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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Structure of the human SAGA coactivator complex

The SAGA complex is a regulatory hub involved in gene regulation, chromatin modification, DNA damage repair and signaling. While structures of yeast SAGA (ySAGA) have been reported, there are noteworthy functional and compositional differences for this complex in metazoans. Here we present the cryogenic-electron microscopy (cryo-EM) structure of human SAGA (hSAGA) and show how the arrangement of distinct structural elements results in a globally divergent organization from that of yeast, with a different interface tethering the core module to the TRRAP subunit, resulting in a dramatically altered geometry of functional elements and with the integration of a metazoan-specific splicing module. Our hSAGA structure reveals the presence of an inositol hexakisphosphate (InsP 6 ) binding site in TRRAP and an unusual property of its pseudo-(Ψ)PIKK. Finally, we map human disease mutations, thus providing the needed framework for structure-guided drug design of this important therapeutic target for human developmental diseases and cancer.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Machine learning of 27Al NMR electric field gradient tensors for crystalline structures from DFT

NMR crystallography has emerged as a promising technique for the determination and refinement of atomic coordinates in crystal structures. The crystal structure of compounds containing quadrupolar nuclei, such as 27Al, can be improved by directly comparing solid-state NMR measurements to DFT computations of the electric field gradient (EFG) tensor. The non-negligible computational cost of these first-principles calculations limits the applicability of this method to all but the most well-defined structures. We developed a fast, low-cost machine learning model to predict EFG parameters based on local structural motifs and elemental parameters. We computed 8081 EFG tensors from 1681 27Al crystalline solids using DFT and benchmarked them against 105 experimentally measured 27Al sites. Surprisingly, simple local geometric features dominate the predictive performance of the resulting random-forest model, yielding an R2 value of 0.98 and an RMSE of 0.61 MHz for CQ, the quadrupolar coupling constant. This model accuracy should enable pre-refining future structural assignments before finally validating with first-principles calculations. Such a catalogue of 27Al NMR tensors can serve as a tool for researchers assigning complex NMR spectra influenced by the nuclear electric quadrupole interaction.

Sun, He↗
DOE OSTI2021

Crystal structure of the human PRPK–TPRKB complex

Mutations of the p53-related protein kinase (PRPK) and TP53RK-binding protein (TPRKB) cause Galloway-Mowat syndrome (GAMOS) and are found in various human cancers. We have previously shown that small compounds targeting PRPK showed anti-cancer activity against colon and skin cancer. Here we present the 2.53 Å crystal structure of the human PRPK-TPRKB-AMPPNP (adenylyl-imidodiphosphate) complex. The structure reveals details in PRPK-AMPPNP coordination and PRPK-TPRKB interaction. PRPK appears in an active conformation, albeit lacking the conventional kinase activation loop. We constructed a structural model of the human EKC/KEOPS complex, composed of PRPK, TPRKB, OSGEP, LAGE3, and GON7. Disease mutations in PRPK and TPRKB are mapped into the structure, and we show that one mutation, PRPK K238Nfs*2, lost the binding to OSGEP. Our structure also makes the virtual screening possible and paves the way for more rational drug design.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Structures of the ApoL1 and ApoL2 N-terminal domains reveal a non-classical four-helix bundle motif

Apolipoprotein L1 (ApoL1) is a circulating innate immunity protein protecting against trypanosome infection. However, two ApoL1 coding variants are associated with a highly increased risk of chronic kidney disease. Here we present X-ray and NMR structures of the N-terminal domain (NTD) of ApoL1 and of its closest relative ApoL2. In both proteins, four of the five NTD helices form a four-helix core structure which is different from the classical four-helix bundle and from the pore-forming domain of colicin A. The reactivity with a conformation-specific antibody and structural models predict that this four-helix motif is also present in the NTDs of ApoL3 and ApoL4, suggesting related functions within the small ApoL family. The long helix 5 of ApoL1 is conformationally flexible and contains the BH3-like region. This BH3-like α-helix resembles true BH3 domains only in sequence and structure but not in function, since it does not bind to the pro-survival members of the Bcl-2 family, suggesting a Bcl-2-independent role in cytotoxicity. These findings should expedite a more comprehensive structural and functional understanding of the ApoL immune protein family.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Structural and biophysical insights into targeting of claudin-4 by a synthetic antibody fragment

Claudins are a 27-member family of ~25 kDa membrane proteins that integrate into tight junctions to form molecular barriers at the paracellular spaces between endothelial and epithelial cells. As the backbone of tight junction structure and function, claudins are attractive targets for modulating tissue permeability to deliver drugs or treat disease. However, structures of claudins are limited due to their small sizes and physicochemical properties—these traits also make therapy development a challenge. Here we report the development of a synthetic antibody fragment (sFab) that binds human claudin-4 and the determination of a high-resolution structure of it bound to claudin-4/enterotoxin complexes using cryogenic electron microscopy. Structural and biophysical results reveal this sFabs mechanism of select binding to human claudin-4 over other homologous claudins and establish the ability of sFabs to bind hard-to-target claudins to probe tight junction structure and function. The findings provide a framework for tight junction modulation by sFabs for tissue-selective therapies.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2025

Characterizing the vertical structure of forests in the Brazilian Amazon

Little is known about the structure of tropical forests despite its critical role in the provisioning of ecosystem services. Here we assess the vertical structure of forests in the Brazilian Amazon with a large-scale airborne LiDAR dataset. We show that fire has greater impact in the lowest forest strata, differently from selective logging and windthrow. We also find that secondary forests quickly recover or even exceed reference areas at the 1-10 m height stratum but that full recovery for the 20-30 m height stratum has not been achieved even after 35 years. Our modeling results suggest that proximity to roads, elevation, precipitation, soil pH, and proportion of sand in the soil are the most important predictors of forest structure. Finally, we identify 5 forest structural types (FSTs) and use them to visualize the spatial distribution of forest structure. This study provides important information for forest monitoring, management, and conservation.

Biogeography↗
DOE OSTI2020

Harnessing the surface structure to enable high-performance cathode materials for lithium-ion batteries

The ever-increasing demand for high-performance batteries have been driving the fundamental understanding of the crystal/surface structural and electrochemical properties of intercalation cathode materials, among which the olivine-type, spinel and layered lithium transition metal oxide materials have received particular attention in the past decade due to their successful commercialization. While the most current studies focus on the macroscopic and bulk crystal structure of these materials, our previous work suggests that, as a confined region where charge transfer takes place, the interfacial structures of cathode materials largely dictate their electrochemical performances due to the structural symmetry breaking from 3D (bulk) to 2D (surface), which leads to reconstructions under the different chemical/electrochemical conditions. By summarizing works in this subject and offering our perspectives, this tutorial review will reveal for the first time the correlation between surface structure and interface reconstruction at atomic/molecular scales and their direct impact on corresponding electrochemical performances. More importantly, by extending the knowledge obtained from these three well-studied system, we believe that the same principles established could universally apply to other cathode materials that have been the frontiers of new battery chemistries.

25 ENERGY STORAGE↗
DOE OSTI2020

EXAFS investigations of temperature-dependent structure in cobalt-59 molecular NMR thermometers

Cobalt-59 nuclei are known for extremely thermally sensitive chemical shifts (δ), which in the long term could yield novel magnetic resonance thermometers for bioimaging applications. In this manuscript, we apply extended X-ray absorption fine structure (EXAFS) spectroscopy for the first time to probe the exact variations in physical structure that produce the exceptional thermal sensitivity of the 59 Co NMR chemical shift. We apply this spectroscopic technique to five Co(III) complexes: [Co(NH3)6]Cl3 (1), [Co(en) 3 ]Cl 3 (2) (en = ethylenediamine), [Co(tn) 3 ]Cl 3 (3) (tn = trimethylenediamine), [Co(tame) 2 ]Cl 3 (4) (tame = 1,1,1-tris(aminomethyl)ethane), and [Co(diNOsar)]Cl 3 (5) (diNOsar = dinitrosarcophagine). The solution-phase EXAFS data reveal increasing Co–N bond distances for these aqueous complexes over a ~50 °C temperature window, expanding by Δr (Co–N) = 0.0256(6) Å, 0.0020(5) Å, 0.0084(5) Å, 0.0006(5) Å, and 0.0075(6) Å for 1–5, respectively. Computational analyses of the structural changes reveal that increased connectivity between the donor atoms encourages complex structural variations. Finally, these results imply that rich temperature-dependent structural variations define 59 Co NMR thermometry in macrocyclic complexes.

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

Nanomorphology of eco-friendly colloidal inks, relating non-fullerene acceptor surface energy to structure formation

Nanoengineered, eco-friendly, solution-processable electroactive materials are in demand for the growing field of printed electronics, and these material requirements can be achieved by the development of waterborne colloidal dispersions. Functionality in these composite materials can be tuned by thermodynamically modifying the material nanomorphology, often by creation of kinetically stabilized aqueous nanoparticle dispersions. In this work we demonstrate that the internal structure of organic nanoparticles is controlled by the surface energy difference between the polymeric donor material and the non-fullerene acceptor (NFA) material. Nanoparticles of the following donor–acceptor combinations, suitable for printed organic photovoltaics, have been synthesized: TQ1:N2200, TQ1:PNDIT10, P3HT:N2200, P3HT:o-IDTBR and P3HT:eh-IDTBR. Advanced synchrotron-based X-ray spectroscopy and microscopy are used to correlate the formation of core–shell nanoparticle morphology to the material surface energy. We subsequently present a viable avenue for customizing the blended nanoparticle structure into (i) core–shell, (ii) molecularly intermixed, or (iii) inverted shell–core structures. Our results showed that TQ1:PNDIT10 and P3HT:o-IDTBR nanoparticles were comprised of a donor-rich shell and an NFA-rich core, however, interestingly we show a reversal to the inverse NFA shell/donor core structure for TQ1:N2200, P3HT:N2200 and P3HT:eh-IDTBR nanoparticles, driven by the low surface energy of N2200 (23.7 mJ m –2 ) and eh-IDTBR (18.3 mJ m –2 ). This article is the first report of a flipped nanoparticle core–shell morphology comprising an NFA-rich shell for the miniemulsion synthesis route. The composition of the shells and cores was able to be controlled by the differential mismatch in the surface energy of the donor and acceptor materials, with ΔG surface > 0, ΔG surface = 0, and ΔG surface < 0 for acceptor core–donor shell, molecularly intermixed, and acceptor shell–donor core, respectively. Accordingly, we introduce an entirely overlooked new figure of merit (FoM) for customizing nanoparticulate colloidal inks: tunable surface energy of non-fullerene-based semiconductors. Finally, the establishment of this FoM opens up electroactive material design to a wide range of functional printing applications with varying device and ink structure requirements, thereby reshaping the nanoengineering toolkit for waterborne colloidal dispersions and hence printed electronics.

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

Intercalation–exfoliation processes during ionic exchange reactions from sodium lepidocrocite-type titanate toward a proton-based trititanate structure

Topochemical reactions involving ionic exchange have been used to assess a large number of metastable compositions, particularly in layered metal oxides. This method encompasses complex reactions that are poorly explored, yet are of prime importance to understand and control the materials’ properties. In this work, we embark on investigating the reactions involved during the ionic exchange between a layered Na-titanate (lepidocrocite-type structure) and an acidic solution (HCl), leading to a protonic (H 3 O + ) titanate (trititanate structure). The reactions involve an ionic exchange provoking a structural change from the lepidocrocite-type to the trititanate structure as shown by real-space refinements of ex situ pair distribution function data. Mobile Na + ions are exchanged by hydronium ions inducing high proton mobility in the final structure. Moreover, the reaction was followed by ex situ 23 Na and 1 H solid-state MAS NMR which allowed, among other things, confirming that the Na + ions are in the interlayer space and specifying their local environment. Strikingly, the ionic exchange reaction induces progressive exfoliation of the Na-titanate particles leading to 2–5 nm thin elongated crystallites. To further understand the different steps associated with the ionic exchange, the evolution of the electrolytic conductivity, using conductimetric titration, has been monitored upon HCl addition, enabling characterization of the intercalation(H + )/de-intercalation(Na + ) reactions and assessing kinetic parameters. Accordingly, it is hypothesized that the exfoliation of the particles is due to the accumulation of charges at the particle level in relation to the rapid intercalation of protons. This work provides novel insights into ionic exchange reactions involved in layered oxide compounds.

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

Computational investigations of Dienes defect- and vacancy-induced changes in the electronic and vibrational properties of carbon fiber structural units

Carbon fiber (CF) is a promising lightweight alternative to steel and is of significant interest for energy applications. As CF continues to find new uses and is exposed to new external conditions, a noninvasive method of monitoring its structural integrity is critical. Raman spectroscopy is a commonly used method for this monitoring; however, it is highly inferential, and the interpretation of the data is not always straightforward. In this work, we perform density functional theory (DFT) calculations to investigate changes in the vibrational properties of CF structural units (i.e., graphene and graphite) caused by monovacancy and Dienes defects as a foundation for modeling more complex defects that move our model toward that of realistic CF. Using large computational supercells, we can understand how these defects change the electronic structure and vibrational properties of graphene and graphite for interdefect distances near those of the lower experimental limit. The monovacancy opens an electronic bandgap at the K point. Although no such electronic gap is opened by the Dienes defect, both defects introduce flat defect bands near the Fermi energy. The Dienes defect creates long-range deviations of the phonons, leading to substantial broadening of the highest frequency optical modes in the band structure compared to that of the pristine material. In contrast, the phonon changes caused by the monovacancy are short range, and only minor changes in the band structure or phonon density of states were observed. These findings can assist in the interpretation of experimental results by providing atomic-scale insight into key electronic and vibrational features.

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

Broadband light emitting zero-dimensional antimony and bismuth-based hybrid halides with diverse structures

Low-dimensional organic–inorganic metal halides have recently attracted extensive attention because of their various structures and distinguished photoelectric properties. Herein, we report a series of new zero-dimensional organic–inorganic hybrid metal halides: (TMEDA) 3 Bi 2 Cl 12 ·H 2 O, (TMEDA) 3 Bi 2 Br 12 ·H 2 O, (TMEDA) 3 Sb 2 Br 12 ·H 2 O, and (TMEDA) 5 Sb 6 Cl 28 ·2H 2 O [TMEDA = N,N,N'·trimethylethylenediamine]. (TMEDA) 3 M 2 X 12 ·H 2 O (M = Bi or Sb, X = Cl or Br) crystallizes in the monoclinic space group P2 1 /n, and (TMEDA) 5 Sb 6 Cl 28 ·2H 2 O crystallizes in the orthorhombic space group Pnma. (TMEDA) 3 M 2 X 12 possesses a zero-dimensional structure with the metal halide ions of [MBr 6 ] 3- isolated by the organic TMEDA 2+ cations. Interestingly, the (TMEDA) 5 Sb 6 Cl 28 ·2H 2 O structure consists of a combination of corner-connected octahedra [Sb 4 Cl 18 ] 6- and edge-shared [Sb 2 Cl 10 ] 4- , which is quite rare. The light emission of all these compounds was measured, and (TMEDA) 3 Sb 2 Br 12 ·H 2 O exhibits the most intense luminescence. Upon 400 nm ultraviolet light excitation, (TMEDA) 3 Sb 2 Br 12 ·H 2 O exhibited strong broadband yellow emission centered at 625 nm with a full-width at half-maximum of ~150 nm originating from self-trapped excitons. In conclusion, this work suggests the possibility of new types of hybrid halides by introducing different metal centers and probing the structural evolution and photoluminescent properties, serving as a reference for the relationship between structure and luminescent performance and demonstrating their potential use as phosphors in light-emitting diodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Revisiting the K-edge X-ray absorption fine structure of Si, Ge–Si alloys, and the isoelectronic series: CuBr, ZnSe, GaAs, and Ge

Extended X-ray absorption fine structure (EXAFS) has evolved into an unprecedented local-structure technique that is routinely used to study materials’ problems in the biological, chemical, and physical sciences. Like many other experimental techniques, EXAFS also requires that several key atomic parameters must be known a priori before structural information can be quantitatively determined. Utilizing current analytical methods, we revisit the isoelectronic series CuBr, ZnSe, GaAs, and Ge originally studied by Stern et al. during the early development of EXAFS. We demonstrate that the ab initio EXAFS code FEFF accurately predicts the atomic phase shifts and backscattering amplitudes that are primarily functions of the sum of atomic numbers Z along an EXAFS scattering path. We also investigate quantitative fitting and first- and second-shell phase transferability together with problems that arise if a backscattering atom is identified incorrectly in an EXAFS fitting model. Features in the near-edge region, on the other hand, are shown to require a comprehensive treatment of the band structure and density-of-states, including effects of the screened Coulomb interaction between the photoelectron and core hole. Here, we demonstrate that the Bethe–Salpeter equation (BSE) accurately captures the NEXAFS (or XANES) portion of the spectrum for the isoelectronic series in addition to Si and Ge–Si alloys, including within a few eV of the absorption edge, where band structure and excitonic effects are most important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The solution structures and relative stability constants of lanthanide–EDTA complexes predicted from computation

Ligand selectivity to specific lanthanide (Ln) ions is key to the separation of rare earth elements from each other. Ligand selectivity can be quantified with relative stability constants (measured experimentally) or relative binding energies (calculated computationally). The relative stability constants of EDTA (ethylenediaminetetraacetic acid) with La 3+ , Eu 3+ , Gd 3+ , and Lu 3+ were predicted from relative binding energies, which were quantified using electronic structure calculations with relativistic effects and based on the molecular structures of Ln–EDTA complexes in solution from density functional theory molecular dynamics simulations. The protonation state of an EDTA amine group was varied to study pH ~7 and ~11 conditions. Further, simulations at 25 °C and 90 °C were performed to elucidate how structures of Ln–EDTA complexes varying with temperature are related to complex stabilities at different pH conditions. Relative stability trends are predicted from computation for varying Ln 3+ ions (La, Eu, Gd, Lu) with a single ligand (EDTA at pH ~11), as well as for a single Ln 3+ ion (La) with varying ligands (EDTA at pH ~7 and ~11). Changing the protonation state of an EDTA amine site significantly changes the solution structure of the Ln–EDTA complex resulting in a reduction of the complex stability. As a result, increased Ln–ligand complex stability is correlated to reduced structural variations in solution upon an increase in temperature.

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

Structural elucidation of polydopamine facilitated by ionic liquid solvation

Minimal understanding of the formation mechanism and structure of polydopamine (pDA) and its natural analogue, eumelanin, impedes the practical application of these versatile polymers and limits our knowledge of the origin of melanoma. The lack of conclusive structural evidence stems from the insolubility of these materials, which has spawned significantly diverse suggestions of pDA's structure in the literature. We discovered that pDA is soluble in certain ionic liquids. Using these ionic liquids (ILs) as solvents, we present an experimental methodology to solvate pDA, enabling us to identify pDA's chemical structure. The resolved pDA structure consists of self-assembled supramolecular aggregates that contribute to the increasing complexity of the polymer. The underlying molecular energetics of pDA solvation and a macroscopic picture of the disruption of the aggregates using IL solvents have been investigated, along with studies of the aggregation mechanism in water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Materials design, synthesis, and transport properties of disordered rare-earth Zintl bismuthides with the anti -Th 3 P 4 structure type

The synthesis, structural elucidation, and transport properties of the extended series Ca 4–x RE x Bi 3 (RE = Y, La–Nd, Sm, Gd–Tm, and Lu; x ≈ 1) and Ca 4–x RE x Bi 3–δ Sb δ (RE = La, Ho, Er, and Lu; x ≈ 1, δ ≈ 1.5) are presented. Structural elucidation is based on single-crystal X-ray diffraction data and confirms the chemical drive of Ca4Bi3 with the cubic anti-Th 3 P 4 structure type (space group I4 ¯3d, no. 220, Z = 4) into a Zintl phase by the introduction of trivalent rare-earth atoms. The structure features complex bonding, heavy elements, and electron count akin to that of valence-precise semiconductors, making it an ideal target for thermoelectrics development. Introducing crystallographic site disorders at the cation site for the Ca 4–x RE x Bi 3 phase and both the cation and anion sites for the Ca 4–x RE x Bi 3–δ Sb δ phase brings about additional desirable characteristics for thermoelectric materials in the context of tuning knobs for lowering thermal conductivity. Electronic structure calculations of idealized Ca 3 YBi 3 and Ca 3 LaBi 3 compounds indicate the opening of indirect bandgaps at the Fermi level with magnitudes Eg = 0.38 eV and 0.57 eV, respectively. The electrical resistivity ρ(T) of some of the investigated phases measured on single crystals evolve in a metallic manner with magnitudes of order 1.4 mΩ cm near 500 K, thus supporting the notion of a degenerate semiconducting state, with the temperature dependence of the Seebeck coefficient α(T) suggesting the p-type behavior. Furthermore, the low electrical resistivity and the realization of a degenerate semiconducting state in the title phases present a window of opportunity for optimizing their carrier concentrations for enhanced thermoelectric performance.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Apex hydrogen bonds in dendron assemblies modulate close-packed mesocrystal structures

The close-packed mesocrystal structures from soft-matter assemblies have recently received attention due to their structural similarity to atomic crystals, displaying various sphere-packing Frank–Kasper (FK) and quasicrystal structures. Herein, diverse mesocrystal structures are explored in second-generation dendrons (G2-X) designed with identical wedges, in which the terminal functionalities X = CONH 2 and CH 2 NH 2 represent two levels of the strong and weak hydrogen-bonding apexes, respectively. The cohesive interactions at the core apex, referred to as the core interactions, are effectively modulated by forming heterogeneous hydrogen bonds between these two functional units. For the dendron assemblies compositionally close to each pure component of G2-CONH 2 and G2-CH 2 NH 2 , their own FK A15 and C14 phases dominate other phases, respectively. We show the existence of the wide-range FK σ including the dodecagonal quasicrystal (DDQC) phases from the dendron mixtures between G2-CONH 2 and G2-CH 2 NH 2 , providing an experimental phase sequence of A15–σ–DDQC–C14 as the core interactions are alleviated. Intriguingly, the temperature dependence of particle sizes shows that the high plateau values of particle sizes are maintained equivalently until each threshold temperature (T th ), followed by a prompt decrease above the T th . A decrease in T th by alleviating the core interactions and its composition dependence suggest that the more size-dispersed particles, the more susceptibility to chain exchange with increasing temperature. Here our results on the formation of supramolecular dendron assemblies provide a guide to understand the core-interaction-dependent mesocrystal structures toward the fundamental principle underlying the temperature dependence of their particle sizes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

NaCo 2 (SeO 3 ) 2 (OH): competing magnetic ground states of a new sawtooth structure with 3d 7 Co 2+ ions

While certain magnetic sublattices have long been known theoretically to give rise to emergent physics via competing magnetic interactions and quantum effects, finding such configurations in real materials is often deeply challenging. Here we report the synthesis and characterization of a new such material, NaCo 2 (SeO 3 ) 2 (OH) which crystallizes with a highly frustrated sublattice of sawtooth Co 2+ chains. Single crystals of NaCo 2 (SeO 3 ) 2 (OH) were synthesized using a low-temperature hydrothermal method. X-ray single crystal structure analysis reveals that the material crystallizes in orthorhombic space group of Pnma (no. 62). Its crystal structure exhibits one-dimensional chains of corner-sharing isosceles triangles that are made of two crystallographically distinct 3d 7 Co 2+ sites (Co(1) and Co(2)). The chains run along the b-axis and are interconnected via [SeO 3 ] groups to form a three-dimensional structure mediating super-exchange interactions. The temperature dependent magnetization data show a ferromagnetic-like (FM) transition at 11 K (T 1 ) followed by an antiferromagnetic (AFM) transition at about 6 K (T 2 ). Neutron-powder diffraction measurements reveal that at T 1 = 11 K only the Co(2) site orders magnetically, forming ferromagnetic zigzag chains along the b-axis. Below T 2 = 6 K, both Co(1) and Co(2) sites order in a nearly orthogonal configuration, with Co(1) moments lying inside the plane of the sawtooth chain while Co(2) moments cant out of the plane. The canting of the magnetic moments leads to a net ferromagnetic component along b-axis, parallel to the chain direction. The ordered moments are fully compensated in the ac-plane. Inelastic neutron scattering measurements reveal crystal field excitations that are consistent with the presence of a spin–orbital entangled pseudo-spin state J eff = 1/2 for the Co 2+ ions. Low-energy spin-wave excitations are observed below the second magnetic transition. The analysis of powder excitation spectrum suggests complex exchange interactions that go beyond a Heisenberg Hamiltonian model with nearest neighbor couplings. Our results demonstrate the richness of the magnetic properties of sawtooth-type structures and encourage the study of similar structures with different oxyanion groups.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗