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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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116 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Magnetoelastic distortion of multiferroic BiFeO 3 in the canted antiferromagnetic state

Using THz spectroscopy, here we show that the spin-wave spectrum of multiferroic BiFeO 3 in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly antiparallel spins. The revised spin model for BiFeO 3 contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Electronic structure and lattice dynamics of 1⁢𝑇−VSe 2 : Origin of the three-dimensional charge density wave

To characterize in detail the charge density wave (CDW) transition of 1⁢𝑇−VSe 2 , its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the 𝐾 edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at 𝑇 > 𝑇⁢ CDW , which leads to a depletion of the density of states in the ML-M'L' plane at 𝑇 < 𝑇⁢ CDW , and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon (𝜔 1 ) with propagation vector (0.25 0 −0.3) r.l.u. Here, we show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the 𝜔 1 soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of VSe 2 reported here adds important pieces of information to the understanding of the electronic modulations in the family of transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Local lattice distortions and electronic phases in perovskite manganite Pr 0.5 Sr 0.5 MnO 3

We use variable temperature and magnetic field total x-ray scattering to study the crystal structure of the strongly correlated Pr 0.5 Sr 0.5 MnO 3 perovskite, which is a paramagnetic insulator at room temperature, becomes a ferromagnetic metal at 272 K and, upon further decreasing the temperature, turns into an antiferromagnetic insulator at 105 K. We find that a model featuring a monoclinic symmetry captures the structure and its temperature and field evolution well, eliminating the need to evoke a phase segregation scenario as done in prior studies. It appears that coupled variations in Mn–oxygen bonding distances and angles from their values in an undistorted perovskite lattice, i.e., coupled local lattice distortions, assist the phase transitions in Pr 0.5 Sr 0.5 MnO 3 , contributing to its unique physical properties. Local structural distortions thus emerge as an important degree of freedom in strongly correlated systems, in particular perovskite manganates, and, therefore, they should be fully accounted for when their fascinating physics is considered.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Interplay of Crystal Structure and Magnetic Properties of the Eu 5.08-x Sr x Al 3 Sb 6 Solid Solution

Zintl phases containing rare-earth metals have gained attention due to their magnetic, electronic, and thermoelectric properties. Eu 5.08 Al 3 Sb 6 is a new structure type (monoclinic space group C2/m) that can be described as a pseudorock-salt EuSb motif with the Eu-centered Sb octahedra at the origin of the unit cell, and on the C-face center, containing either Eu (8%) or an Al 4 tetrahedron modeled as a dual tetrahedron (37.5%). The complete solid solution of Eu 5.08–x Sr x Al 3 Sb 6 can be prepared; however, the cation totals vacillate from 5 to 5.24 depending on the Al content. Al K-edge XANES shows a shift to higher energy relative to the Al metal but at slightly lower energy relative to AlSb, indicating an intermediate oxidation state closer to +3 than 0. The lack of an Al K-edge shift with the incorporation of Sr suggests that changes in Sr content do not have a meaningful impact on the electronics of the Al tetrahedra. Investigation of the solid solution structures provides evidence for classifying this structure type as a polar intermetallic phase with variable composition. Magnetization measurements were collected for the solid solution and show complex magnetic ordering with competing ferromagnetic and antiferromagnetic interactions as the Sr content increases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Structural, magnetic, optical, dielectric and electronic properties of R 2 NiIrO 6 (R = Pr and Nd): A comprehensive experimental and theoretical investigation

Double perovskites are highly promising materials capable of exhibiting a wide variety of phenomena. In this work, we perform a comprehensive experimental and theoretical study of polycrystalline R 2 NiIrO 6 (R = Pr and Nd) compounds. Both compounds were synthesised using the solid-state reaction method. Rietveld refinement confirmed a monoclinic structure with the P2 1 /n space group for both compounds. The scanning electron images showed the average grain sizes of 0.55 μm for R = Pr and 0.46 μm for R = Nd. Fourier transform infrared ra- diation spectra of the two compounds presented two intense bands at 470 cm -1 and 540 cm -1 . The optical measurements revealed that the band gaps of the compounds were in the visible absorption range. The field- cooled magnetisation - field hysteresis measurements indicated exchange bias properties in the synthesised compounds at low temperatures. Both temperature and frequency variation of dielectric constant and loss tangent measurements were conducted. The frequency-dependent ac conductivity measurements indicated that the conductivity increases with the increase of frequency as well as temperature. The Nd 2 NiIrO 6 compound showed lower ac conductivities compared to its isostructural Pr 2 NiIrO 6 compound. The atomic and electronic structures of Nd 2 NiIrO 6 and Pr 2 NiIrO 6 were explored using the spin-polarised calculations performed within the DFT+U method. Our results suggested that the inclusion of on-site correlations and repulsions for the d-states of atoms was necessary in order to obtain finite band gaps of Nd 2 NiIrO 6 and Pr 2 NiIrO 6 systems.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structural and double magnetic transitions in the frustrated spin- 1 2 capped-kagome antiferromagnet ( RbCl ) Cu 5 P 2 O 10

The structural and magnetic properties of the geometrically frustrated spin- 1 2 capped-kagome antiferromagnet (RbCl) Cu 5 P 2 O 10 are investigated via temperature-dependent x-ray diffraction, magnetization, heat capacity, and 31 P NMR experiments on a polycrystalline sample. It undergoes a structural transition at around T t ≃ 310 K from a high-temperature trigonal (P ¯3m1) to a low-temperature monoclinic (C2/c) unit cell, where the low-temperature structure features the capped-kagome geometry of Cu 2+ ions. Interestingly, it shows the onset of two successive magnetic transitions at T N1 ≃ 20 K and T N2 ≃ 7 K. The shape of the 31 P NMR spectra unfolds the possible nature of the transitions below T N1 and T N2 to be of incommensurate and commensurate antiferromagnetic type, respectively. A large value of the Curie-Weiss temperature as compared to T N1 sets the frustration parameter f ≃ 8, ensuring strong magnetic frustration in the compound. From the 31 P NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange coupling is estimated to be J/k B ≃ 117 K. Furthermore, these unusual double magnetic transitions make this compound beguiling for further investigations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Evolution of the structural transition in Mo 1–x W x Te 2

The composition dependence of the structural transition between the monoclinic 1T' and orthorhombic Td phases in the Mo 1–x W x Te 2 Weyl semimetal was investigated by elastic neutron scattering on single crystals up to x ≈ 0.54. First observed in MoTe 2 , the transition from T d to 1T' is accompanied by an intermediate pseudo-orthorhombic phase, T d *. Upon doping with W, the T d * phase vanishes by x ≈ 0.34. Above this concentration, a phase coexistence behavior with both T d and 1T' is observed instead. The interlayer in-plane positioning parameter δ, which relates to the 1T' β angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. As a result, the temperature width of the phase coexistence remains almost constant up to x ≈ 0.54, in contrast to the broadening reported under pressure.

36 MATERIALS SCIENCE↗
NASA NTRS2017

Partitioning of F and Cl Between Apatite and a Synthetic Shergottite Liquid (QUE 94201) at 4 Gpa from 1300 TO 1500 C

Apatite [Ca5(PO4)3(F,Cl,OH)] is present in a wide range of planetary materials. Due to the presence of volatiles within its crystal structure (Xsite), many recent studies have attempted to use apatite to constrain the volatile contents of planetary magmas and mantle sources. In order to use the volatile contents of apatite to accurately determine the abundances of volatiles in coexisting silicate melt or fluids, thermodynamic models for the apatite solid solution and for the apatite components in multicomponent silicate melts and fluids are required. Although some thermodynamic models for apatite have been developed, they are incomplete. Furthermore, no mixing model is available for all of the apatite components in silicate melts or fluids, especially for F and Cl components. Several experimental studies have investigated the apatite-melt and apatite-fluid partitioning behavior of F, Cl, and OH in terrestrial and planetary systems, which have determined that apatite-melt partitioning of volatiles are best described as exchange equilibria similar to Fe-Mg partitioning between olivine and silicate melt. However, McCubbin et al., recently reported that the exchange coefficients vary in portions of apatite compositional space where F, Cl, and OH do not mix ideally in apatite. In particular, solution calorimetry data of apatite compositions along the F-Cl join exhibit substantial excess enthalpies of mixing, and McCubbin et al. reported substantial deviations in the Cl-F exchange Kd along the F-Cl apatite join that could be explained by the preferential incorporation of F into apatite. In the present study, we assess the effect of apatite crystal chemistry on F-Cl exchange equilibria between apatite and melt at 4 GPa over the temperature range of 1300-1500 C. The goal of these experiments is to assess the variation in the Ap-melt Cl-F exchange Kd over a broad range of F:Cl ratios in apatite. The results of these experiments could be used to understand at what composition apatite shifts from a hexagonal unit cell with space group P63/m to a unit cell with monoclinic symmetry within space group P21/b. We anticipate that this transition occurs at >70% chlorapatite based on solution calorimetry data.

McCubbin, F. M.↗