa-interstitial sites in Y{sub 2}Ti{sub 2}O{sub 7}, before forming small clusters in Y{sub 2}Ti{sub 2}O{sub 7}. Helium partitioning in NFAs depends on the number and dispersion of Y{sub 2}Ti{sub 2}O{sub 7}; and thus initially, bubble formation and growth in iron matrix can be largely suppressed. Charge transfer occurs from helium to neighboring oxygen anions, but not to neighboring metal cations, suggesting a general effectiveness of trapping helium in oxides. Reasons for the ultimate fate of helium to form small nm-scale interface bubbles are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Do Ag{sub n} (up to n = 8) clusters retain their identity on graphite? Insights from first-principles calculations including dispersion interactions

Adsorption of pre-formed Ag{sub n} clusters for n = 1 − 8 on a graphite substrate is studied within the density functional theory employing the vdW-DF2 functional to treat dispersion interactions. Top sites above surface layer carbon atoms turn out to be most favorable for a Ag adatom, in agreement with experimental observations. The same feature is observed for clusters of almost all sizes which have the lowest energies when the Ag atoms are positioned over top sites. Most gas phase isomers retain their structures over the substrate, though a couple of them undergo significant distortions. Energetics of the adsorption can be understood in terms of a competition between energy cost of disturbing Ag–Ag bonds in the cluster and energy gain from Ag–C interactions at the surface. Ag{sub 3} turns out to be an exceptional candidate in this regard that undergoes significant structural distortion and has only two of the Ag atoms close to surface C atoms in its lowest energy structure.

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

Pressure induced tetragonal to monoclinic transition in RbN{sub 3} studied from first principles theory

Alkali metal azides are well known for their application as explosives and gas generators. They are used as precursors in synthesis of polymeric nitrogen, an ultimate green high energy density material. Among the alkali metal azides, rubidium azide RbN{sub 3} crystallizes in tetragonal structure with linear azide ions arranged in layers and binds through weak dispersive interactions. In this present work, we have studied the structural stability, electronic structure and optical properties of solid RbN{sub 3} by using van der Waals corrected density functional theory. We find that the ambient tetragonal structure undergoes a structural transition to monoclinic structure at 0.72 GPa, which is in good agreement with the experimental transition pressure of less than 1 GPa. The phonon frequencies at the gamma point are calculated and found that the lattice mode Eg softens under pressure which may supports the structural phase transition. The electronic band structure and optical properties are calculated by using Tran Blaha-modified Becke Johnson (TB-mBJ) functional and found that solid RbN{sub 3} is an insulator with a gap of 5.976 eV and the optical absorption starts with the UV light of wave length 207.5 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Platinum atomic wire encapsulated in gold nanotubes: A first principle study

The nanotubes of gold incorporated with platinum atomic wire have been investigated by means of firstprinciples density functional theory with plane wave pseudopotential approximation. The structure with zig-zag chain of Pt atoms in side gold is found to be 0.73 eV lower in energy in comparison to straight chain of platinum atoms. The Fermi level of the composite tube was consisting of d-orbitals of Pt atoms. Further interaction of oxygen with these tubes reveals that while tube with zig-zag Pt prefers dissociative adsorption of oxygen molecule, the gold tube with linear Pt wire favors molecular adsorption.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Study of dynamic properties for NaK binary liquid alloy using first principle and theoretical predictions of isothermal bulk modulus using elastic constants

Study of atomic motions in the binary liquid alloys have been studied in terms of dynamical variables like velocity auto correlation, power spectrum and mean square displacement. Elastic constants and isothermal bulk modulus have been calculated to see the effeectiveness of ab-initio pseudopotentials which has been used in this paper. This appraoch is free from the fitting parameters and results obtained using this appraoch have been found very close to the average values.

36 MATERIALS SCIENCE↗
DOE OSTI2014

First principles electronic band structure and phonon dispersion curves for zinc blend beryllium chalcogenide

A detailed theoretical study of structural, electronic and Vibrational properties of BeX compound is presented by performing ab-initio calculations based on density-functional theory using the Espresso package. The calculated value of lattice constant and bulk modulus are compared with the available experimental and other theoretical data and agree reasonably well. BeX (X = S,Se,Te) compounds in the ZB phase are indirect wide band gap semiconductors with an ionic contribution. The phonon dispersion curves are represented which shows that these compounds are dynamically stable in ZB phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Comment on “Rethinking first-principles electron transport theories with projection operators: The problems caused by partitioning the basis set” [J. Chem. Phys. 139, 114104 (2013)]

In a recent paper Reuter and Harrison [J. Chem. Phys. 139, 114104 (2013)] question the widely used mean-field electron transport theories, which employ nonorthogonal localized basis sets. They claim these can violate an “implicit decoupling assumption,” leading to wrong results for the current, different from what would be obtained by using an orthogonal basis, and dividing surfaces defined in real-space. We argue that this assumption is not required to be fulfilled to get exact results. We show how the current/transmission calculated by the standard Greens function method is independent of whether or not the chosen basis set is nonorthogonal, and that the current for a given basis set is consistent with divisions in real space. The ambiguity known from charge population analysis for nonorthogonal bases does not carry over to calculations of charge flux.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Response to “Comment on ‘Rethinking first-principles electron transport theories with projection operators: The problems caused by partitioning the basis set’” [J. Chem. Phys. 140, 177103 (2014)]

The thesis of Brandbyge's comment [J. Chem. Phys. 140, 177103 (2014)] is that our operator decoupling condition is immaterial to transport theories, and it appeals to discussions of nonorthogonal basis sets in transport calculations in its arguments. We maintain that the operator condition is to be preferred over the usual matrix conditions and subsequently detail problems in the existing approaches. From this operator perspective, we conclude that nonorthogonal projectors cannot be used and that the projectors must be selected to satisfy the operator decoupling condition. Because these conclusions pertain to operators, the choice of basis set is not germane.

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

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

We report on density functional theory investigations of the electronic properties of monofunctional ligands adsorbed on ZnO-(1010) surfaces and ZnO nanowires using semi-local and hybrid exchange-correlation functionals. We consider three anchor groups, namely thiol, amino, and carboxyl groups. Our results indicate that neither the carboxyl nor the amino group modify the transport and conductivity properties of ZnO. In contrast, the modification of the ZnO surface and nanostructure with thiol leads to insertion of molecular states in the band gap, thus suggesting that functionalization with this moiety may customize the optical properties of ZnO nanomaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗