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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Oxygen trapped by rare earth tetrahedral clusters in Nd{sub 4}FeOS{sub 6}: Crystal structure, electronic structure, and magnetic properties

Single crystals of Nd{sub 4}FeOS{sub 6} were grown from an Fe–S eutectic solution. Single crystal X-ray diffraction analysis revealed a Nd{sub 4}MnOSe{sub 6}-type structure (P6{sub 3}mc, a=9.2693(1) Å, c=6.6650(1)Å, V=495.94(1) Å{sup 3}, Z=2), featuring parallel chains of face-sharing [FeS{sub 6×1/2}]{sup 4−} trigonal antiprisms and interlinked [Nd{sub 4}OS{sub 3}]{sup 4+} cubane-like clusters. Oxygen atoms were found to be trapped by Nd{sub 4} clusters in the [Nd{sub 4}OS{sub 3}]{sup 4{sub +}} chains. Structural differences among Nd{sub 4}MnOSe{sub 6}-type Nd{sub 4}FeOS{sub 6} and the related La{sub 3}CuSiS{sub 7}− and Pr{sub 8}CoGa{sub 3}-type structures have been described. Magnetic susceptibility measurements on Nd{sub 4}FeOS{sub 6} suggested the dominance of antiferromagnetic interactions at low temperature, but no magnetic ordering down to 2 K was observed. Spin-polarized electronic structure calculations revealed magnetic frustration with dominant antiferromagnetic interactions. - Graphical abstract: Trapping of oxygen in Nd{sub 4} tetrahedral clusters results in the formation of the Nd{sub 4}MnOSe{sub 6}-type Nd{sub 4}FeOS{sub 6}, in contrast to the La{sub 3}CuSiS{sub 7}-type oxygen-free Nd{sub 4}FeS{sub 7} and related Pr{sub 8}CoGa{sub 3}-type structures. Complex magnetic frustration inhibits magnetic ordering at low temperature. - Highlights: • Single crystals of Nd{sub 4}FeOS{sub 6} were grown using self-flux method. • Oxygen was found trapped by Nd{sub 4} tetrahedral clusters. • Comparison with two closely related structural types were discussed. • Magnetic measurements revealed antiferromagnetic (AFM) interaction. • VASP calculations confirmed strong magnetic frustration in AFM model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Geometric structure, electronic structure, and spin transition of several Fe{sup II} spin-crossover molecules

We present a density functional study on the geometric structure, electronic structure, and spin transition of a series of Fe{sup II} spin-crossover (SCO) molecules, i.e., [Fe(abpt){sub 2}(NCS){sub 2}] (1), [Fe(abpt){sub 2}(NCSe){sub 2}] (2), and [Fe(dpbo)(HIm){sub 2}] (3) with dpbo diethyl(E,E)-2,2'-[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate](2-), N',O{sup 3},O{sup 3}', and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole in order to explore more about the way to control SCO behavior of transition metal complexes. Our calculated results show that the spin transition of these Fe{sup II} molecules is accompanied with charge transfer between the Fe atom and ligands. This causes change in the electrostatic energy ({Delta}U) as well as the total electronic energy of SCO molecules. Moreover, our calculated results demonstrate an important contribution of the interionic interactions to {Delta}U, and there is the relation between {Delta}U and the thermal hysteresis behavior of SCO molecules. These results should be helpful for developing new SCO molecules.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

A novel Bi-based phosphomolybdate photocatalyst K{sub 2}Bi(PO{sub 4})(MoO{sub 4}): Crystal structure, electronic structure and photocatalytic activity

Graphical abstract: - Highlights: • A new type of phosphomolybdate K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) photocatalyst was successfully synthesized. • The products synthesized at 600 °C were mainly composed of nano-cubes. • The indirect band gap of K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) has been determined to be 2.93 eV. • K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) synthesized at 600 °C exhibits the highest photocatalytic activity. • The electronic structure was calculated by density functional calculations. - Abstract: A novel phosphomolybdate photocatalyst K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) has been successfully developed via a solid-state reaction. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), diffuse reflectance spectrum (DRS) and photoluminescence (PL) spectra. The photocatalytic activities of the samples prepared at different temperature were determined by the photooxidative decomposition of methylene blue (MB) in aqueous solution. The results revealed that K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) can be used as an effective photocatalyst under UV–vis irradiation and the nanocubes obtained at 600 °C exhibits the highest photocatalytic activity. The photodegradation of MB by K{sub 2}Bi(PO{sub 4})(MoO{sub 4}) nanocrystals followed the first-order kinetics. Theoretical calculations on electronic structure confirmed the indirect optical transitions property in the absorption edge region of K{sub 2}Bi(PO{sub 4})(MoO{sub 4}), and the orbital constitutions of CB and VB were also analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structure, electronic and magnetic characterization, and calculated electronic structures of two oxyhalide hexagonal perovskites

Herein we report the crystal structures, initial magnetic and charge transport characterization, and calculated electronic structures of the hexagonal oxyhalide perovskites, Ba 7 Ru 4 O 15 Cl 2 and Ba 7 Ru 4 O 15 Br 2 . The experimental information is obtained through the study of single crystals. Face-sharing Ru O 6 octahedra form Ru 2 O 9 dimers in a layered triangular geometry in these materials; minor amounts of off-magnetic-site structural disorder are present in our crystals. Both are magnetically isotropic, with 2.5 μB/mol-Ru, Curie-Weiss theta –185 K, and 2.9 μB/mol-Ru, Curie-Weiss theta –168 K, respectively. Broad features in the magnetic susceptibility and heat capacity associated with magnetic ordering (at 35 and 37 K, respectively) are observed. The charge transport band gaps are 0.03 eV for the oxychloride and 0.006 eV for the oxybromide. There is no gap at the Fermi level for either of these compounds in density-functional theory electronic structure calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Size effects on the structural, electronic, and optical properties of (5,0) finite-length carbon nanotube: An ab-initio electronic structure study

We use density functional computations to study the zero temperature structural, electronic, magnetic, and optical properties of (5,0) finite carbon nanotubes (FCNT), with length in the range of 4–44 Å. It is found that the structural and electronic properties of (5,0) FCNTs, in the ground state, converge at a length of about 30 Å, while the excited state properties exhibit long-range edge effects. We discuss that curvature effects enhance energy gap of FCNTs, in contrast to the known trend in the periodic limit. It is seen that compensation of curvature effects in two special small sizes may give rise to spontaneous magnetization. The obtained cohesive energies provide some insights into the effects of environment on the growth of FCNTs. The second-order difference of the total energies reveals an important magic size of about 15 Å. The optical and dynamical magnetic responses of the FCNTs to polarized electromagnetic pulses are studied by time dependent density functional theory. The results show that the static and dynamic magnetic properties mainly come from the edge carbon atoms. The optical absorption properties are described in terms of local field effects and characterized by Casida linear response method.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Crystal structure, electronic structure, and photoluminescent properties of SrMoO{sub 4}:Tb{sup 3+} phosphors

Highlights: • SrMoO{sub 4}:Tb{sup 3+} phosphor samples were synthesized at different temperatures. • The crystal and electronic structures, and luminescence properties were studied. • The excitation peak shifts to red with increasing the sintering temperature. • The luminescence mechanism of SrMoO{sub 4}:Tb{sup 3+} was suggested. - Abstract: The experimental and theoretical studies of the optical properties of SrMoO{sub 4}:Tb{sup 3+} phosphors were carried out. The structural, optical, and electronical properties of the phosphors were systematically studied. The phosphor samples were crystallized at different temperatures via a sol–gel method. Excitation spectra of SrMoO{sub 4}:Tb{sup 3+} powder samples exhibited gradual red shift and luminescent intensity changed with increasing the sintering temperature. Such spectral changes depend strongly on the crystallographic properties such as lattice parameters and crystallinity. The shift of the excitation spectra is mainly ascribed to the covalent bond interaction between Mo−O bonds. An energy band model was demonstrated to describe the luminescence mechanism in the material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Crystal structure, electronic structure, temperature-dependent optical and scintillation properties of CsCe 2 Br 7

CsCe 2 Br 7 is a self-activated inorganic scintillator that shows promising performance, but the understanding of the important structure-property relationships is lacking. In this work, we conduct a comprehensive study on CCsCe 2 Br 7 . The crystal structure of CsCe 2 Br 7 is refined using single crystal X-ray study for the first time. It crystallizes into the orthorhombic crystal system with Pmnb space group. Its electronic structure is revealed by Density Functional Theory (DFT) calculations. Two cerium emission centers are identified and the energy barriers related to the thermal quenching to 4f ground states of Ce 3+ for these two Ce centers are evaluated. CsCe 2 Br 7 single crystal has better light yield and energy resolution than CsCe 2 Cl 7 , but with an additional slow decay component of 1.7 s. The existence of a deep trap with a depth of 0.9 eV in CsCe 2 Cl 7 contributes to its higher afterglow level in comparison to that of CsCe 2 Br 7 . The most possible point defects in CsCe 2 Cl 7 and CsCe 2 Br 7 are proposed by considering the vapour pressure in the growth atmosphere upon melting point.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural, electronic structure, and band alignment properties at epitaxial NiO/Al{sub 2}O{sub 3} heterojunction evaluated from synchrotron based X-ray techniques

The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al{sub 2}O{sub 3} heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al{sub 2}O{sub 3} substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al{sub 2}O{sub 3} substrate. Surface related feature in Ni 2p{sub 3/2} core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al{sub 2}O{sub 3} substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al{sub 2}O{sub 3} layers. A type-I band alignment at NiO and Al{sub 2}O{sub 3} heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based light emitting devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Crystal structure, electronic structure, optical and scintillation properties of self-activated Cs 4 YbI 6 [Crystal structure, optical and scintillation properties of self-activated Cs 4 YbI 6 ]

A self-activated Cs 4 YbI 6 single crystal was grown by the vertical Bridgman method. Crystal structure refinements verified the phase purity and the trigonal crystal system with a space group of $\bar{R}$ 3 c . By using differential scanning calorimetry, the melting and crystallization points were determined to be 550 and 510 °C, respectively. Luminescence and scintillation properties were systematically studied. Upon ultraviolet light (360 nm) excitation, the Cs 4 YbI 6 crystal exhibits bluish-green emission centered at 450 and 480 nm due to spin-allowed and spin-forbidden transitions of Yb 2+ activators. The lifetimes of the corresponding emission bands at room temperature are tens and hundreds of nanoseconds, respectively. X-ray excited radioluminescence spectrum is dominated by the spin-forbidden transition of Yb 2+ at 480 nm. The absolute light yield is 2700 ± 200 photons/MeV with a principal scintillation decay time of 33 ns. In conclusion, the physical explanation for the low light yield observed is proposed from experimental and theoretical insights.

36 MATERIALS SCIENCE↗
DOE OSTI2013

β-Technetium Dichloride: Solid-State Modulated Structure, Electronic Structure, and Physical Properties

A second polymorph of technetium dichloride, β-TcCl 2 , has been synthesized from the reaction of Tc metal and chlorine in a sealed tube at 450 °C. The crystallographic structure and physical properties of β-TcCl 2 have been investigated. The structure of β-TcCl 2 consists of infinite chains of face sharing [Tc 2 Cl 8 ] units; within a chain, the T c ≡T c vectors of two adjacent [T c 2 Cl 8 ] units are ordered in the long-range where perpendicular and/or parallel arrangement of Tc≡Tc vectors yields a modulated structure. Resistivity and Seebeck measurements performed on a β-T c Cl 2 single crystal indicate the compound to be a p-type semiconductor while a magnetic susceptibility measurement shows technetium dichloride to be diamagnetic. A band gap of 0.12(2) eV was determined by reflectance spectroscopy measurements. Theoretical calculations at the density functional level were utilized for the investigation of other possible stable forms of T c Cl 2 .

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

Structural stability, electronic structure and mechanical properties of actinide carbides AnC (An = U, Np)

Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of actinide carbides AnC (An=U, Np) for three different crystal structures, namely NaCl, CsCl and ZnS. Among the considered structures, NaCl structure is found to be the most stable structure for these carbides at normal pressure. A pressure induced structural phase transition from NaCl to ZnS is observed. The electronic structure reveals that these carbides are metals. The calculated elastic constants indicate that these carbides are mechanically stable at normal pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural stability, electronic structure and mechanical properties of alkali gallium hydrides AGaH{sub 4} (A = Li, Na)

Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of alkali gallium hydrides AGaH{sub 4} (A = Li, Na) for three different crystal structures, namely tetragonal (P42{sub 1}c), tetragonal (P4{sub 2}/nmc) and monoclinic (P2{sub 1}/c). Among the considered structures, tetragonal (P42{sub 1}c) phase is found to be the most stable phase for these hydrides at normal pressure. A pressure induced structural phase transition from tetragonal (P42{sub 1}c) to tetragonal (P4{sub 2}/nmc) is observed. The electronic structure reveals that these hydrides are insulators. The calculated elastic constants indicate that these ternary imides are mechanically stable at normal pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A first principles study of structural stability, electronic structure and mechanical properties of beryllium alanate BeAlH{sub 5}

Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of BeAlH{sub 5} for monoclinic crystal structures with two different types of space group namely P2{sub 1} and C{sub 2}/c. Among the considered structures monoclinic (P2{sub 1}) phase is found to be the most stable at ambient condition. The structural phase transition from monoclinic (P2{sub 1}) to monoclinic (C{sub 2}/c) phase is observed in BeAlH{sub 5}. The electronic structure reveals that this compound is insulator. The calculated elastic constants indicate that this material is mechanically stable at ambient condition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A first principles study of structural stability, electronic structure and mechanical properties of ABeH{sub 3} (A = Li, Na)

Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of ABeH{sub 3} (A = Li, Na) for three different crystal structures, namely orthorhombic (Pnma), monoclinic (P2{sub 1}/c) and triclinic (P-1) phase. Among the considered structures monoclinic (P2{sub 1}/c) phase is found to be the most stable one for all the three hydrides at ambient condition. The electronic structure reveals that these materials are wide band gap semiconductors. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗