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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Electronic, Magnetic, and Theoretical Characterization of (NH 4 ) 4 UF 8 , a Simple Molecular Uranium(IV) Fluoride

The simple system of tetraammonium octafluorouranate is employed to derive a fundamental understanding of the uranium–fluorine interaction. The structure is composed of isolated molecules, enabling a detailed examination of the U 4+ (ƒ 2 ) ion. Characterization of single-crystals by X-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is combined with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method (IQA) is used to examine the nature of the U–F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) in [UF8] 4- . For the structural and theoretical reasons discussed herein, it is proposed that the structure of (NH 4 ) 4 UF 8 may be appropriately employed as a benchmark compound for future theoretical characterization of U(IV).

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2025

EuAuSb: An odd-parity helical variation of altermagnetism

EuAuSb is a triangular-lattice Dirac semimetal in which a topological Hall effect has been observed to develop in association with a magnetically ordered phase. Our single-crystal neutron diffraction measurements have identified an incommensurate helical order in which individual ferromagnetic Eu 2+ layers rotate in-plane by ∼ 120° from one layer to the next. An in-plane magnetic field distorts the incommensurate order, eventually leading to a first order transition to a state that is approximately commensurate and that is continuously polarized as the bulk magnetization approaches saturation. From an analysis of the magnetic diffraction intensities versus field, we find evidence for a dip in the ordered in-plane moment at the same field where the topological Hall effect is a maximum, and we propose that this is due to field-induced quantum spin fluctuations. Our electronic structure calculations yield exchange constants compatible with the helical order and show that the bands near the Fermi level lose their spin degeneracy via a mechanism similar to that in the collinear altermagnets. In conclusion, we find that, unlike the even symmetry seen in the altermagnets, the spin splitting in EuAuSb has odd-wave symmetry similar to that recently found in a number of coplanar magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic Pr 2 Ti 2 O 7

Little is so far known about the magnetism of the A 2 B 2 O 7 monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for r A /r B > 1.78. We show that high quality monoclinic Pr 2 Ti 2 O 7 single crystals with a three-dimensional network of non-Kramers Pr 3+ ions that interact through edge-sharing superexchange interactions, form a singlet ground-state quantum paramagnet that does not undergo any magnetic phase transitions down to, at least, 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite Pr 2 Ti 2 O 7 were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of Pr 2 Ti 2+x O 7 (–0.16 ≤ x ≤ 0.16 ) showed that deviations from the Pr 2 Ti 2 O 7 stoichiometry lead to secondary phases of related structures including the perovskite phase Pr 2/3 TiO 3 and the orthorhombic phases Pr 4 Ti 9 O 24 and Pr 2 TiO 5 . No indications of site disordering (stuffing and antistuffing) or vacancy defects were observed in the Pr 2 Ti 2 O 7 majority phase. A procedure for growth of high-structural-quality stoichiometric single crystals of Pr 2 Ti 2 O 7 by the traveling solvent floating zone method is reported. Thermomagnetic measurements of single-crystalline Pr 2 Ti 2 O 7 reveal an isolated singlet ground state that we associate with the low-symmetry crystal electric-field environments that split the (2J + 1 = 9)-fold degenerate spin-orbital multiplets of the four differently coordinated Pr 3+ ions into 36 isolated singlets resulting in an anisotropic temperature-independent van Vleck susceptibility at low T. Here, a small isotropic Curie term is associated with 0.96(2)% noninteracting Pr 4+ impurities.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Coherent control of strongly interacting spins in the solid-state

The major goal of this project is to develop coherent interacting spin systems in the solid state, based on rare earth ions. By leveraging the ability to controllably incorporate rare earth ions such as Er into a solid-state host, and maintain individual-ion addressability below the diffraction limit based on inhomogeneous broadening, we aim to create strongly interacting electron spin clusters with individual-site control. Performing spectroscopy of these strongly interacting systems will shed light on their fundamental properties, as well as enable applications to quantum information science, in particular quantum networks that are also enabled by a telecom-band spin-photon interface in Er.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Theory of x-ray scattering from laser-driven electronic systems

We describe, within the framework of quantum electrodynamics, an interaction between a nonresonant hard x-ray pulse and an electronic system in the presence of a temporally periodic laser field driving electron dynamics in this system. Here we apply Floquet theory to describe the laser-driven electronic system, and then obtain the scattering probability of an arbitrary nonresonant x-ray pulse from such a system employing the density-matrix formalism. We show that the scattering probability can be connected to the time-dependent electron density of the driven electronic system only under certain conditions, in particular, if the bandwidth of the probe x-ray pulse is sufficiently narrow to spectroscopically resolve transitions to different final states. A special focus is laid on application of the theory to laser-driven crystals in a strongly nonperturbative regime. We show how the time-dependent electron density of a crystal can be reconstructed from energy-resolved scattering patterns. This is illustrated by a calculation of a diffraction signal from a driven MgO crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Molecular self-assembled designing and characterization of TiO2 NPs-CdS QDs-dye composite for photoanode materials

Highlights: • Initially TiO{sub 2} NPs were synthesized by sol-gel method and CdS QDs by one pot synthesis method. • The connectivity between TiO{sub 2} NPs and CdS QDs was well revealed by zeta potential and FTIR techniques. • This binary system was sensitized with N719 dye molecule for better photoanode material. • Ternary TiO{sub 2} NPs-CdS QDs-N719 dye photoanode showed higher conversion efficiency than its binary photoanode devices. - Abstract: Ternary based photoanodes have been prepared by using titanium dioxide (TiO{sub 2}) nanoparticles (NPs), cadmium sulfide (CdS) quantum dots (QDs) and N719 dye. Initially, CdS QDs and TiO{sub 2} NPs were synthesized by one pot synthesis and sol-gel method, respectively. CdS QDs were connected electrostatically to the surfaces of TiO{sub 2} NPs, and after that, this binary hybrid further self-assembled with N719 dye to form the composites. The NPs, QDs and the resultant composites are characterized by using different techniques. The connectivity of these anatase TiO{sub 2} NPs with cubic CdS QDs are confirmed by FTIR; while the broad coverage of these materials is studied through UV–Visible diffuse reflectance spectroscopy (DRS). The surface topology and the cross-sectional morphology of thin films are studied by high resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy (FE-SEM) techniques; while the structural properties of NPs and QDs are studied by X-ray diffractometer (XRD). The synergetic co-sensitization of CdS and N719 dye significantly enhancing the overall power conversion efficiency (η) of TiO{sub 2} up to 2.37% (J{sub sc} = 10.31 mA/cm{sup 2} and V{sub oc} of 550 mV) (100 mW/cm{sup 2}). The J{sub sc}, V{sub oc}, and η values along with Incident photon to current conversion efficiency (IPCE) value of ternary photoelectrode-based solar devices, is almost double to that of either binary TiO{sub 2} NPs-CdS QDs or binary TiO{sub 2} NPs-N719 binary systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Neutron scattering study of the quasi-one-dimensional antiferromagnet Ba 2 CoSi 2 O 7

Magnetization and neutron scattering measurements have been carried out on an antiferromagnet Ba 2 CoSi 2 O 7 . The observed magnetic excitation is almost dispersionless, and the neutron intensity is only modulated along the [101] direction. The dispersionless magnetic excitation suggest that the Ba 2 CoSi 2 O 7 system is a quasi-one-dimensional antiferromagnet. Classical spin-wave theory for a one-dimensional antiferromagnet can explain the dispersionless spin excitation. Additionally, the magnetic structure determined by the measurement of the neutron powder diffraction is consistent with no observation of the multiferroic property in this system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Defects in C d 3 A s 2 epilayers via molecular beam epitaxy and strategies for reducing them

Molecular beam epitaxy offers an exciting avenue for investigating the behavior of the topological semimetal Cd 3 As 2 , by providing routes for doping, alloying, strain engineering, and heterostructure formation. To date, however, minimal exploration has been devoted to the impact of defects that are incorporated into epilayers due to constraints imposed by the substrate and narrow growth window. Here, we use a combination of lattice-matched Zn x Cd 1-x Te buffer layers, miscut substrates, and broadband illumination to study how dislocations, twins, and point defects influence the electron mobility of Cd 3 As 2 . In conclusion, a combination of defect suppression approaches produces Cd 3 As 2 epilayers with electron mobilities upwards of 15000 cm 2 /Vs at room temperature.

14 SOLAR ENERGY↗
DOE OSTI2022

Superconducting phase of Ti x O y thin films grown by molecular beam epitaxy

Here we investigate the complex relationship between the growth conditions and the structural and transport properties of Ti x O y thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure P Ox for films grown on (0001)-Al 2 O 3 substrates at 850° C. A cubic c-TiO 1±δ buffer layer is formed for low O/Ti ratios, while a corundum cr-Ti 2 O 3 layer is formed under higher-oxidizing conditions. Metallicity is observed for c-TiO 1-δ buffer layers. The superconducting γ -Ti 3 O 5 Magnéli phase is found to nucleate on a c-TiO 1-δ buffer for intermediate POx conditions, and an insulator-superconducting transition is observed at 4.5 K (T$^{onset}_{C}$ = 6K) for 85 nm thick films. Strain relaxation of γ -Ti 3 O 5 occurs with increasing film thickness and correlates with a thickness-dependent increase in T C observed for Ti x O y thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Structure and Photophysics of Indigoids for Singlet Fission: Cibalackrot

Here, we report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. Here, in order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model.

14 SOLAR ENERGY↗
DOE OSTI2022

Photoinduced evolution of lattice orthorhombicity and conceivably enhanced ferromagnetism in LaMnO 3 membranes

Ultrashort laser pulses have been utilized to dynamically drive phase transitions in correlated quantum materials. Of particular interest is whether phases not achievable in thermal equilibrium can be induced in complex oxides with intricately coupled lattice, electron and spin degrees of freedom. Here, we tracked atomic motions in LaMnO 3 following photoexcitation with MeV ultrafast electron diffraction (MeV-UED) technique. We found that the light excited state exhibits numerous signatures different from thermal equilibrium ones, including nearly conserved Bragg intensities, strongly suppressed La cation and oxygen anion displacements, and the long-range lattice orthorhombicity evolution. Furthermore, using first-principles calculations, we predict that the ferromagnetic ordering and conductivity are both enhanced upon laser excitation due to the reduction of the lattice orthorhombicity. This work benefits from recent advance in fabrication of membrane films with high epitaxial quality and in MeV-UED with large momentum space access and high temporal resolution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tri-molybdenum phosphide (Mo 3 P) and multi-walled carbon nanotube junctions for volatile organic compounds (VOCs) detection

Detection and analysis of volatile organic compounds' (VOCs) biomarkers lead to improvement in healthcare diagnosis and other applications such as chemical threat detection and food quality control. Here, we report on tri-molybdenum phosphide (Mo3P) and multiwalled carbon nanotube (MWCNT) junction-based vapor quantum resistive sensors (vQRSs), which exhibit more than one order of magnitude higher sensitivity and superior selectivity for biomarkers in comparison to pristine MWCNT junctions based vQRSs. Transmission electron microscope/scanning tunneling electron microscope with energy dispersive x-ray spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy studies reveal the crystallinity and the presence of Mo and P elements in the network. The presence of Mo3P clearly enhanced the performance of vQRS as evidenced in sensitivity and selectivity studies. The vQRSs are stable over extended periods of time and are reproducible, making them a potential candidate for sensing related applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Real Space Visualization of Competing Phases in La 0.6 Sr 2.4 Mn 2 O 7 Single Crystals

We report that correlated quantum materials are expected to provide the foundation for the next generation of information or energy technologies. A key feature of these materials is the proximity of multiple ground states close in energy, which results in the ability to tune properties with small changes in an external parameter such as magnetic field, composition, or temperature. For example, the colossal magnetoresistance exhibited by manganites is related to charge and orbital ordering and results from a metallic ferromagnetic phase in proximity to a paramagnetic insulating phase. The presence of competing ground states, at the heart of the physics and functionality of these materials, often results in nanoscale phase separation. Probing nanoscale phase separation with conventional diffraction techniques alone is not adequate, particularly when the domains are small or nanosized. In the present work we use a scanning transmission electron microscopy image-based technique of picometer precision strain maps (PPSM) to directly visualize the competing nanoscale phases with charge and orbital ordering in a double-layer manganite. Finally, this work underscores the role of subtle structural distortions in determining the electron physics in correlated quantum materials and provides insights into designing new functionalities via spatially tuning multiple competing ground states.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis, Electronic Properties and Reactivity of [B 12 X 11 (NO 2 )] 2– (X=F–I) Dianions

Nitro-functionalized undecahalogenated closo-dodecaborates [B 12 X 11 (NO 2 )] 2– were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X-diffraction, mass spectrometry, and gas-phase ion vibrational spectroscopy. The NO 2 substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B 12 X 12 ] 2– (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum-chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO . ). The homolytic NO . elimination from the dianion under very soft collisional excitation in gas-phase ion experiments results in the formation of the radical [B 12 X 11 O] 2–. . Theoretical investigations suggest that the loss of NO . proceeds via the rearrangement product [B 12 X 11 (ONO)] 2– . The O-bonded nitrosooxy structure is thermodynamically more stable than the N-bonded nitro structure and its formation by radical recombination of [B 12 X 11 O] 2–. and NO . is demonstrated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Plutonium Hybrid Materials: A Platform to Explore Assembly and Metal–Ligand Bonding

In this work, we report the synthesis of five new hybrid materials containing the [PuCl 6 ] 2- anion and charge balancing, non-covalent interaction donating 4-X-pyridinium (X = H, Cl, Br, I) cations. Single crystals of the title compounds were grown and harvested from acidic, chloride-rich, aqueous media and their structures were determined via X-ray diffraction. Compounds 1-4, (4XPyH) 2 [PuCl 6 ] and 5, (4IPyH) 4 [PuCl 6 ] · 2Cl, exhibit two distinct sheet-like structure types. Structurally relevant non-covalent interactions were tabulated from crystallographic data and verified computationally using electrostatic surface potential maps and the quantum theory of atoms in molecules (QTAIM) approach. The strength of the hydrogen and halogen bonds was quantified using Kohn-Sham density functional theory and a hierarchy of acceptor-donor pairings established. In turn, the PuIV-Cl bonds were studied using the QTAIM and natural localized molecular orbital (NLMO) approaches to delineate the underlying bond mechanism and hybrid atomic orbital contributions therein. Energy decomposition (ED) and natural ED analyses were also explored to probe the bond mechanism and, more broadly, explore the efficacy of these techniques in studying these anionic systems. The results of the PuIV-Cl bond analyses were compared across composition via analogous treatments of previously reported [PuO 2 Cl 4 ] 2- and [PuCl 3 (H 2 O) 5 ] molecular units. In summary, our study indicates that the Pu-Cl bonds are predominately ionic, yet exhibit small varying degrees of covalent character that increase from [PuCl 3 (H 2 O) 5 ], [PuO 2 Cl 4 ] 2- , to [PuCl 6 ] 2- , while the participation of the Pu based s/d and f orbitals concurrently decrease and increase, respectively.

transuranic↗
DOE OSTI2018

High Pressure Induced in Situ Solid-State Phase Transformation of Nonepitaxial Grown Metal@Semiconductor Nanocrystals

Considering the large lattice mismatch induced interface strain between nonepitaxial grown monocrystalline semiconductor shell and metal core, we studied the solid-state phase transformation of such nonepitaxial grown Au@CdS core/shell NCs under high pressure in this paper. Consistent with HRTEM characterizations, the high resolution Raman spectra and synchrotron angle-dispersive X-ray diffraction (ADXRD) spectra evolution were utilized to investigate the hydrostatic pressure (0–24 GPa) induced gradual phase transformation. Here, due to the strong lattice interactions between Au core and CdS shell, the different behavior and improved stability under high pressure appeared compared to single quantum dots (QDs).

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

Ultra-wide-bandgap AlGaN homojunction tunnel diodes with negative differential resistance

The power efficiencies of state-of-the-art Al x Ga 1-x N deep-ultraviolet (UV) emitters operating in the <300 nm wavelength region are currently limited to a few percent in part due to limitations in the series and contact resistance which result in excessive drive voltages. Al x Ga 1-x N tunnel contacts and tunnel junctions in deep-UV devices are a promising route toward increasing these efficiencies by improving the contact resistances, hole injection, and reducing optical absorption by removing undesirable p-GaN contact layers. However, due to doping inefficiencies, standalone tunnel diodes have not been realized in the form of homojunction Al x Ga 1-x N. In this work, Al x Ga 1-x N (0.19 ≤ x ≤ 0.58) homojunction tunnel diodes are fabricated with high reverse bias current densities, and one device with x = 0.19 demonstrates a negative differential resistance at ~2.4 V. Al x Ga 1-x N p++/n++/n tunnel diodes are compared to reference p++/i/n diodes to provide clarity about the role of tunneling conduction vs leakage conduction. Transmission electron microscopy verifies that heavy doping does not result in visible defects such as Mg precipitates and allows for subsequent epitaxy, critical for buried tunnel junction structures. Increasing the bandgap energy of Al x Ga 1-x N for higher Al content tunnel junctions decreases the tunnel current, but still allows sufficient conduction necessary for future improvements in deep UV emitter efficiencies.

Nitrides↗