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At least 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Mixed Nd 4+/3+ and Cluster Magnetism in Hexagonal Perovskite 12R-Ba 4 NdMn 3 O 12−γ

Hexagonal perovskite oxides with 12R stacking host well-separated face-sharing octahedral metal trimers with short metal–metal distances, leading in some cases to large degrees of magnetic frustration and cluster magnetism. Introducing magnetic ions in proximity to these trimers can influence the degree of frustration and cluster magnetism, but the extent to which the magnetism can be tuned by varying neighboring metal cations remains an open question. In this work, we test the impacts of using Nd in proximity to Mn trimers in the hexagonal perovskite 12R-Ba 4 NdMn 3 O 12−γ . Given this stoichiometry, Nd should assume the 4+ oxidation state with a spin state of S = 1, which would be the first realization of Nd 4+ in an oxide environment. Through detailed bulk magnetic, X-ray absorption spectroscopic, and powder neutron diffraction (PND) measurements, we find that Nd 4+ is realized in this material, but there is also partial reduction to Nd 3+ , which is charge-balanced by O vacancies. Magnetometry measurements indicate an antiferromagnetic ordering temperature T N ≈ 16 K, and PND measurements reveal a surprising collinear antiferromagnetic structure with magnetic space group Pc2/m (no. 10.49 in BNS notation), which has not previously been seen in this class of materials. Our results represent a comprehensive analysis of the structural, electronic, and magnetic properties of 12R-Ba 4 NdMn 3 O 12−γ , showing the first observation of partial Nd 4+ in an oxide, and demonstrating that this structural class can host a broad range of magnetic structures which are not easily predicted based on compositional trends.

14 SOLAR ENERGY↗
DOE OSTI2018

Stabilization of Hexaaminobenzene in a 2D Conductive Metal–Organic Framework for High Power Sodium Storage

Redox-active organic materials have gained growing attention as electrodes of rechargeable batteries. However, their key limitations are the low electronic conductivity and limited chemical and structural stability under redox conditions. Herein, we report a new cobalt-based ;2D conductive metal-organic framework (MOF), Co-HAB, having stable, accessible, dense active sites for high-power energy storage device through conjugative coordination between a redox-active linker, hexaaminobenzene (HAB), and a Co(II) center. Given the exceptional capability of Co-HAB for stabilizing reactive HAB, a reversible three-electron redox reaction per HAB was successfully demonstrated for the first time, thereby presenting a promising new electrode material for sodium-ion storage. Specifically, through synthetic tunability of Co-HAB, the bulk electrical conductivity of 1.57 S cm(-1) was achieved, enabling an extremely high rate capability, delivering 214 mAh g(-1) within 7 min or 152 mAh g(-1) in 45 s. Meanwhile, an almost linear increase of the areal capacity upon increasing active mass loading up to 9.6 mg cm(-2) was obtained, demonstrating 2.6 mAh cm(-2) with a trace amount of conducting agent.

Electrochemistry↗
DOE OSTI2024

Pressure-tuned quantum criticality in the large-D antiferromagnet DTN

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl 2 · 4SC(NH 2 ) 2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Competing magnetic ground states and their coupling to the crystal lattice in CuFe 2 Ge 2

Identifying and characterizing systems with coupled and competing interactions is central to the development of physical models that can accurately describe and predict emergent behavior in condensed matter systems. This work demonstrates that the metallic compound CuFe 2 Ge 2 has competing magnetic ground states, which are shown to be strongly coupled to the lattice and easily manipulated using temperature and applied magnetic fields. The temperature-dependent magnetization M measurements reveal a ferromagnetic-like onset at 228 (1) K and a broad maximum in M near 180 K. Powder neutron diffraction confirms antiferromagnetic ordering below T N ≈ 175 K, and an incommensurate spin density wave is observed below ≈125 K. Coupled with the small refined moments (0.5–1 μ B/Fe), this provides a picture of itinerant magnetism in CuFe 2 Ge 2 . Furthermore, the neutron diffraction data reveal a coexistence of two magnetic phases that further highlights the near-degeneracy of various magnetic states. Our results demonstrate that the ground state in CuFe 2 Ge 2 can be easily manipulated by external forces, making it of particular interest for doping, pressure, and further theoretical studies.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synthetic, structural, and luminescence study of uranyl coordination polymers containing chelating terpyridine and trispyridyltriazine ligands

In this work, seven uranyl coordination polymers [(UO 2 )(BrC 8 H 3 O 4 )(C 15 H 11 N 3 )] ( 1 ), [(UO 2 )(ClC 8 H 3 O 4 )(C 15 H 11 N 3 )] ( 2 ), [(UO 2 )(BrC 8 H 3 O 4 )(C 18 H 12 N 6 )]·H 2 O ( 3 ), [(UO 2 )(ClC 8 H 3 O 4 )(C 18 H 12 N 6 )]·2H 2 O ( 4 ), [(UO 2 )(C 8 H 3 IO 4 )(C 18 H 12 N 6 )]·2C 3 H 7 O ( 5 ), [(UO 2 )(C 9 H 6 O 4 )(C 18 H 12 N 6 )]·H 2 O ( 6 ), and [(UO 2 )(C 6 H 2 O 4 S)(C 18 H 12 N 6 )]·2H 2 O ( 7 ) containing N-donor ligands 2,2':6',2''-terpyridine (TPY) and 2,4,6-tripyridyl s-triazine (TPTZ); and O-donor linkers thiophene 2,5-dicarboxylic acid (TDC) and X-1,4-benzene dicarboxylic acid (X-BDC, where X = Me, Cl, Br, I) were synthesized and characterized using single crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), and fluorescence spectroscopy. Each compound crystallizes as binuclear pseudo dimers containing a UO 2 2+ pentagonal bipyramidal primary building unit. Fluorescence spectra of 1–4 , and 6–7 show characteristic UO 2 2+ emission whereas 5 is non-emissive. Red shifts within the emission of 1–4 are noted as a function of N-donor ligands TPY and TPTZ whereas emission shifts are not observed in TPTZ bearing compounds ( 3 , 4 , 6 , 7 ) containing different O-donor linkers. Luminescent lifetimes of 1–7 were obtained and fitted with either bi-exponential or tri-exponential components.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Direct structural evidence of commensurate-to-incommensurate transition of hydrocarbon adsorption in a microporous metal organic framework

Here, the efficiency of physisorption-based separation of gas-mixtures depends on the selectivity of adsorbent which is directly linked to size, shape, polarizability and other physical properties of adsorbed molecules. Commensurate adsorption is an interesting and important adsorption phenomenon, where the adsorbed amount, location, and orientation of an adsorbate are commensurate with the crystal symmetry of the adsorbent. Understanding this phenomenon is important and beneficial as it can provide vital information about adsorbate–adsorbent interaction and adsorption–desorption mechanism. So far, only sporadic examples of commensurate adsorption have been reported in porous materials such as zeolites and metal organic frameworks (MOFs). In this work we show for the first time direct structural evidence of commensurate-to-incommensurate transition of linear hydrocarbon molecules (C 2 –C7) in a microporous MOF, by employing a number of analytical techniques including single crystal X-ray diffraction (SCXRD), in situ powder X-ray diffraction coupled with differential scanning calorimetry (PXRD-DSC), gas adsorption and molecular simulations.

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

The nickel battery positive electrode revisited: stability and structure of the β-NiOOH phase

In this work, the crystal structure of the nickel battery positive electrode material, β-NiOOH, is analyzed through a joint approach involving NMR and FTIR spectroscopies, powder neutron diffraction and DFT calculations. The obtained results confirm that structural changes occur during the β-Ni(OH) 2 /β-NiOOH transformation leading to a metastable crystal structure with a TP2 host lattice. This structure involves two types of hydrogen atoms both forming primary and secondary hydrogen bonds. The formation of TP2 NiOOH as opposed to the more stable P3 host type during β-Ni(OH) 2 /β-NiOOH transformation has a kinetic origin that can be understood by a lower strain penalty involved in the transformation.

25 ENERGY STORAGE↗
DOE OSTI2020

A new halospinel superionic conductor for high-voltage all solid state lithium batteries

We report a new Li-superionic chloride, Li 2 Sc 2/3 Cl 4 , that crystallizes in a disordered spinel structure, and exhibits an ionic conductivity of 1.5 mS cm –1 with a low activation energy barrier for Li + ion diffusion of 0.34 eV. Here, this material is the first spinel-type superionic halide. Structural elucidation via powder neutron diffraction reveals a significantly disordered Li + -ion distribution over available tetrahedral and octahedral sites within the lattice, forming an infinitely 3D connected Li + ion diffusion pathway comprised of face-sharing octahedra and tetrahedra. Due to the high oxidative stability of Li 2 Sc 2/3 Cl 4 , all solid state lithium batteries employing Li 2 Sc 2/3 Cl 4 and high voltage cathodes (LiCoO 2 , LiNi 0.6 Mn 0.2 Co 0.2 O 2 or high-Ni LiNi 0.85 Mn 0.1 Co 0.05 O 2 ) – without any coating – exhibit excellent electrochemical performance up to 4.6 V in terms of capacity retention and cycle life.

25 ENERGY STORAGE↗
DOE OSTI2019

Pushing data quality for laboratory pair distribution function experiments

Over the past decade, some studies with laboratory pair distribution function (PDF) data emerged. Yet, limited Q max or instrumental resolution impeded in-depth structural refinements. With more advanced detector technologies, the question arose how to design novel PDF equipment for laboratories that will allow decent PDF refinements over r = 1–70 Å. It is crucial to reflect the essential requirements, namely, monochromatic X-rays, suppression of air scattering, instrumental resolution, and overall measurement times. The result is a novel PDF setup based on a STOE STADI P powder diffractometer in transmission-/Debye-Scherrer geometry with monochromatic Ag Kα 1 radiation, featuring a MYTHEN2 4K detector covering a Q range of 0.3–20.5 Å –1 . PDF data are collected in a moving PDF mode within 6 h. Structural signatures of liquids can be satisfactorily resolved in the PDF as shown for the ionic liquid hmimPF 6 . The high instrumental resolution is mirrored in low q damp values determined from LaB 6 measurements. PDF data from a powder sample of ca. 7 nm TiO 2 nanoparticles were successfully refined over up to 70 Å with goodness-of-fit values R w < 0.22 (respectively R w = 0.18 over 30 Å), thanks to the low background and high instrumental resolution, hereby enlarging the accessible r range by several tens of Angstroms compared to previous laboratory PDF studies.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

VERDI: VERsatile DIffractometer with wide-angle polarization analysis for magnetic structure studies in powders and single crystals

The VERsatile DIffractometer will set a new standard for a world-class magnetic diffractometer with versatility for both powder and single crystal samples and capability for wide-angle polarization analysis. The instrument will utilize a large single-frame bandwidth and will offer high-resolution at low momentum transfers and excellent signal-to-noise ratio. A horizontal elliptical mirror concept with interchangeable guide pieces will provide high flexibility in beam divergence to allow for a high-resolution powder mode, a high-intensity single crystal mode, and a polarized beam option. A major science focus will be quantum materials that exhibit emergent properties arising from collective effects in condensed matter. The unique use of polarized neutrons to isolate the magnetic signature will provide optimal experimental input to state-of-the-art modeling approaches to access detailed insight into local magnetic ordering.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Low temperature tetragonal polymorph of CaZrF 6

A new tetragonal polymorph of CaZrF 6 can be prepared by high energy ball milling of a CaF 2 /ZrF 4 mixture, followed by heat treatment at 325 °C. This polymorph is thermodynamically stable with respect to the well-known cubic form at low temperatures. However, it readily transforms to the cubic form on heating above ~400 °C. The tetragonal (β) CaZrF 6 is not isostructural with any previously known alkaline earth AZrF 6 phase. Unlike the cubic form, which shows strong negative thermal expansion over a wide temperature range, the tetragonal form displays positive thermal expansion in all directions (100–400 K: α l ~ +17 × 10 –6 K –1 and +13 × 10 –6 K –1 along the a- and c-axes, respectively).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Flat bands in the CoSn-type compounds

Quantum interference on the kagome lattice generates electronic bands with narrow bandwidth, called flat bands. Crystal structures incorporating this lattice can host strong electron correlations with nonstandard ingredients, but only if these bands lie at the Fermi level. In the six compounds with the CoSn structure type (FeGe, FeSn, CoSn, NiIn, RhPb, and PtTl) the transition metals form a kagome lattice. The two iron variants are robust antiferromagnets so we focus on the latter four and investigate their thermodynamic and transport properties. We consider these results and calculated band structures to locate and characterize the flat bands in these materials. Finally, we propose that CoSn and RhPb deserve the community's attention for exploring flat-band physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Fluctuating Ru trimer precursor to a two-stage electronic transition in RuP

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T c , is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became ''usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, nonsuperconducting parent of the maximum- T c branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic structure and Ising-like antiferromagnetism in the bilayer triangular lattice compound NdZnPO

Here, the complex interplay of spin frustration and quantum fluctuations in low-dimensional quantum materials leads to a variety of intriguing phenomena. This research focuses on a detailed analysis of the magnetic behavior exhibited by NdZnPO, a bilayer spin-1/2 triangular lattice antiferromagnet. The investigation employs magnetization, specific heat, and powder neutron scattering measurements. At zero field, a long-range magnetic order is observed at T N = 1.64 K. Powder neutron diffraction experiments show the Ising-like magnetic moments along the c-axis, revealing a stripe like magnetic structure with magnetic propagation vector (1/2, 0, 1/2). Application of a magnetic field along the c-axis suppresses the antiferromagnetic order, leading to a fully polarized ferromagnetic state above B c = 4.5 T. This transition is accompanied by notable enhancements in the nuclear Schottky contribution. Moreover, the absence of spin frustration and expected field-induced plateau-like phases are remarkable observations. Detailed calculations of magnetic dipolar interactions revealed complex couplings reminiscent of a honeycomb lattice, suggesting the potential emergence of Kitaev-like physics within this system. This comprehensive study of the magnetic properties of NdZnPO highlights unresolved intricacies, underscoring the imperative for further exploration to unveil the underlying governing mechanisms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Anharmonicity and atomic distribution of SnTe and PbTe thermoelectrics

The structure and lattice dynamics of rock-salt thermoelectric materials SnTe and PbTe are investigated with single crystal and powder neutron diffraction, inelastic neutron scattering (INS), and first-principles simulations. Our first-principles calculations of the radial distribution function (RDF) in both SnTe and PbTe show a clear asymmetry in the first nearest-neighbor (1NN) peak, which increases with temperature, in agreement with experimental reports (Ref. 1,2). We show that this peak asymmetry for the 1NN Sn–Te or Pb–Te bond results from large-amplitude anharmonic vibrations (phonons). No atomic off-centering is found in our simulations. In addition, the atomic mean square displacements derived from our diffraction data reveal stiffer bonding at the anion site, in good agreement with the partial phonon densities of states from INS, and first-principles calculations. In conclusion, these results provide clear evidence for large-amplitude anharmonic phonons associated with the resonant bonding leading to the ferroelectric instability.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTe 1-x Se x

It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular choices of four-spin plaquettes, where the appropriate choice changes as the parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of FeTe 1–x Se x . We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); these can both be described with the same local plaquette, but with different interplaquette correlations. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response upon cooling. Lastly, we also present powder neutron diffraction results for lattice parameters in FeTe 1–x Se x indicating that the tetrahedral bond angle tends to increase towards the ideal value upon cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe 3d orbitals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗