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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Computation of entropies and phase equilibria in refractory V-Nb-Mo-Ta-W high-entropy alloys

Here, we have applied the first-principles phonon method to the refractory V-Nb-Mo-Ta-W high-entropy alloys (HEAs) to predict the major phase separations in the temperature-compositional space and hence the associated entropy changes within the systems, taking into account vibrational, electronic, and configurational contributions to the total entropy. The first-principles calculations covered 178 phases ranging from pure elements, the ordered B2, B32, B2 3 , B2 2 , hR8, hR7, tI6, C15, and D0 3 binary phases, two ordered MoNbTaW quaternary phases, and the partially disordered and completely disordered bcc phases. By sorting their relative phase stabilities with the Dantzig's simplex minimization algorithm, the possibilities of phase separation for the refractory quaternary and quinary HEAs were thermodynamically found in the temperature range of 500e907 K.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Uncovering the role of nanoscale precipitates on martensitic transformation and superelasticity

Here we characterize the role of coherent nanoscale B2 Ni 50 Al 50 precipitates on the temperature- and stress-induced martensitic phase transformation in nanocrystalline Ni 63 Al 37 shape memory alloys using multi-million-atoms molecular dynamics (MD) simulations. We studied two types of precipitates: one with single crystal precipitates (SXP) and a second where grain boundaries cut through precipitates (PXP). Simulations reveal that the presence of B2 precipitates stabilizes the cyclic flag-shaped stress-strain response, characteristic of superelasticity, and reduces remnant strain. In contrast, single-phase nanocrystalline Ni 63 Al 37 exhibits degradation of the reverse transformation during cyclic loading and, eventually, incomplete reversible transformation within a few cycles. This is consistent with previous experimental findings of ultra-low fatigue in Ni-Ti-Cu alloys with Ti 2 Cu precipitates. The simulations reveal that the presence of precipitates significantly improves the reversibility of the transformation by acting as elastic zones that partially shield the martensitic transformation and drive the reverse transformation. A detailed analysis of the MD trajectories reveals that the martensitic transformation of the matrix induces ultra-large elastic deformation in some of the B2 precipitates (approximately 12%) to the point of resulting in a martensite-like atomic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Decoupling size and surface effects of intermetallic CuPd nanocrystals for electrocatalytic nitrate reduction to ammonia

Nitrate pollution poses a major environmental challenge, but its electrochemical conversion to ammonia offers a sustainable waste-to-value solution. Here, in this study, we synthesized monodisperse, size-tunable B2-phase CuPd intermetallic nanocrystals (6–46 nm) and studied their performance in the electrochemical nitrate reduction reaction (eNO 3 RR). By using bromide ions to modulate Pd reduction and applying mild annealing, we achieved phase-pure B2 structures across all sizes. Catalytic testing revealed a volcano-like trend in ammonia yield, peaking at 33 nm nanocubes with a rate of 6.97 mol h −1 g −1 at −0.6 V vs. reversible hydrogen electrode (RHE). This optimum reflects a balance between the increased surface area of smaller particles and the enhanced exposure of active (100) facets in larger ones. Theoretical calculations indicated that the B2-CuPd (100) facet is favorable for nitrate adsorption, thereby supporting the high activity of nanocubes. Our results highlight the critical role of tuning both nanoparticle size and surface structure to maximize eNO 3 RR efficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A study of stress-induced phase transformation and micromechanical behavior of CuZr-based alloy by in-situ neutron diffraction

The stress-induced phase transformation and micromechanical behavior of CuZr-based alloy were investigated by in-situ neutron diffraction. The pseudoelastic behavior with a pronounced strain-hardening effect is observed. The retained martensite nuclei and the residual stress obtained from the 1st cycle reduce the stress threshold for the martensitic transformation. A critical stress level is required for the reverse martensitic transformation from martensite to B2 phase. An increase of intensity for the B2 (110) plane in the 1st cycle is caused by the twinning along the {112}<111> twinning system. The convoluted stress partitioning influenced by the elastic and transformation anisotropy along with the newly formed martensite determines the microstress partitioning of the studied CuZr-based alloy. The reversible martensitic transformation is responsible for the pseudoelasticity. The macro mechanical behavior of the pure B2 phase can be divided into 3 stages, which are mediated by the evolvement of the martensitic transformation. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effect of synthesis temperature on the phase formation of NiTiAlFeCr compositionally complex alloy thin films

Here, the synthesis temperature dependent phase formation of Ni 10 Ti 10 Al 25 Fe 35 Cr 20 thin films is compared to a bulk processed sample of identical composition. The as-cast alloy exhibits a dual-phase microstructure which is composed of a disordered BCC phase and AlNiTi-based B2- and/or L2 1 -ordered phase(s). Formation of the BCC phase as well as an ordered AlNi-based B2 phase is observed for a thin film synthesised at 500 °C (ratio of synthesis temperature of thin film to melting temperature of bulk alloy: T/T m = 0.49), which is attributed to both surface and bulk diffusion mediated growth. Post deposition annealing at 900 °C (T/T m = 0.75) of a thin film deposited without intentional heating results in the formation of NiAlTi-based B2 and/or L2 1 -phase(s) similar to the bulk sample, which is attributed to bulk diffusion. Depositions conducted at room temperature without intentional substrate heating (T/T m = 0.20) resulted in the formation of an X-ray amorphous phase, while a substrate temperature increase to 175 °C (T/T m = 0.28) causes the formation of a BCC phase. Atom probe tomography of the thin films deposited without intentional substrate heating and at 175 °C indicates the formation of ~5 nm and ~10 nm FeAl-rich domains, respectively. This can be rationalized based on the activation energy for surface diffusion, as Ti and Ni exhibt 2.5 to 4 times larger activation energy barriers than Al, Fe and Cr. It is evident from the homologous temperature that the phase formation observed at 500 °C (T/T m = 0.49) is a result of both surface and bulk diffusion. As the temperature is reduced, the formation of FeAl-rich domains can be understood based on the differences in activation energy for surface diffusion and is consistent with kinetically limited thin film growth.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-pressure high-temperature melting and recrystallization of nanolamellar high-entropy alloys

Additively manufactured (AM) High Entropy Alloys (HEAs) are notable for their exceptional high-yield strength and large tensile ductility. The nanolamellar Eutectic HEA (EHEA) AlCoCrFeNi 2.1 was fabricated by laser powder bed fusion (L-PBF) in the as-printed form (EHEA1) and subsequently annealed at 1000oC (EHEA2) and 600oC (EHEA3) to achieve a broad range of mechanical properties. EHEA2 and EHEA3 samples were studied using Scanning transmission electron microscopy (STEM), energy dispersive X-ray diffraction (EDXRD) at high-pressures and high temperatures, nanoindentation hardness and modulus measurements. According to EDXRD EHEA2 and EHEA3 are composed of B2 and L1 2 phases. High-pressure high-temperature EDXRD studies show melting for EHEA2 at 1698 ± 25 K at a pressure of 6.5 GPa and melting for EHEA3 at 1598 ± 25 K at a pressure of 5.8 GPa. Post-melt and recrystallized samples were recovered at ambient conditions, and XRD analysis showed retention of B2 and L1 2 phases, although a new σ phase appeared for both EHEA2 and EHEA3 samples due to high-pressure and high-temperature melting and recrystallization experiment. SEM analysis also demonstrated the preservation of the nanolamellar morphology. Nanoindentation studies revealed that recrystallized EHEAs retain their original mechanical property hierarchy, with EHEA3 being 47% harder than EHEA2, largely related to higher content of B2-phase and retention of nanolamellar morphology. We also present Pressure-Volume-Temperature (P-V-T) data for 3-D printed and annealed eutectic high entropy alloys and extract mechanical and thermal properties data.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Structural and microstructural study of nanostructured Fe{sub 50}Al{sub 40}Ni{sub 10} powders produced by mechanical alloying

A nanostructured Fe{sub 50}Al{sub 40}Ni{sub 10} mixture was prepared by mechanical alloying of elemental Fe, Al and Ni powders in a planetary ball mill. Structural and microstructural changes during the milling process were followed by X-ray diffraction technique. The patterns so obtained were analyzed using the Maud program. An ordered B2 FeAl phase is formed after 1 h of milling. The observed lattice expansion is related to the production of antisite defects; Fe{sub Al} and Al{sub Fe}. During the intermediate stages of milling, the mechanical alloying process gives rise to a mixture of two BCC {alpha}{sub i}-Fe(Al,Ni) (i = 1,2) structures with the same crystallite size but different lattice parameters, microstrains and proportions. The BCC {alpha}{sub 2}-Fe(Al,Ni) disappeared after 4 h, only the B2 FeAl and BCC {alpha}{sub 1}-Fe(Al,Ni) solid solution persist over prolonged milling times. - Research highlights: {yields} Fe{sub 50}Al{sub 40}Ni{sub 10} was prepared by MA from Fe, Al and Ni powders in a planetary ball mill. {yields} B2 FeAl is formed after 1 h of MA. Lattice expansion is related to Fe{sub Al} and Al{sub Fe} defects. {yields} MA gives rise to 2 BCC structures with the same L but different a, <{sigma}{sup 2}>{sup 1/2} and %.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure and phase composition characterization in a Co{sub 38}Ni{sub 33}Al{sub 29} ferromagnetic shape memory alloy

Transmission electron microscopy was performed to investigate the microstructures of a secondary phase and its surrounding matrix in a Co{sub 38}Ni{sub 33}Al{sub 29} ferromagnetic shape memory alloy. The secondary phase shows a γ′ L1{sub 2} structure exhibiting a dendritic morphology with enclosed B2 austenite regions while the matrix shows the L1{sub 0} martensitic structure. A secondary phase-austenite-martensite sandwich structure with residual austenite ranging from several hundred nanometers to several micrometers wide is observed at the secondary phase-martensite interface due to the depletion of Co and enrichment of Al in the chemical gradient zone and the effect of the strong martensitic start temperature dependency of the element concentrations. The crystallographic orientation relationship of the secondary phase and the B2 austenite fits the Kurdjumov-Sachs relationship. - Highlights: •The secondary phase has a γ′ L1{sub 2} structure exhibiting a dendritic morphology. •A secondary phase-austenite-martensite sandwich structure is observed. •The structural sandwich structure is due to elemental composition variation. •The secondary phase and the B2 austenite fit the Kurdjumov-Sachs relationship.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Selective growth of Ni4Ti3 precipitate variants induced by complicated cyclic stress during laser additive manufacturing of NiTi-based composites

Highlights: • The NiTi-based composites were in-situ synthesized by SLM. • Ni{sub 4}Ti{sub 3} could tailor the phase transformation temperature of TiC/NiTi composites. • A three-dimensional stress field modeling of TiC/NiTi composites was established. • The relationship between growth of Ni{sub 4}Ti{sub 3} variants and cyclic stress was disclosed. - Abstract: NiTi-based composites were in-situ synthesized by selective laser melting (SLM) additive manufacturing technology with formation of a great many Ni{sub 4}Ti{sub 3} precipitates. The differential scanning calorimetry (DSC) were carried out to exhibit the similar phase transformation temperature (A{sub s} = 51.82 °C, A{sub f} = 94.02 °C, M{sub s} = 55.87 °C and M{sub f} = 18.93 °C) by comparing with that of SLM-fabricated equi-atomic NiTi alloy, which could be attributed to the compensation of Ni{sub 4}Ti{sub 3} precipitates for the effect of an addition of TiC particles. Three different Ni{sub 4}Ti{sub 3} precipitate variants ((111){sub R}//(1 1-bar 1-bar ){sub B2}, (111){sub R}//( 1-bar 1-bar 1){sub B2}, and (111){sub R}//( 1-bar 1 1-bar ){sub B2}) with various precipitate diameters were observed within SLM fabricated part, showing apparently selective growth behavior. By high-resolution transmission electron microscopy, severe lattice defects formed surrounding the smaller variant, while relatively intact lattice structure was found around the larger one, which indicated that high strain energy was produced in the interface between the matrix and the smaller variant. Furthermore, finite element simulation method was applied to disclose the evolution features of complicated cyclic stress during SLM processing of TiC/NiTi composites. By predicting, the main constraint mode within as-fabricated track was tensile stress effect. Based on the redistribution behavior of thermal stress and the coupling effect between the thermal stress and strain field induced by Ni{sub 4}Ti{sub 3} precipitate, the selective growth mechanism of Ni{sub 4}Ti{sub 3} variant was revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Nb/Ti/V/Ta on phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic stainless steels

Highlights: • Effects of Nb/Ti/V/Ta on phase precipitation at 1073 K in AFA alloys are studied. • The precipitation of σ-FeCr can be accelerated by the formation of B2-NiAl. • The additions of V and Ta can improve the oxidation resistance of AFA alloys. • The precipitation of σ-FeCr will deteriorate the oxidation resistance. • Good performance can be achieved by M co-alloying and an appropriate M/C ratio. - Abstract: The present work investigated the effects of strong carbide-forming elements M (M = Nb, Ti, V, Ta) on the phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic (AFA) stainless steels (Fe-20Ni-(18–23)Cr-2.5Al-2.3Mo-0.08C-Nb/Ti/V/Ta wt%). The designed alloy rods were solid-solutioned at 1523 K for 1.5 h, and then aged at 1073 K for 24 h or oxidized at 1073 K for 500 h, respectively. Besides primary MC carbides, B2-NiAl, Fe{sub 2}M, and σ-FeCr phases would be precipitated from the γ-austenitic matrix in these aged AFA alloys. Furthermore, the precipitation of σ-FeCr could be accelerated by the B2-NiAl formation, which is related to the γ stability. The formation of MC and Fe{sub 2}M can be controlled via adjusting the ratio of M/C. The oxidation resistance of these AFA alloys can be improved effectively by V and Ta addition due to the formation of compact Al{sub 2}O{sub 3} protective scales. This work will provide a new approach to obtain a good combination of creep strength and oxidation resistance through co-alloying with Nb/Ta/V and controlling the M/C ratio within 1.0–2.0 (in molar ratio) simultaneously.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior of Widmanstätten O phase associated with interface in aged Ti2AlNb-based alloys

Highlights: • Ball milling tunes the morphology of primary O phase in Ti{sub 2}AlNb alloys. • The interface of Widmanstatten B2 and O affects the hardness of the alloys. • The entire precipitation process of Widmanstatten O phase is reported. - Abstract: Ti{sub 2}AlNb-based alloys, spark plasma sintered from ball-milled octahedral Ti-22Al-25Nb powder, are aged for 0–3 h in this study. Ball milling tunes the morphology of primary orthorhombic (O) phase to be columnar, and improves the mechanical properties of the alloys. The evolution of the Widmanstätten O phase, which brought main precipitation strengthening effect, is then investigated during the ageing process. The hardness of the alloys is improved by an increasing content of O phase and the reduced width of the Widmanstätten O phase; in addition, the length of Widmanstätten O phase also brings positive effect on the hardness, which indicates that the hardness is strongly coupled with the interface of the Widmanstätten O precipitates and B2 matrix. By means of transmission electron microscopy observation, the precipitation of O initials with an amorphous state, followed by a coherent interface between (110){sub B2} and (002){sub O}, which is destroyed to be semi-coherent and incoherent during the next ageing stage.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Transformation behavior and shape memory characteristics of thermo-mechanically treated Ti–(45−x)Ni–5Cu–xV (at%) alloys

Transformation behavior, shape memory characteristics and superelasticity of thermo-mechanically treated Ti–(45−x)Ni–5Cu–xV (at%) (x = 0.5–2.0) alloys were investigated by means of differential scanning calorimetry, transmission electron microscopy, X-ray diffractions, thermal cycling tests under constant load and tensile tests. The B2–B19′ transformation occurred when V content was 0.5 at%, above which the B2–B19–B19′ transformation occurred. The B2–B19 transformation was not separated clearly from the B19–B19′ transformation. Thermo-mechanically treated Ti–(45−x)Ni–5Cu–xV alloys showed perfect shape memory effect and transformation hysteresis(ΔT) of Ti–43.5Ni–5.0Cu–1.5V and Ti–43.0Ni–5.0Cu–2.0V alloys was about 9 K which was much smaller than that of a Ti–44.5Ni–5.0Cu–0.5V alloy(23.3 K). More than 90% of superelastic recovery ratio was observed in all specimens and transformation hysteresis (Δσ) of a Ti–44.5Ni–5.0Cu–0.5V alloy was about 70 MPa, which was much larger than that of a Ti–43.0Ni–5.0Cu–2.0V alloy (35 MPa).

36 MATERIALS SCIENCE↗
DOE OSTI2013

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Graphical abstract: - Highlights: • Amorphous Si-coated TiNi powders were prepared successfully by ball milling. • Ti{sub 4}Ni{sub 4}Si{sub 7} was formed at the interface between Si and TiNi after annealing. • Si-coated Ti–Ni powders displayed the R phase after annealing. - Abstract: Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO{sub 2} occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powders were recovered and Ti{sub 4}Ni{sub 4}Si{sub 7} was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The effect of the target-organ geometric complexity on the choice of delivery between RapidArc and sliding-window IMRT for nasopharyngeal carcinoma

We attempted to assess the effect of target-organ geometric complexity on the plan quality of sliding-window intensity-modulated radiotherapy (IMRT), double-arc (RA2), and triple-arc (RA3) RapidArc volumetric-modulated arc radiotherapy for nasopharyngeal carcinoma (NPC). Plans for 9-field sliding-window IMRT, RA2, and RA3 were optimized for 36 patients with NPC ranging from T1 to T4 tumors. Initially the patients were divided into 2 groups, with group A representing the most simple early stage (T1 and T2) cases, whereas group B represented the more complex advanced cases (T3 and T4). Evaluation was performed based on target conformity, target dose homogeneity, organ-sparing capability, and delivery efficiency. Based on the plan quality results, a subgroup of advanced cases, group B2, representing the most demanding task was distinguished and reported separately from the rest of the group B cases, B1. Detailed analysis was performed on the anatomic features for each group of cases, so that planners can easily identify the differences between B1 and B2. For the group A cases, RA3 plans were superior to the IMRT plans in terms of organ sparing, whereas target conformity and dose homogeneity were similar. For the group B1 cases, the RA3 plans produced almost equivalent plan quality as the IMRT plans. For the group B2 cases, for most of which large target volumes were adjacent to (5 mm or less) and wrapping around the brain stem, RA2 and RA3 were inferior to the IMRT regarding both target dose homogeneity and conformity. RA2 plans were slightly inferior to IMRT and RA3 plans for most cases. The plan comparison results depend on the target to brain stem distances and the target sizes. The plan quality results together with the anatomic information may allow the evaluation of the 3 treatment options before actual planning.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2025

Friction surface layer deposition of triple-phase Al 10 Cr 12 Fe 35 Mn 23 Ni 20 high entropy alloy: Process optimization and microstructural evolution

A high-strength Co-free triple-phase Al 10 Cr 12 Fe 35 Mn 23 Ni 20 high-entropy alloy (HEA) was successfully fabricated using Friction Surface Layer Deposition (FSLD), a bulk manufacturing method. Multiple single-layer deposits were produced by varying forging force (F) and traverse speeds (T r ) to optimize the process parameters. The optimized conditions (F = 40 kN & T r = 200 mm/min) were then applied to manufacture a scaled-up multi-layer specimen. The initial microstructure of the HEA consisted of coarse grains of the soft FCC-phase, long columnar dendrites of the hard BCC-phase, and small precipitates of the harder B2-phase within the BCC-dendrites. During FSLD, the FCC-matrix underwent continuous dynamic recrystallization due to high-temperature severe plastic deformation, forming finer equiaxed grains. Simultaneously, the BCC-dendrites fractured into smaller fragments, some of which experienced partial growth and coarsening under applied stress, resulting in an hourglass morphology. In contrast, the small B2-precipitates within the BCC-fragments dissolved during the elevated temperatures of FSLD and reprecipitated as substantially finer precipitates during continuous cooling post-FSLD. Additionally, the orientation relationships between the FCC and BCC/B2 phases were completely destroyed by the severe thermoplastic deformation inherent to FSLD. The microstructural refinements led to a substantial improvement in hardness from 177 HV to 283 HV, driven by Hall-Petch strengthening. The increased number of interfaces, including coherent BCC-B2 interfaces, potentially enhances the sink strength and radiation tolerance of the HEA, making it a promising candidate for nuclear applications. In conclusion, this study also highlights FSLD as a versatile technique for achieving tunable properties in HEAs, with detailed schematics illustrating the complex mechanisms of phase transformations during processing.

Additive Manufacturing↗
DOE OSTI2018

Tumor-endothelial cell interaction in an experimental model of human hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a densely vascularized tumor that is highly dependent on angiogenic pathways to direct arterial blood flow to the growing neoplasm, though little is known about how the interaction of tumor and endothelial cells drives these processes and the degree of clinical importance. To this end, we examined the intercellular cross-talk between HepG2 (human HCC) and human endothelial progenitor cells (EPC) in a co-culture system that mimics some aspects of initial tumor parenchyma and stroma interactions. The results showed that the remote cell-to-cell (paracrine) interactions between HepG2 cells and EPC play a critical role in the differentiation and angiogenic activity of endothelial cells, possibly through intercellular signaling function of the exosomes released in the medium by HepG2 cells. The tumor-cell activated phenotype of EPC was associated with increased migration and elevated expression of ephrin-B2, and Delta-like 4 ligand (DLL4). Furthermore, ephrin-B2 was found to be overexpressed in HCC and cholangiocarcinoma tissue samples taken from humans. Overall, our results demonstrate that ephrin-B2 and Dll4 mediated co-dependence of HCC and EPC intercellular crosstalk in the initial stages of HCC establishment and development, a promising target for new clinical strategies.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn 2 O 4

Spinel compounds AB 2 X 4 consist of both tetrahedral (AX 4 ) and octahedral (BX 6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn 2 O 4 has been reported to exhibit one structural transition and two successive magnetic transitions. In this work, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn 2 O 4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be ($Mn_{0.9}^{2+}Fe_{0.1}^{3+}$)$_{A}$($Mn^{3+}Fe_{0.9}^{3+}Mn_{0.1}^{2+}$)$_{B}$O 4 . While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at T S ≈ 750 K, driven by the Jahn–Teller effect of the orbital active B-site Mn 3+ cation. Strong magnetoelastic coupling is unveiled at T N1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to T N2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn 3+ /Fe 3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ~25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O–B–O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn 2 O 4 . Our study indicates that FeMn 2 O 4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

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

Chemically related 4,5-linked aminoglycoside antibiotics drive subunit rotation in opposite directions

>Dynamic remodelling of intersubunit bridge B2, a conserved RNA domain of the bacterial ribosome connecting helices 44 (h44) and 69 (H69) of the small and large subunit, respectively, impacts translation by controlling intersubunit rotation. Here we show that aminoglycosides chemically related to neomycin—paromomycin, ribostamycin and neamine—each bind to sites within h44 and H69 to perturb bridge B2 and affect subunit rotation. Neomycin and paromomycin, which only differ by their ring-I 6'-polar group, drive subunit rotation in opposite directions. This suggests that their distinct actions hinge on the 6'-substituent and the drug’s net positive charge. By solving the crystal structure of the paromomycin–ribosome complex, we observe specific contacts between the apical tip of H69 and the 6'-hydroxyl on paromomycin from within the drug’s canonical h44-binding site. These results indicate that aminoglycoside actions must be framed in the context of bridge B2 and their regulation of subunit rotation.

Science & Technology - Other Topics↗