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DOE OSTI2022

Investigating Nanoscale Electron Transfer Processes at the Cell-Mineral Interface in Cobalt-Doped Ferrihydrite Using Geobacter sulfurreducens: A Multi-Technique Approach

Cobalt is an essential element for life and plays a crucial role in supporting the drive to clean energy, due to its importance in rechargeable batteries. Co is often associated with Fe in the environment, but the fate of Co in Fe-rich biogeochemically-active environments is poorly understood. To address this, synchrotron-based scanning X-ray microscopy (SXM) was used investigate the behaviour of cobalt at the nanoscale in Co-Fe(III)-oxyhydroxides undergoing microbial reduction. SXM can assess spatial changes in metal speciation and organic compounds helping to elucidate the electron transfer processes occurring at the cell-mineral interface and inform on the fate of cobalt in redox horizons. G. sulfurreducens was used to reduce synthetic Co-ferrihydrite as an analogue of natural cobalt-iron-oxides. Magnetite [Fe(II)/Fe(III) 3 O 4 ] production was confirmed by powder X-ray diffraction (XRD), SXM and X-ray magnetic circular dichroism (XMCD) data, where best fits of the latter suggested Co-bearing magnetite. Macro-scale XAS techniques suggested Co(III) reduction occurred and complementary SXM at the nanoscale, coupled with imaging, found localised biogenic Co(III) reduction at the cell-mineral interface via direct contact with outer membrane cytochromes. No discernible localised changes in Fe speciation were detected in the reordered cobalt-iron-oxides that were formed and at the end point of the experiment only 11% Co and 1.5% Fe had been solubilised. The solid phase retention, alongside the highly localised and preferential cobalt bioreduction observed at the nanoscale is consistent with retention of Co in redox zones. This work improves our fundamental molecular-scale understanding of the fate of Co in complex environmental systems and supports the development of biogenic Co-doped magnetite for industrial applications from drug delivery systems to magnetic recording media.

58 GEOSCIENCES↗

DOE OSTI2020

Discovery of single-atom alloy catalysts for CO 2 -to-methanol reaction by density functional theory calculations

The transformations of CO 2 molecules into valuable products are of increasing interest due to the negative impact of anthropogenic CO 2 emissions on global warming. The CO 2 -to-methanol hydrogenation is an economically profitable reaction of carbon fixation, but it still steps away from widespread industrialization because of the lack of efficient and selective catalysts. Recently, single-atom alloy (SAA) catalysts have been developed to work remarkably in CO 2 hydrogenation reactions. Doping isolated single atoms into metallic catalyst can dramatically alter the catalytic performance of the host. Here, we have performed a screening discovery on Ru and 6 RuX (X = Fe, Co, Ni, Cu, Ir and Pt) SAAs using density functional theory (DFT) computations. We considered 13 possible elementary reactions in 4 possible reaction pathways on Ru and all RuX surfaces. In the computed mechanisms, we found that the formation of *H 2 COOH and *HCOO intermediates plays a critical role in determining catalysts’ activities. Doping Co and Pt isolated single atoms into Ru surface can thermodynamically and kinetically facilitate these intermediates formation processes, eventually promoting the production of methanol. The combination of weak binding and enhanced charge redistribution on RuCo and RuPt surfaces give them improved catalytic activities over pure Ru. This work will ultimately facilitate the discovery and development of SAAs for CO 2 to methanol, serving as guidance to experiments and theoreticians alike.

25 ENERGY STORAGE↗

DOE OSTI2022

Processing and properties of hafnium-doped tin-modified lead zirconate titanate 95/5 ceramics

Niobium (Nb)-doped lead-tin-zirconate-titanate (PSZT) ceramics near the lead-zirconate-titanate 95/5 orthorhombic AFE-rhombohedral FE morphotropic phase boundary (PSZT 13.5/81/5.5 -1.6Nb) were prepared with up to 10 mol.% of hafnium (Hf) substituted for zirconium. The ceramics were prepared by a traditional solid-state synthesis route and sintered to near full density at 1150°C for 6 h in sealed alumina crucibles with self-same material as the lead vapor source. All compositions were ~98% dense with no detectable secondary phases by X-ray diffraction. The grain size was ~3 μm for all compositions, consisting of equiaxed grains with intergranular porosity. The compositions exhibited remnant polarization values of ~32 μC/cm 2 . Depolarization by the hydrostatic pressure-induced FE-AFE phase transition occurred at 310 MPa for all compositions, resulting in a total depolarization output of 32.4 μC/cm 2 for the PSZT ceramics. Evaluation of the R3c-R3m and R3m-Pm $\bar{3}$ m phase transition temperatures by impedance spectroscopy showed temperatures on heating ranging from 86 to 92°C and 186 to 182°C, respectively, for increasing nominal Hf content. Thermal hysteresis of the phase transitions was also observed in the ceramics, with the transition temperature on cooling being 1–4°C lower. The study demonstrated that the PSZT ceramics are relatively insensitive to variations in Hf content in the range of 0 to 10 mol.%.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Stripe antiferromagnetism and disorder in the Mott insulator NaFe 1₋ x Cu x As ( x ≲0.5)

Neutron scattering measurements have demonstrated that the heavily Cu doped NaFe 1₋ x Cu x As compound behaves like a Mott insulator exhibiting both real-space Fe-Cu stripes and antiferromagnetism below a Néel temperature for x ≲ 0.5. In this work, we have investigated the evolution of structural and magnetic ordering using 23 Na and 75 As NMR for single crystals ( x = 0.39 and 0.48), confirming antiferromagnetism in the form of magnetic stripes. We show that end-chain defects in these stripes are the principal source of magnetic disorder and are responsible for cluster spin-glass transitions in both compounds, in the latter case coexistent with antiferromagnetism. Aided by our numerical simulation of the 75 As spectra, we show that a staggered magnetization at the Fe sites is induced by nonmagnetic Cu dopants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Excess-iron driven spin glass phase in Fe 1 + y Te 1 – x Se x

The iron-chalcogenide superconductor FeTe 1– x Se x displays a variety of exotic features distinct from iron pnictides. Although much effort has been devoted to understanding the interplay between magnetism and superconductivity near x = 0.5, the existence of a spin glass phase with short-range magnetic order in the doping range ( x ~ 0.1–0.3) has rarely been studied. Here, we use DC/AC magnetization and (quasi) elastic neutron scattering to confirm the spin-glass nature of the short-range magnetic order in a Fe 1.07 Te 0.8 Se 0.2 sample. The AC-frequency dependent spin-freezing temperature T f generates a frequency sensitivity Δ T f ( ω )/[ T f ( ω )Δlog 10 ω ] ≈ 0.028 and the description of the critical slowing down with τ = τ 0 ( T f / T SG – 1) - z v gives T SG ≈ 22 K and zv ≈ 10, comparable to that of a classical spin-glass system. We have also extended the frequency-dependent T f to the smaller time scale using energy-resolution-dependent neutron diffraction measurements, in which the T N of the short-range magnetic order increases systematically with increasing energy resolution. We find that by removing the excess iron through annealing in oxygen, the spin-freezing behavior disappears, and bulk superconductivity is realized. Thus, the excess Fe is the driving force for the formation of the spin-glass phase detrimental to bulk superconductivity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

Strong pinning and slow flux creep relaxation in Co-doped CaFe 2 As 2 single crystals

In this paper we report on measurements of critical current densities J c and flux creep rates S of freestanding Ca(Fe 1–x Co x ) 2 As 2 (x ≈ 0.033) single crystals with T c ≈ 15.7 K by performing magnetization measurements. The magnetic field dependences of J c at low temperature display features related to strong pinning. In addition, we find that the system displays small flux creep rates. The characteristic glassy exponent, μ, and the pinning energy, U 0 , display exceptional high values for pristine crystals. We find that for magnetic fields between 0.3 T and 1 T, μ decreases from ≈ 2.8 to ≈ 2 and U 0 remains ≈ 300 K. Analysis of the pinning force indicates that the mechanism is similar to the observed in polycrystalline systems in which grain boundaries and random disorder produce the vortex pinning. Considering the large U 0 observed in the single crystal, we attempt to improve the pinning by adding random point disorder by 3 MeV proton irradiation with a fluence of 2 × 10 16 proton/cm 2 . The results show that, unlike other iron-based superconductors, the superconducting fraction is sharply reduced by irradiation. This fact indicates that the superconductivity in the system is extremely fragile to an increment in the disorder. The superconducting volume fraction in the irradiated crystal systematically recovers after removal disorder by thermal annealing, which evidences as to the observation of critical state in curves of magnetization versus magnetic field. No features related to a reentrant antiferromagnetic transition are observed for the irradiated sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Slope of the upper critical field at T c in two-band superconductors with nonmagnetic disorder: s + + superconductivity in Ba 1 − x K x Fe 2 As 2

A recent theory of the disorder-dependent slope of the upper critical field at the superconducting transition temperature T c , defined as S ≡ |dH c2 /dT| T→T c , is extended to multiband superconductors aimed at iron-based superconductors, considering two constant gaps of different magnitudes and, potentially, different signs. Here, we show that the slope S decreases with increasing nonmagnetic scattering rate P in the s ± pairing state and increases in the s ++ superconductor for a reasonable range of parameters. The experiment shows that in a typical iron-based superconductor, Ba 1–x K x Fe 2 As 2 (BaK122), the nonmagnetic disorder induced by electron irradiation leads to an increasing S(P) across the superconducting “dome,” at different x. This implies that Ba 1–x K x Fe 2 As 2 is likely an s ++ superconductor with two effective gaps of different magnitudes, at least at moderate doping levels, x< 0.6. This work reopens a decade-long discussion about the nature of the superconducting order parameter in iron pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Strong Purcell enhancement at telecom wavelengths afforded by spinel Fe 3 O 4 nanocrystals with size-tunable plasmonic properties

Developments in the field of nanoplasmonics have the potential to advance applications from information processing and telecommunications to light-based sensing. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the infrared (IR). Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging class of materials with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. In this work, we report the plasmonic properties of Fe 3 O 4 sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting quantum dots (QDs). We establish the synthetic conditions for tuning sp-MO NC size, composition and doping characteristics, resulting in unprecedented tunability of electronic behavior and plasmonic response over 450 nm. In particular, with diameter-dependent variations in free-electron concentration across the Fe 3 O 4 NC series, we introduce a strong NC size dependency onto the optical response. In addition, our observation of plasmonics-enhanced decay rates from telecom-emitting QDs reveals Purcell enhancement factors for simple plasmonic-spacer-emitter sandwich structures up to 51-fold, which are comparable to values achieved previously only for emitters in the visible range coupled with conventional noble metal NCs.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Predicting Li-ion Battery Performance for Impurity-doped NMC Cathodes Using Deep Learning

With the electric vehicle (EV) market expansion and the energy sector's shift towards electrification, the demand for battery metals, including lithium (Li), cobalt (Co), and nickel (Ni), is set to surpass supply. A critical knowledge gap exists in the purity standards for battery precursors and the impact of impurities on battery performance. Addressing this, our study employs a Deep Machine Learning (DL) based multi-objective optimization approach to interpret the relationship between metal impurities in domestic battery resources and their effects on battery performance. We analyze experimental data from Li-ion batteries with NMC (Nickel-Manganese-Cobalt oxide) cathodes over 1000 cycles, representing approximately ~6-8 months of operation, to establish a baseline of performance without impurities. Leveraging this data, we develop a Physics-Informed Deep Learning (PIDL) framework to extend our findings to cases that include metal impurities (e.g., Fe, Cu, Al) ranging from (0.001 - 0.01) %, respectively. By incorporating physics-based features, our PIDL model can accurately estimate the performance of NMC cathodes doped with various metal impurities to provide rapid design decisions. This research paves the way for informed decisions in Li-ion battery material design and optimization, ensuring the sustainable growth of the EV market and the broader energy sector.

25 ENERGY STORAGE↗

DOE OSTI2024

Layer Resolved Cr Oxidation State Modulation in Epitaxial SrFe 0.67 Cr 0.33 O 3−δ Thin Films

Understanding how doping influences physicochemical properties of ABO 3 perovskite oxides is critical for tailoring their functionalities. In this study, SrFe 0.67 Cr 0.33 O 3−δ epitaxial thin films were used to examine the effects of Fe and Cr competition on structure and B-site cation oxidation states. The films exhibit a perovskite-like structure near the film/substrate interface, while a brownmillerite-like structure with horizontal oxygen vacancy channels predominates near the surface. Electron energy loss spectroscopy shows Fe remains Fe 3+ , while Cr varies from ∼Cr 3+ (tetrahedral layers) to ∼Cr 4+ (octahedral layers) within brownmillerite phases and becomes ∼Cr 4.5+ in perovskite-like phases. Theoretical simulations indicate that Cr–O bond arrangements and the way oxygen vacancies interact with Cr and Fe drive Cr charge disproportionation. High-valent Cr cations introduce additional densities of states near the Fermi level, reducing the optical bandgap from ∼2.0 eV (SrFeO 2.5 ) to ∼1.7 eV (SrFe 0.67 Cr 0.33 O 3−δ ). In conclusion, these findings offer insights into B-site cation doping in the perovskite oxide framework.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

CO-promoted low-temperature conversion of CH 4 to hydrogen and carbon nanotubes on Nanocrystalline Cr-doped ferrite catalyst

Catalytic decomposition of methane (CDM) to H 2 and multiwalled carbon nanotubes (MWCNTs) was achieved by a nanocrystalline Cr-doped ferrite (FeCr) catalyst at 500 °C and atmospheric pressure with minor cofed CO. The exothermic Boudouard reaction increased the temperature and H 2 from CDM at catalyst surface that induced Fe 2+ reduction to Fe 0 . The Fe 0 clusters along with the CO-originated surface oxygens enabled transfer of C and H to sustain the surface CDM and CO reactions. The metallic Fe-enabled C transfer led to the formation of MWCNTs. The Cr 6+/3+ dopants facilitated the Fe redox cycles and maintained surface oxygens for high catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

THE IMPACT OF OXIDATION-INDUCED DEGRADATION ON MATERIALS USED IN HYDROGEN-FIRED MICROTURBINES

Hydrogen-fueled microturbines are being considered as part of the future green microgrid. However, the use of hydrogen as a fuel presents new challenges for selection and development of suitable high temperature materials for hydrogen combustion. The burning of hydrogen is expected to result in higher operating temperatures and higher than typically observed water vapor contents in exhaust gases than after burning natural gas. In the present work, foil specimens of various Fe- and Ni-based alloys were oxidized in air + 10vol.% H2O and air + 60vol.% H2O for up to 5,000 h at 700 °C to simulate the exhaust atmosphere of natural gas and hydrogen-fueled microturbines. The impact of alloy composition and water vapor content on the oxidation/ volatilization induced loss of wall thickness was experimentally evaluated. Enhanced external oxidation and volatilization of Cr¬2O3 and Ti-doped Cr2O3 scales was observed in 60 % H2O compared to 10% H2O. No significant impact of the higher water vapor content was observed on Al2O3 scales formed on Fe-based alumina forming alloys. Lifetime modeling was employed to predict the combined effects of water vapor content, gas flow rates, temperature and alloy composition on the oxidation-induced lifetime of the investigated materials

Romedenne, Marie↗

DOE OSTI2022

Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet

The existence of long-range magnetic order in low-dimensional magnetic systems, such as the quasi-two-dimensional van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe N GeTe 2 (N = 3, 4, 5; FGT) family is exceptional, owing to its vast tunability of magnetic properties. In particular, a ferromagnetic ordering temperature (T C ) above room temperature at N = 5 (F5GT) is observed. Here, our work shows that, by nickel (Ni) substitution of iron in F5GT, a record high T C =478(6) K is achieved. Importantly, pervasive, beyond room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, including increased 3D magnetic couplings due to the structural alterations.

2-dimensional systems↗

DOE OSTI2024

Broadband and Tunable Microwave Absorption Properties from Large Magnetic Loss in Ni–Zn Ferrite

Highly effective electromagnetic (EM) wave absorber materials with strong reflection loss (RL) and a wide absorption bandwidth (EBW) in gigahertz (GHz) frequencies are crucial for advanced wireless applications and portable electronics. Traditional microwave absorbers lack magnetic loss and struggle with impedance matching, while ferrites are stable, exhibit excellent magnetic and dielectric losses, and offer better impedance matching. However, achieving the desired EBW in ferrites remains a challenge, necessitating further composition design. In this study, impedance matching is successfully enhanced and EBW in Ni–Zn ferrite is broadened by successive doping with Mn and Co , without incorporation of any polymer filler. It is found that Ni 0.4 Co 0.1 Zn 0.5 Fe 1.9 Mn 0.1 O 4 material exhibits exceptional EM wave absorption, with a maximum RL of −48.7 dB. It also featured a significant EBW of 10.8 GHz, maintaining a 90% absorption rate (RL < −10 dB) for a thickness of 4.5 mm. These outstanding properties result from substantial magnetic losses and favorable impedance matching. These findings represent a significant step forward in the development of microwave absorber materials, addressing EM wave pollution concerns within GHz frequencies, including the frequency band used in popular 5G technology.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Existence of La-site antisite defects in LaMO 3 (M = Mn, Fe, and Co) predicted with many-body diffusion quantum Monte Carlo

The properties of LaMO 3 (M: 3d transition metal) perovskite crystals are significantly dependent on point defects, whether introduced accidentally or intentionally. The most studied defects in La-based perovskites are the oxygen vacancies and doping impurities on the La and M sites. Here, we identify that intrinsic antisite defects, the replacement of La by the transition metal, M, can be formed under M-rich and O-poor growth conditions, based on results of an accurate many-body ab initio approach. Our fixed-node diffusion Monte Carlo (FNDMC) calculations of LaMO 3 (M = Mn, Fe, and Co) find that such antisite defects can have low formation energies and are magnetized. Complementary density functional theory (DFT)-based calculations show that Mn antisite defects in LaMnO 3 may cause the p-type electronic conductivity. These features could affect spintronics, redox catalysis, and other broad applications. Our bulk validation studies establish that FNDMC reproduces the antiferromagnetic state of LaMnO 3 , whereas DFT with PBE (Perdew–Burke–Ernzerhof), SCAN (strongly constrained and appropriately normed), and the LDA+U (local density approximation with Coulomb U) functionals all favor ferromagnetic states, at variance with experiment.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Electronic and magnetic structures that hinder the superconducting state in the collapsed phase of $\mathrm{SrCr_2}$ $\mathrm{As_2}$

The lack of superconductivity in the family of materials $AB_2$ $\mathrm{As_2}$ ($A$ = $\mathrm{Sr, Ba}$ and $B$ = $\mathrm{Cr, Mn}$) under doping or pressure is an intriguing issue, considering the resemblance of these materials to their cousin materials and superconductors $\mathrm{(Ba, Sr) Fe_2}$ $\mathrm{As_2}$. In this context, the suppression of magnetism together with the presence of electron and hole pockets in the Brillouin zone seem to be fundamental ingredients to explain superconductivity. In this paper, we report a tetragonal to collapsed-tetragonal phase transition in $\mathrm{SrCr_2}$ $\mathrm{As_2}$ under high pressure with the appearance of hole and electron pockets in the Brillouin zone. In this collapsed-tetragonal phase, a residual local magnetic moment in the Cr-ion site and a finite global magnetic energy are derived. Importantly, this scenario would suggest why superconductivity is prevented in this compound. Our observations were obtained from the analysis of synchrotron x-ray diffraction measurements of polycrystalline samples of $\mathrm{SrCr_2}$ $\mathrm{As_2}$, and using first-principles simulations, under pressures $P$ in the range $\mathrm{1.4}$ $\mathrm{GPa}$ < $P$ < $\mathrm{20 GPa}$ at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2026

Bistable Optical Semiconductor Switching Based on C-Doped GaN

Highly resistive gallium nitride (GaN) is an essential material for power optoelectronic applications. While carbon doping is widely used to achieve semi-insulating properties in GaN, the persistent photoconductivity (PPC) arising from deep-level defect traps remains a major obstacle for high-speed power switching. This study demonstrates a novel approach: leveraging the ultrahigh photoresponsivity of GaN:C (up to 2.1 A ⋅ cm/W ⋅ kV, surpassing alternatives such as GaN:Fe) and employing defect-selective optical control to effectively quench the PPC. By synchronizing a short infrared (1064 nm) quenching pulse with UV (385 nm) excitation in an epitaxially grown GaN:C layer on a heavily doped n-type GaN substrate, we achieve a dramatic reduction in photocurrent fall time by approximately 293× (from 470 to 1.6 μ s), increasing modulation bandwidth from 745 Hz to nearly 218 kHz. Here, this advancement not only establishes a new pathway for controlling PPC in GaN:C but also enables the practical integration of GaN:C in fast power switching devices. Enhanced modulation bandwidth, along with GaN:C excellent photoresponsivity, makes it a promising candidate for optically controlled high-voltage, high-power electronic systems, such as photoconductive semiconductor switches (PCSSs) used in pulsed-power drivers, high-power microwave (HPM) sources, and high-voltage gate drivers for wide bandgap (WBG) power electronics.

Carbon-dopped Gallium nitride (GaN:C)↗

DOE OSTI2022

Activating Single‐Atom Ni Site via First‐Shell Si Modulation Boosts Oxygen Reduction Reaction

Abstract Atomically dispersed nitrogen‐coordinated 3d transition‐metal site on carbon support (M‐NC) are promising alternatives to Pt group metal‐based catalysts toward oxygen reduction reaction (ORR). However, despite the excellent activities of most of M‐NC catalysts, such as Fe‐NC, Co‐NC et al., their durability is far from satisfactory due to Fenton reaction. Herein, this work reports a novel Si‐doped Ni‐NC catalyst (Ni‐SiNC) that possesses high activity and excellent stability. X‐ray absorption fine structure and aberration‐corrected transmission electron microscopy uncover that the single‐atom Ni site is coordinated with one Si atom and three N atoms, constructing Ni‐Si 1 N 3 moiety. The Ni‐SiNC catalyst exhibits a half‐wave potential (E 1/2 ) of 0.866 V versus RHE, with a distinguished long‐term durability in alkaline media of only 10 mV negative shift in E 1/2 after 35 000 cycles, which is also validated in Zn‐air battery. Density functional theory calculations reveal that the Ni‐Si 1 N 3 moiety facilitates ORR kinetics through optimizing the adsorption of intermediates.

Chemistry↗