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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Highly Efficient Proton Conduction in the Metal–Organic Framework Material MFM-300(Cr)·SO 4 (H 3 O) 2

The development of materials showing rapid proton conduction with a low activation energy and stable performance over a wide temperature range is an important and challenging line of research. Here, we report confinement of sulfuric acid within porous MFM-300(Cr) to give MFM-300(Cr)·SO 4 (H 3 O) 2 , which exhibits a record-low activation energy of 0.04 eV, resulting in stable proton conductivity between 25 and 80 °C of >10 –2 S cm –1 . In situ synchrotron X-ray powder diffraction (SXPD), neutron powder diffraction (NPD), quasielastic neutron scattering (QENS), and molecular dynamics (MD) simulation reveal the pathways of proton transport and the molecular mechanism of proton diffusion within the pores. Confined sulfuric acid species together with adsorbed water molecules play a critical role in promoting the proton transfer through this robust network to afford a material in which proton conductivity is almost temperature-independent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Layered Semiconductor Cr 0.32 Ga 0.68 Te 2.33 with Concurrent Broken Inversion Symmetry and Ferromagnetism: A Bulk Ferrovalley Material Candidate

The valleytronic state found in group-VI transition-metal dichalcogenides such as MoS2 has attracted immense interest since its valley degree of freedom could be used as an information carrier. However, valleytronic applications require spontaneous valley polarization. Such an electronic state is predicted to be accessible in a new ferroic family of materials, i.e., ferrovalley materials, which features the coexistence of spontaneous spin and valley polarization. Although many atomic monolayer materials with hexagonal lattices have been predicted to be ferrovalley materials, no bulk ferrovalley material candidates have been reported or proposed. Here, in this work, we show that a new non-centrosymmetric van der Waals (vdW) semiconductor Cr 0.32 Ga 0.68 Te 2.33 , with intrinsic ferromagnetism, is a possible candidate for bulk ferrovalley material. This material exhibits several remarkable characteristics: (i) it forms a natural heterostructure between vdW gaps, a quasi-two-dimensional (2D) semiconducting Te layer with a honeycomb lattice stacked on the 2D ferromagnetic slab comprised of the (Cr, Ga)-Te layers, and (ii) the 2D Te honeycomb lattice yields a valley-like electronic structure near the Fermi level, which, in combination with inversion symmetry breaking, ferromagnetism, and strong spin-orbit coupling contributed by heavy Te element, creates a possible bulk spin-valley locked electronic state with valley polarization as suggested by our DFT calculations. Further, this material can also be easily exfoliated to 2D atomically thin layers. Therefore, this material offers a unique platform to explore the physics of valleytronic states with spontaneous spin and valley polarization in both bulk and 2D atomic crystals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Proton irradiation-decelerated intergranular corrosion of Ni-Cr alloys in molten salt

The effects of ionizing radiation on materials often reduce to "bad news". Radiation damage usually leads to detrimental effects such as embrittlement, accelerated creep, phase instability, and radiation-altered corrosion. Here we report that proton irradiation decelerates intergranular corrosion of Ni-Cr alloys in molten fluoride salt at 650 °C. We demonstrate this by showing that the depth of intergranular voids resulting from Cr leaching into the salt is reduced by proton irradiation alone. Interstitial defects generated from irradiation enhance diffusion, more rapidly replenishing corrosion-injected vacancies with alloy constituents, thus playing the crucial role in decelerating corrosion. Our results show that irradiation can have a positive impact on materials performance, challenging our view that radiation damage usually results in negative effects.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atomic-scale understanding of oxide growth and dissolution kinetics of Ni-Cr alloys

Aqueous corrosion of metals is governed by formation and dissolution of a passivating, multi-component surface oxide. Unfortunately, a detailed atomistic description is challenging due to the compositional complexity and the need to consider multiple kinetic factors simultaneously. To this end, we combine experiments with a first-principles-derived, multiscale computational framework that transcends thermodynamic descriptions to explicitly simulate the kinetic evolution of surface oxides of Ni-Cr alloys as a function of composition, temperature, pH, and applied voltage. In the absence of pitting, we identify three distinct voltage regimes, which are kinetically dominated by oxide growth, dissolution, and competitive dissolution and reprecipitation. Evolving compositional gradients and oxide thickness are revealed, including a transition between a metastable Ni-Cr mixed oxide and a thick, porous Ni-dominated oxide. Beyond elucidating the underlying physics, we highlight the need for competing kinetics in models to properly predict the transition from passivation to corrosion. Our results provide a key step towards co-design of alloy composition alongside environmental conditions for sustainable use across a variety of critical energy and infrastructure applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multifunctional two-dimensional van der Waals Janus magnet Cr-based dichalcogenide halides

Two-dimensional van der Waals Janus materials and their heterostructures offer fertile platforms for designing fascinating functionalities. Here, by means of systematic first-principles studies on van der Waals Janus monolayer Cr-based dichalcogenide halides CrYX (Y = S, Se, Te; X = Cl, Br, I), we find that CrSX (X = Cl, Br, I) are the very desirable high T C ferromagnetic semiconductors with an out-of-plane magnetization. Excitingly, by the benefit of the large magnetic moments on ligand S 2- anions, the sought-after large-gap quantum anomalous Hall effect and sizable valley splitting can be achieved through the magnetic proximity effect in van der Waals heterostructures CrSBr/Bi 2 Se 3 /CrSBr and MoTe 2 /CrSBr, respectively. Additionally, we show that large Dzyaloshinskii–Moriya interactions give rise to skyrmion states in CrTeX (X = Cl, Br, I) under external magnetic fields. Our work reveals that two-dimensional Janus magnet Cr-based dichalcogenide halides have appealing multifunctionalities in the applications of topological electronic and valleytronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Visualizing oxygen transport pathways during intergranular oxidation in Ni-Cr

The transport paths of O during intergranular oxidation in binary Ni-Cr were investigated. To isolate the selective oxidation of Cr, oxidation was performed with a CO/CO 2 gas mixture in which the oxygen partial pressure was kept under the NiO dissociation pressure. A combination of electron microscopy and atom probe tomography (APT) was used to study the nanometer-scale details of the passivation and penetrative intergranular oxidation processes at high-energy grain boundaries. Oxygen transport towards the terminating oxidation front is elucidated with dedicated usage of oxygen tracer exchange experiments. Secondary ion mass spectroscopy and APT support classical theories of internal oxidation, revealing preferred transport paths at the oxide/alloy interface with sub-nanometer resolution.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Competitive and cooperative electronic states in Ba(Fe1−xTx)2As2 with T = Co, Ni, Cr

Abstract The electronic inhomogeneities in Co, Ni, and Cr doped BaFe 2 As 2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni 0.04 and Co 0.08 ) and holes (Cr 0.04 ) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

In-beam spectroscopy reveals competing nuclear shapes in the rare isotope 62 Cr

In recent decades, rareisotope facilities have enabled the study of short-lived, neutron-rich nuclei. Their measured properties indicate that shell structure changes in the regime of unbalanced neutron-to-proton ratios compared to that of stable nuclei. In the so-called islands of inversion in the nuclear chart – around the neutron-rich nuclei 32 Mg, 42 Si and 64 Cr, for example – the textbook shell-model picture predicts the presence of spherical shapes due to the respective magic neutron numbers of 20, 28, and 40 of these nuclei. However, nuclei in these regions turn out to be deformed in their ground states. Another hallmark of these islands is shape coexistence, where a nucleus assumes different shapes with excitation energy. Furthermore, we present evidence for this phenomenon from the observation of an excited 0 + state in 62 Cr, two neutrons away from the heart of the island of inversion around neutron number N = 40. We use large-scale shell-model calculations to interpret the results and report extrapolations for the doubly-magic nucleus 60 Ca.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Entangled core/shell magnetic structure driven by surface magnetic symmetry-breaking in Cr 2 O 3 nanoparticles

Bulk Cr 2 O 3 is an antiferromagnetic (AFM) oxide that exhibits the magnetoelectric effect at room temperature, with neither spontaneous magnetization nor net electric polarization. These physical properties stem from a subtle competition between exchange and crystal field interactions. In this article, we exploit the symmetry breaking at the surface of Cr 2 O 3 nanoparticles for unbalancing this delicate physical equilibrium. The emerging weak ferromagnetic signal we observe persists up to near room temperature (≈ 270 K) at which the antiferromagnetic order disappears. In addition, an exchange-bias effect, that rapidly decreases on heating from low temperature up to 30 K, is resistant to thermal disorder above 200 K. In conclusion, our findings point to the possible formation of an entangled core/shell magnetic structure, where pinned uncompensated spins at the shell are randomly distributed in a low-temperature spin-glass ordering, with low net magnetic moment and an ordering temperature governed by the AFM Néel temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Absence of a Dirac gap in ferromagnetic Cr x (Bi 0.1 Sb 0.9 ) 2-x Te 3

Magnetism breaks the time-reversal symmetry expected to open a Dirac gap in 3D topological insulators that consequently leads to the quantum anomalous Hall effect. The most common approach of inducing a ferromagnetic state is by doping magnetic 3d elements into the bulk of 3D topological insulators. In Cr 0.15 (Bi 0.1 Sb 0.9 ) 1.85 Te 3 , the material where the quantum anomalous Hall effect was initially discovered at temperatures much lower than the ferromagnetic transition, T C , the scanning tunneling microscopy studies have reported a large Dirac gap of ~20–100 meV. The discrepancy between the low temperature of quantum anomalous Hall effect (< C ) and large spectroscopic Dirac gaps (>>T C ) found in magnetic topological insulators remains puzzling. Here, we used angle-resolved photoemission spectroscopy to study the surface electronic structure of the pristine and potassium doped surface of Cr 0.15 (Bi 0.1 Sb 0.9 ) 1.85 Te 3 . Upon potassium deposition, the p-type surface state of the pristine sample was turned into an n-type, allowing the spectroscopic observation of Dirac point. Finally, we find a gapless surface state, with no evidence of a large Dirac gap reported in tunneling studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Enhanced ferromagnetism in monolayer Cr 2 Te 3 via topological insulator coupling

Exchange-coupled interfaces are pivotal in exploiting two-dimensional (2D) ferromagnetism. Due to the extraordinary correlations among charge, spin, orbital and lattice degrees of freedom, layered magnetic transition metal chalcogenides (TMCs) bode well for exotic topological phenomena. Here we report the realization of wafer-scale Cr 2 Te 3 down to monolayer (ML) on insulating SrTiO 3 (111) and/or Al 2 O 3 (001) substrates using molecular beam epitaxy. Robust ferromagnetism persists in the 2D limit. In particular, the Curie temperature T C of 2 ML Cr 2 Te 3 increases from 100 K to ∼120 K when proximitized to topological insulator (TI) (Bi,Sb) 2 Te 3 , with substantially boosted magnetization as observed via polarized neutron reflectometry. Our experiments and theory strongly indicate that the Bloembergen–Rowland interaction is likely universal underlying T C enhancement in TI-coupled magnetic heterostructures. The topological-surface-enhanced magnetism in 2D TMC enables further exchange coupling physics and quantum hybrid studies, including paving the way to realize interface-modulated topological electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effect of a Cr-rich surface layer on D retention in various RAFM steels

In this work, deuterium (D) retention is compared between various reduced-activation ferritic/martensitic steels, including CLF-1, Eurofer, F82H, and Rusfer, exposed to pure D plasma (ion flux ~1.5–2 × 10 21 m -2 s -1 , fluence, &straightphi; D , ~1 × 10 25 m -2 , sample temperature ~373 K, and incident ion energy ~100 eV). It is found that there is a large difference (up to ~30x) in the D retention between CLF-1 and F82H even with the same plasma exposure condition and similar nominal composition. The &straightphi; D dependence of the D retention exhibits a peculiar, counterintuitive feature: the D retention decreases with an increase in &straightphi; D from 2 × 10 23 m -2 to 1 × 10 25 m -2 . Surface analyses reveal that a Cr-rich surface layer is formed during outgassing at 773 K for 1 h before plasma exposure, and that there exists a clear correlation between the D retention and the Cr content in the surface layer.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Investigation of magnetic and electrical transport properties of YMB 4 (M = Cr, Fe, and Co) compounds

We have studied the structural, magnetic, and electrical transport properties of the YMB 4 series of compounds (M= Cr, Fe, and Co). These materials exhibit high stability and possess notable refractory and thermoelectric properties. Furthermore, recent theoretical predictions suggest that some of these compounds may display quantum magnetism and dimer formation among the magnetic ions. We have synthesized almost single phase YMB 4 compounds in the orthorhombic crystal structure (space group Pbam) using arc-melting and annealing. The Rietveld analysis of the room-temperature X-ray diffraction patterns reveals a clear change in the lattice parameter c, which correlates with the atomic radii of Cr, Fe, and Co. The temperature variation of resistivity measured between 2 K and 300 K reveals a metallic electron transport in all three YMB 4 compounds. However, no abrupt change in resistivity due to structural or magnetic phase transitions is observed. Additionally, a weak positive magnetoresistance of 1%–2% has been measured at 2.5 K, the contribution from the metallic impurity cannot be ruled out. The carrier concentration, on the order of 10 21 cm −3 , has been determined using conventional Hall measurements. The thermomagnetic curves recorded between 2 K and 350 K reveal the non-magnetic behavior of all these compounds. However, a small magnetic moment from paramagnetic/ferromagnetic impurity phases is detected at low temperatures. Above room temperature, no magnetic transition associated with the breaking of a dimer is observed.

borides↗
DOE OSTI2025

Microscopic scattering approach to in-gap states: Cr adatoms on superconducting 𝛽−Bi 2 ⁢Pd

We develop a microscopic scattering formalism to describe Yu-Shiba-Rusinov (YSR) states due to a single Cr adatom on the Bi-terminated surface of 𝛽−Bi 2 ⁢Pd, by combining ab initio Wannier functions with a real-space Green's function approach in the Bogoliubov–de Gennes formalism. Our framework reproduces key scanning tunneling spectroscopy features, including a single particle-hole asymmetric YSR peak and isotropic 𝑑⁢𝐼/𝑑⁢𝑉 maps around the impurity. Decomposing the YSR states reveals contributions from four nearly degenerate 𝐶 4⁢𝑣 representations, with energy broadening masking their individual signatures. Spin-orbit coupling induces partial spin polarization, while the spatial asymmetry between particle and hole components arises from Cr 𝑑−Bi 𝑝 hybridization. These results highlight the importance of realistic band structures and microscopic modeling for interpreting STM data and provide a foundation for studying impurity chains hosting topological excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Selected magnetoelectrical phonon stiffening in Cr 2 ⁢O 3

A thorough understanding of lattice dynamics, particularly how they couple with spin through spin-phonon interactions, is crucial for pioneering new spin-caloritronic applications. Here, despite extensive studies on Cr 2 ⁢O 3 , the origin of nonlinear thermal expansions and the unusual stiffening of optical phonon modes is still elusive. Through inelastic neutron scattering and atomistic calculations, we found that these behaviors can be attributed to the renormalization of electron states owing to the magnetic transition, and we ruled out the effects from thermal expansion, phonon anharmonicity, magnetostriction, or electron-phonon interactions. Importantly, our quantitative modeling suggests that the common belief that dynamic spin-phonon interactions are the origin of anomalous phonon energy stiffening in magnetic compounds does not apply in Cr 2 ⁢O 3 .

DFT+U↗
DOE OSTI2021

Electronic structure of rhombohedral Cr X 3 ( X = Br, Cl, I) van der Waals crystals

We perform a comprehensive analysis of both the chemical and correlated electronic structure reconstruction of rhombohedral Cr X 3 (X = Br, Cl, I) van der Waals bulk crystals. Using the generalized gradient approximation (GGA) plus dynamical mean-field theory we explicitly demonstrate the importance of local dynamical correlations for a consistent understanding of emergent Kondo quasiparticles and Mott localized electronic states, showing the interplay between material-dependent one-electron GGA line-shape and multiorbital electronic interactions. To probe the correlated paramagnetic electronic state we performed x-ray absorption spectroscopy measurements for CrCl 3 and CrBr 3 bulk crystals. Furthermore, our correlated many-body study is relevant to understanding the electronic structure reconstruction of paramagnetic Cr-trihalide crystals and should be widely applicable to other van der Waals magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Charge doping effects on magnetic properties of single-crystal La 1-x Sr x (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O 3 (0 ≤ x ≤ 0.5) high-entropy perovskite oxides

The observed influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ABO 3 perovskite La 1-x Sr x (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O 3 (0 ≤ x ≤ 0.5). It is found that aliovalent A -site substitution is a viable means to manipulate the magnetically active B -site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary LaT M O 3 (T M =Cr,Mn,Fe,Co,Ni) oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electronic and vibrational properties of bulk Cr 2 Ge 2 Te 6 from first-principles calculations

Here we use density functional theory based ab initio calculations to investigate the structural, vibrational, magnetic, and electronic properties of the layered ferromagnet Cr 2 Ge 2 Te 6 (CGT) that has attracted attention for potential spintronic applications. We optimize the structure for a fixed c/a ratio. Our results are in excellent agreement with experimental data on structure, phonons, and electronic properties. The use of van der Waals interactions and relativistic spin-orbit coupling yields accurate lattice constants and interlayer distances. Simultaneously, the adopted theoretical methods lead to a rigorous description of the vibrational normal modes, as well as the valence bands, that are in excellent agreement with Raman spectroscopy and angle-resolved photoemission spectroscopy spectra, respectively. The magnetic moment is slightly overestimated, and the magnetic anisotropy has the correct sign but is greater in magnitude than that in experiment. We find that inclusion of the on-site Coulomb repulsion on Cr−3d orbitals worsens agreement with experiment, in contrast to previous studies. Our results provide a path toward ab initio analysis of magnetic heterostructures based on CGT.

36 MATERIALS SCIENCE↗