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Two-dimensional covalent organic frameworks (2D-COFs) are a class of crystalline porous organic polymers that consist of covalently linked, two-dimensional sheets that can stack together through noncovalent interactions. Here we report the synthesis of a novel COF, called PyCOFamide, which has an experimentally observed pore size that is greater than 6 nm in diameter. This is among the largest pore size reported to date for a 2D-COF. PyCOFamide exhibits permanent porosity and high crystallinity as evidenced by the nitrogen adsorption, powder X-ray diffraction, and high-resolution transmission electron microscopy. We show that the pore size of PyCOFamide is large enough to accommodate fluorescent proteins such as Superfolder green fluorescent protein and mNeonGreen. This work demonstrates the utility of noncovalent structural reinforcement in 2D-COFs to produce larger and persistent pore sizes than previously possible.

The modulator 2-fluorobenzoic acid (2-fba) is widely used to prepare RE clusters in metal-organic frameworks (MOFs). In contrast to known RE MOF structures containing hydroxide bridging groups, we report for the first time the possible presence of fluoro bridging groups in RE MOFs. In this report we discuss the synthesis of a holmium-UiO-66 analogue as well as a novel holmium MOF, where evidence of fluorinated clusters is observed. The mechanism of fluorine extraction from 2-fba is discussed as well as the implications that these results have for previously reported RE MOF structures.

The BaAl4 prototype structure and its derivatives have been identified to host several topological quantum materials and noncentrosymmetric superconductors. Single crystals up to ∼3 mm × 3 mm × 5 mm of Ln2Co3Ge5 (Ln = Pr, Nd, and Sm) are obtained via flux growth utilizing Sn as metallic flux. The crystal structure is isostructural to the Lu2Co3Si5 structure type in the crystallographic space group C2/c. The temperature-dependent magnetization indicates magnetic ordering at 30 K for all three compounds. Pr2Co3Ge5 and Nd2Co3Ge5 exhibit complex magnetic behavior with spin reorientations before ordering antiferromagnetically around 6 K, whereas Sm2Co3Ge5 shows a clear antiferromagnetic behavior at 26 K. The structures and properties of Ln2Co3Ge5 (Ln = Pr, Nd, and Sm) are compared to those of the ThCr2Si2 and BaNiSn3 structure types. Herein, we present the optimized crystal growth, structure, and physical properties of Ln2Co3Ge5 (Ln = Pr, Nd, and Sm).

Interlayer excitons, or bound electron-hole pairs whose constituent quasiparticles are located in distinct stacked semiconducting layers, are being intensively studied in heterobilayers of two dimensional semiconductors. They owe their existence to an intrinsic type-II band alignment between both layers that convert these into p-n junctions. Here, we unveil a pronounced interlayer exciton (IX) in heterobilayers of metal monochalcogenides, namely gamma-InSe on epsilon-GaSe, whose pronounced emission is adjustable just by varying their thicknesses given their number of layers dependent direct bandgaps. Time-dependent photoluminescense spectroscopy unveils considerably longer interlayer exciton lifetimes with respect to intralayer ones, thus confirming their nature. The linear Stark effect yields a bound electron-hole pair whose separation d is just (3.6 \pm 0.1) {\AA} with d being very close to dSe = 3.4 {\AA} which is the calculated interfacial Se separation. The envelope of IX is twist angle dependent and describable by superimposed emissions that are nearly equally spaced in energy, as if quantized due to localization induced by the small moir\'e periodicity. These heterostacks are characterized by extremely flat interfacial valence bands making them prime candidates for the observation of magnetism or other correlated electronic phases upon carrier doping., Comment: ACS Nano, in press 42 pages plus, 4 figures, plus supplementary information file, containing 21 supplementary figures

Supercritical carbon dioxide (scCO2) activation provides milder conditions to process covalent organic frameworks (COFs) without compromising their crystallinity and porosity. To this end, three hydrazone COFs (TFPB-DHz COF, TFPT-DHz COF, Py-DHz COF) were synthesized with a terephthaloyl dihydrazide linker (DHz) which has no substituents. To date, the synthesis of hydrazone COFs without a narrow range of alkoxy linkers has not been possible. The scCO2-activated hydrazone-linked COFs in this study were crystalline and had high surface areas (surface areas of TFPB-DHz COF, TFPT-DHz COF, and Py-DHz COF were 790, 1199, and 932 m2/g, respectively). This study shows the significance of using milder activation methods for making hydrazone-linked COF structures that were previously inaccessible.

The recent discovery of the A

Nickel catalysts represent a low cost and environmentally friendly alternative to palladium-based catalytic systems for Suzuki–Miyaura cross-coupling (SMC) reactions. However, nickel catalysts have suffered from poor air, moisture, and thermal stabilities, especially at high catalyst loading, requiring controlled reaction conditions. In this report, we examine a family of mono- and dinuclear Ni(II) and Pd(II) complexes with a diverse and versatile α-diimine ligand environment for SMC reactions. To evaluate the ligand steric effects, including the bite angle in the reaction outcomes, the structural variation of the complexes was achieved by incorporating iminopyridine- and acenaphthene-based ligands. Moreover, the impact of substrate bulkiness was investigated by reacting various aryl bromides with phenylboronic acid, 2-naphthylboronic acid, and 9-phenanthracenylboronic acid. Yields were the best with the dinuclear complex, being nearly quantitative (93–99%), followed by the mononuclear complexes, giving yields of 78–98%. Consequently, α-diimine-based ligands have the potential to deliver Ni-based systems as sustainable catalysts in SMC.

Single crystals of the ternary Yb0.5Co3Ge3 have been grown out of molten Sn flux and characterized by single crystal and synchrotron powder X-ray diffraction. Yb0.5Co3Ge3 adopts a hybrid YCo6Ge6/Co...

A new ternary nonstoichiometric Zr6.5Pt6Se19 has been discovered as a part of effort to dope Zr into the layered transitional metal chalcogenide PtSe2. With a new structure type (oC68), it is the f...

There is growing interest in Holmium carriers for radiotherapeutic applications. In this work, a holmium-based metal-organic framework (MOF) using the 4,4′-biphenyldicarboxylic acid (H2BPDC) linker was synthesized and characterized to explore its potential as a radiotherapeutic carrier. The 3D MOF [Ho(BPDC)2]·(CH3)2NH2 was characterized by single crystal X-ray diffraction, FTIR, TGA and PXRD. A challenge to overcome in lanthanide-based MOFs is the deformation or collapse of the framework that can occur after evacuation of the pores. This structure displays high thermal stability and no collapse was observed when the molecules confined in the pores were removed. The coordination around the holmium center (CN = 8) is the key to this stability since only the organic linker and no solvent molecules coordinate to the metallic center. The porosity of the material was confirmed by high-pressure carbon dioxide (CO2) adsorption–desorption analysis. The stability of the MOF, its holmium content (28 wt%) and its porosity are features that make this material a potential holmium carrier for radiotherapeutic applications.

Low-dimensional solids are highly anisotropic by nature and show promise as new quantum materials, leading to exotic physical properties not realized in three-dimensional materials. To discover cor...

The Kagome lattice is an important fundamental structure in condensed matter physics for investigating the interplay of electron correlation, topology, and frustrated magnetism. Recent work on Kagome metals in the AV3Sb5 (A = K, Rb, and Cs) family has shown a multitude of correlation-driven distortions, including symmetry breaking charge density waves and nematic superconductivity at low temperatures. Here, we study the new Kagome metal Yb0.5Co3Ge3 and find a temperature-dependent kink in the resistivity that is highly similar to the AV3Sb5 behavior and is commensurate with an in-plane structural distortion of the Co Kagome lattice along with a doubling of the c-axis. The symmetry is lower below the transition temperature, with a breaking the in-plane mirror planes and C6 rotation, while gaining a screw axis along the c-direction. At very low temperatures, anisotropic negative magnetoresistance is observed, which may be related to anisotropic magnetism. This raises questions about the types of the distortions in Kagome nets and their resulting physical properties including superconductivity and magnetism.

Two dimensional covalent organic frameworks (2D-COFs) are a class of crystalline porous organic polymers that consist of covalently linked, two dimensional sheets that can stack together through non-covalent interactions. Here we report the synthesis of a novel COF, called PyCOFamide, which has an experimentally observed pore size that is greater than 6 nm in diameter. This is among the largest pore size reported to date for a 2D-COF. PyCOFamide exhibits permanent porosity and high crystallinity as evidenced by the nitrogen adsorption, powder X-ray diffraction, and high-resolution transmission electron microscopy. We show that the pore size of PyCOFamide is large enough to accommodate fluorescent proteins such as Superfolder green fluorescent protein and mNeonGreen. This work demonstrates the utility of non-covalent structural reinforcement in 2D-COFs to produce larger, persistent pore sizes than previously possible.

Significance π-stacking of polynuclear aromatic rings encompasses fundamental scientific phenomena in chemical bonding and photophysics with strong impacts on societal aspects that include electronic devices, DNA/RNA intercalation, and optical sensing of environmental pollutants. This work describes unusually strong π-stacking in a recently synthesized naphthalene-diimide derivative, as well as a prototypical π-stacked molecule, pyrene. The infinite/cooperative π-stacking in the former and dimer stacking in the latter are much stronger than previously thought to be possible for this type of supramolecular interaction, reaching ∼50 and 20 kcal ⋅ mol −1 , respectively. This has led to unusually large anisotropic thermal expansion and multicycle/multicolor thermochromism in BNDI-T, whereas multiple unusual binding and photophysical properties have been discovered for the classical organic chromophore pyrene.

The BaAl

The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr2Co3Ge5 (U2Co3Si5-type, Ibam). Our findings show that o-Pr2Co3Ge5 hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges near T sp = 32 K, with a spin reconfiguration transition near T M = 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity of T sp and T M. The magnetic entropy further reveals that crystal electric field splitting lifts the Hund’s rule degeneracy at low temperatures. Taken together, these measurements show that Pr2Co3Ge5 is an environment for complex f state magnetism with potential strongly correlated electron states.

The competition between Ruderman-Kittel-Kasuya-Yosida (RKKY), crystal electric field (CEF), and Kondo energy scales has recently emerged at the heart of complex magnetism in several Ce- or Yb-based intermetallics. Hard axis magnetic order has been observed in a handful of these compounds, independent of the crystal symmetry, size of the ordered moment, or the relative scale of the Kondo and magnetic ordering temperatures. This raises the question of the role of each energy scale in driving the ground state properties. In focusing on a single class of compounds, the rhombohedral R${\mathrm{Rh}}_{3}{\mathrm{Si}}_{7}$, we compare the anisotropy and magnetic ground states in members of this series with only RKKY interactions (R = Gd), or RKKY and CEF effects (R = Tb-Tm), with the behavior of the R = Yb compound, where all three energy scales (RKKY, CEF, Kondo) are at play. Moreover, we extend the comparison to two other isostructural Kondo systems ${\mathrm{YbIr}}_{3}{\mathrm{Si}}_{7}$ and ${\mathrm{YbIr}}_{3}{\mathrm{Ge}}_{7}$, where hard axis magnetic order is also observed. The non-Kondo compounds R${\mathrm{Rh}}_{3}{\mathrm{Si}}_{7}$ (R = Tb-Tm) lack the complexity of magnetic order along the hard CEF axis, pointing to the dominant role of the Kondo effect in driving this magnetic order. However, the CEF-RKKY competition is still responsible for complex magnetic ground states, and it appears that the electronic and magnetic degrees of freedom are entangled in all magnetic members of this series of compounds.

Strongly correlated electrons in layered perovskite structures have been the birthplace of high-temperature superconductivity, spin liquid, and quantum criticality. Specifically, the cuprate materials with layered structures made of corner sharing square planar CuO$_4$ units have been intensely studied due to their Mott insulating grounds state which leads to high-temperature superconductivity upon doping. Identifying new compounds with similar lattice and electronic structures has become a challenge in solid state chemistry. Here, we report the hydrothermal crystal growth of a new copper tellurite sulfate Cu$_3$(TeO$_4$)(SO$_4$)$\cdot$H$_2$O, a promising alternative to layered perovskites. The orthorhombic phase (space group $Pnma$) is made of corrugated layers of corner-sharing CuO$_4$ square-planar units that are edge-shared with TeO$_4$ units. The layers are linked by slabs of corner-sharing CuO$_4$ and SO$_4$. Using both the bond valence sum analysis and magnetization data, we find purely Cu$^{2+}$ ions within the layers, but a mixed valence of Cu$^{2+}$/Cu${^+}$ between the layers. Cu$_3$(TeO$_4$)(SO$_4$)$\cdot$H$_2$O undergoes an antiferromagnetic transition at $T_N$=67 K marked by a peak in the magnetic susceptibility. Upon further cooling, a spin-canting transition occurs at $T^{\star}$=12 K evidenced by a kink in the heat capacity. The spin-canting transition is explained based on a $J_1$-$J_2$ model of magnetic interactions, which is consistent with the slightly different in-plane super-exchange paths. We present Cu$_3$(TeO$_4$)(SO$_4$)$\cdot$H$_2$O as a promising platform for the future doping and strain experiments that could tune the Mott insulating ground state into superconducting or spin liquid states., 29 pages

The reversible phase transitions of surface tethered thermoresponsive polymers were studied at the nanoscale using in situ atomic force microscopy (AFM) by heating and cooling a test platform of po...

A nonstoichiometric ternary antimonide, Zr3.55Pt4Sb4, with a new structure type (hP24), has been synthesized via arc-melting. Its crystal structure was determined by single-crystal X-ray diffraction with hexagonal space group P63/mmc and lattice parameters a = 4.391(3) A, c = 30.53(2) A, and V = 509.7(8) A3. It features the unique Pt4Sb4 slab with Pt-Pt bonds and is reminiscent to hexagonal diamond substructures. Three different Zr atoms, occupying three different sites, aid in the close-packing of the Pt and Sb atoms. Electronic structure calculations show the half occupancy of one Zr site creates a pseudogap at the Fermi level and optimizes the Pt-Sb bonding interactions. This enhances the electronic stability and accounts for the very narrow phase width observed for this nonstoichiometric compound. Furthermore, strong Zr-Pt and Zr-Sb interactions play a crucial role in the chemical bonding of the title compound. Electrical transport measurements show metallic behavior of this compound down to 2 K, consistent with the band structure calculations.

A new structure type of composition Ce6Co5Ge16 was grown out of a molten Sn flux. Ce6Co5Ge16 crystallizes in the orthorhombic space group Cmcm, with highly anisotropic lattice parameters of a = 4.3293(5) A, b = 55.438(8) A, and c = 4.3104(4) A. The resulting single crystals were characterized by X-ray diffraction, and the magnetic and transport properties are presented. The Sn-stabilized structure of Ce6Co5Ge16 is based on the stacking of disordered Ce cuboctahedra and is an intergrowth of existing structure types including AlB2, BaNiSn3, and AuCu3. The stacking of structural subunits has previously been shown to be significant in the fields of superconductivity, quantum materials, and optical materials. Herein, we present the synthesis, characterization, and complex magnetic behavior of Ce6Co5Ge16 at low temperature, including three distinct magnetic transitions.

Single crystals of Ln2Fe4- xCo xSb5- yBi y (Ln = La, Ce; 0 ≤ x < 0.5; 0 ≤ y ≤ 0.2) were grown using Bi flux and self-flux methods. The compounds adopt the La2Fe4Sb5 structure type with tetragonal space group I4/ mmm. The La2Fe4Sb5 structure type is comprised of rare earth atoms capping square Sb nets in a square antiprismatic fashion and two transition-metal networks forming a PbO-type layer with Sb and transition-metal isosceles triangles. Substituting Co into the transition-metal sublattice results in a decrease in the transition temperature and reduced frustration, indicative of a transition from localized to itinerant behavior. In this manuscript, we demonstrated that Bi can be used as an alternate flux to grow single crystals of antimonides. Even with the incorporation of Bi into the Sb square net, the magnetic properties are not significantly affected. In addition, we have shown that the incorporation of Co into the Fe triangular sublattice leads to an itinerant magnetic system.

A novel copper(II) metal-organic framework (MOF) has been synthesized by modifying the reaction conditions of a 1D coordination polymer. The 1D polymer is built by the coordination between copper and 2,2'-(1 H-imidazole-4,5-diyl)di-1,4,5,6-tetrahydropyrimidine (H-L1). The geometry of H-L1 precludes its ability to form extended 3D framework structures. By adding 1,4-benzenedicarboxylic acid (H2BDC), a well-studied linker in MOF synthesis, we achieved the transition from a 1D polymer chain into porous 2D layered structures. Hydrogen bonding between L1 and BDC directs the parallel stacking of these layers, resulting in a 3D structure with one-dimensional channels accessible by two different pore windows. The preferred growth orientation of the crystal produces prolonged channels and a disparity in pore size distribution. This in turn results in slow diffusion processes in the material. Furthermore, an isoreticular MOF was prepared by substituting the BDC linker by 2,6-naphthalenedicarboxylic acid (H2NDC).

Based on the 2,2′-bipyridine-5,5′-dicarboxylic acid ligand (H2bpdc), a new metal–organic framework using holmium as the coordinating metal site was synthesized through a solvothermal reaction. The 3D compound {[Ho2(bpdc)3(DMF)2]·2(DMF)}n (1) was characterized by single crystal X-ray diffraction, FTIR, TGA and PXRD. The coordination between H2bpdc, the solvent, and holmium (CN = 7) allows for the construction of a MOF containing potentially free metal sites and uncoordinated N-donor sites. These features make this MOF an attractive holmium carrier for radiotherapeutic applications.

X-ray diffraction, crystal structure, magnetization, and heat capacity results are presented for the rare earth – chalcogenide ternary system PrCu0.45Te2 and its non-4f analogue LaCu0.40Te2. The crystal structure of PrCu0.45Te2 is characterized by chains of edge and corner-sharing CuTe4 tetrahedra and Pr centered in polyhedra comprised of Cu and Te. The Cu site is partially occupied and exhibits signatures of local disorder. Magnetic susceptibility measurements show a Curie-Weiss temperature dependence consistent with a Pr3+ state. No magnetic ordering is observed down to 1.8 K, but the negative Curie-Weiss temperature suggests an antiferromagnetic exchange interaction. Importantly, the low temperature heat capacity of PrCu0.45Te2 is strongly enhanced by comparison to LaCu0.40Te2, suggesting that there is a build-up of entropy that is associated with the 4f-electrons from the Pr3+ ions. These features reveal possible spin frustration behavior and introduce this family of materials as a template for studying new phenomenon.

Strongly correlated electrons in layered perovskite structures have been the birthplace of high-temperature superconductivity, spin liquids, and quantum criticality. Specifically, the cuprate materials with layered structures made of corner-sharing square-planar CuO

Solid-state thermoelastic behavior-a sudden exertion of an expansive or contractive physical force following a temperature change and phase transition in a solid-state compound-is rare in organic crystals, few are reversible systems, and most of these are limited to a dozen or so cycles before the crystal degrades or they reverse slowly over the course of many minutes or even hours. Comparable to thermosalience, wherein crystal phase changes induce energetic jumping, thermomorphism produces physical work via consistent and near-instantaneous predictable directional force. In this work, we show a fully reversible thermomorphic actuator that is stable at room temperature for multiple years and is capable of actuation for more than 200 cycles at near-ambient temperature. Specifically, the crystals shrink to 90% of their original length instantaneously upon heating beyond 45 °C and expand back to their original length upon cooling below 35 °C. Furthermore, the phase transition occurs instantaneously, with little obvious hysteresis, allowing us to create real-time actuating thermal fuses that cycle between on and off rapidly.

Whereas electron-phonon scattering typically relaxes the electron's momentum in metals, a perpetual exchange of momentum between phonons and electrons conserves total momentum and can lead to a coupled electron-phonon liquid with unique transport properties. This theoretical idea was proposed decades ago and has been revisited recently, but the experimental signatures of an electron-phonon liquid have been rarely reported. We present evidence of such a behavior in a transition metal ditetrelide, NbGe$_2$, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe$_2$ due to weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and calculated curves within a standard Fermi liquid theory. Third, Raman scattering shows anomalous temperature dependence of the phonon linewidths which fits an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential sources of coupled electron-phonon liquids., Comment: Preprint, 18 pages, 4 figures

Exotic multifold topological excitations have been predicted and were recently observed in transition metal silicides like $\ensuremath{\beta}$-RhSi. Herein, we report that interesting topological features of RhSi are also observed in its orthorhombic $\ensuremath{\alpha}$ phase, which displays multiple types of Dirac nodes very close to the Fermi level ${\ensuremath{\varepsilon}}_{F}$. We discuss the symmetry analysis, band connectivity along high-symmetry lines using group representations, band structure, the nature of the Dirac points and of a nodal line occurring near ${\ensuremath{\varepsilon}}_{F}$ which is protected by the crystalline symmetry. The de Haas--van Alphen effect indicates a Fermi surface in agreement with the calculations. We find an elliptically shaped nodal line very close to ${\ensuremath{\varepsilon}}_{F}$ around and near the $S$ point on the ${k}_{y}\text{\ensuremath{-}}{k}_{z}$ plane that results from the intersection of two upside-down Dirac cones. Both Dirac points of the participating Kramers degenerate bands are only 5 meV apart; hence, an accessible magnetic field might induce a crossing between the spin-up partner of the upper Dirac cone and the spin-down partner of the lower Dirac cone, possibly explaining the anomalies observed in the magnetic torque. $\ensuremath{\alpha}$-RhSi is a unique system since all bands crossing ${\ensuremath{\varepsilon}}_{F}$ emerge from Dirac nodes.

Solid state thermosalience—a sudden exertion of an expansive or contractive physical force following a temperature change in a solid state compound—is rare, few are reversible systems, and most of these are limited to a dozen or so cycles before the crystal degrades or they reverse slowly over the course of many minutes or even hours. In this work, we show a fully reversible actuator that is stable at room temperature for multiple years and is capable of actuation for more than two hundred cycles at near ambient temperature. Specifically, the crystals shrink to 90% of its original length instantaneously upon heating beyond 45 °C and expands back to its original length upon cooling below 35 °C. This temperature regime is important because it occurs around physiologically important temperatures. Furthermore, the phase transition occurs instantaneously, with little obvious hysteresis, allowing us to create real-time actuating thermal fuses that cycle between on and off rapidly.

While $f$-electron intermetallics are well-known as hosts for complex valence behavior, magnetism, and unconventional superconductivity, it remains a persistent challenge to design specific phenomena. Some guidance is provided by studies of large families of materials, such as those based on the ${\mathrm{AlB}}_{2}, {\mathrm{AuCu}}_{3}$, and ${\mathrm{BaAl}}_{4}$ structural subunits, where systematic trends in their electronic behavior have been observed. This suggests that an effective approach in developing designer materials is to incorporate multimodal stacking arrangements of well-understood building blocks. Here, we report results for ${\mathrm{Ce}}_{7}{\mathrm{Co}}_{6}{\mathrm{Ge}}_{19}$, the $n=6$ member of a new homologous series, ${{A}_{n}}_{+1}{B}_{n}{X}_{3n+1}$, that consists of subunits previously identified as having potential for unconventional magnetic behavior. We show that ${\mathrm{Ce}}_{7}{\mathrm{Co}}_{6}{\mathrm{Ge}}_{19}$ exhibits complex magnetism with evidence for frustration and Kondo lattice behavior that is directly related to what is seen in other $n$ value analogs. Here, the added structural units introduce additional magnetic degrees of freedom, demonstrating that it is possible to rationally design magnetic complexity, and possibly other phenomena in this family of intermetallics.

In this work, we present a trimetal neodymium complex with two notable qualities. First, the assembly of the complex is templated by peroxide derived from atmospheric oxygen. Second, the bulk material behaves as a superparamagnet, implying that the individual complexes are molecular magnets. Peroxide-templated assembly is possible because of the confluence of the high oxophilicity of neodymium along with the use of an azeotropic distillation synthesis method, which excludes water but admits oxygen. SQUID magnetometry measurements show an extremely high magnetic susceptibility as well as a lack of remanence.

We report the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds. To accomplish this, we have designed monomers with a nonplanar structure that arises from steric crowding, forcing the amide side groups out of plane with the COF sheets orienting the hydrogen bonds between the layers. The presence of these hydrogen bonds provides significant structural stabilization as demonstrated by comparison to control structures that lack hydrogen bonding capability, resulting in lower surface area and crystallinity. We have characterized both azine and imine-linked versions of these COFs, named COFamide-1 and -2, respectively, for their surface areas, pore sizes, and crystallinity. In addition to these more conventional characterization methods, we also used variable temperature infrared spectroscopy methods and van der Waals density functional calculations to directly observe the presence of hydrogen bonding.

Van der Waals (VdW) materials have opened new directions in the study of low dimensional magnetism. A largely unexplored arena is the intrinsic tuning of VdW magnets toward new ground-states. The chromium trihalides provided the first such example with a change of inter-layer magnetic coupling emerging upon exfoliation. Here, we take a different approach to engineer new ground-states, not by exfoliation, but by tuning the spin-orbit coupling (SOC) of the non-magnetic ligand atoms (Cl,Br,I). We synthesize a three-halide series, CrCl$_{3-x-y}$Br$_{x}$I$_{y}$, and map their magnetic properties as a function of Cl, Br, and I content. The resulting triangular phase diagrams unveil a frustrated regime near CrCl$_{3}$. First-principles calculations confirm that the frustration is driven by a competition between the chromium and halide SOCs. Furthermore, we reveal a field-induced change of inter-layer coupling in the bulk of CrCl$_{3-x-y}$Br$_{x}$I$_{y}$ crystals at the same field as in the exfoliation experiments., Comment: 22 pages, 6 figures

We report the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds. To accomplish this, we have designed monomers with a non-planar structure that arises from steric crowding, forcing the amide side groups out of plane with the COF sheets orienting the hydrogen bonds between the layers. The presence of these hydrogen bonds provides significant structural stabilization as demonstrated by comparison to control structures that lack hydrogen bonding capability, resulting in lower surface area and crystallinity. We have characterized both azine and imine-linked versions of these COFs, named COFamide-1 and -2, respectively, for their surface areas, pore sizes and crystallinity. In addition to these more conventional characterization methods, we also used variable temperature infrared spectroscopy (VT-IR) methods and van der Waals density functional calculations to directly observe the presence of hydrogen bonding.

Author(s): Ruiz, A; Nagarajan, V; Vranas, M; Lopez, G; McCandless, GT; Kimchi, I; Chan, JY; Breznay, NP; Frano, A; Frandsen, BA; Analytis, JG | Abstract: We report the existence of a high-temperature magnetic anomaly in the three-dimensional Kitaev candidate material, β-Li2IrO3. Signatures of the anomaly appear in magnetization, heat capacity, and muon spin relaxation measurements. The onset coincides with a reordering of the principal axes of magnetization, which is thought to be connected to the onset of Kitaev-like correlations in the system. The anomaly also shows magnetic hysteresis with a spatially anisotropic magnitude that follows the spin-anisotropic exchange anisotropy of the underlying Kitaev Hamiltonian. We discuss possible scenarios for a bulk and impurity origin.

Single crystals of the ternary Pr3Co2+xGe7 and Ln3Co2+xGe7–ySny (Ln = Pr, Nd, Sm) adopting a disordered version of the La3Co2Sn7 structure type have been prepared via flux-growth methods and characterized by single crystal X-ray diffraction. The structure consists of two lanthanide crystallographic sites with one occupying a cuboctahedral coordination environment and a second in a trigonal prismatic environment. The structure can also be described as an intergrowth of AuCu3 and CeNiSi2 structure types, and the stability of the germanide analogues requires Sn incorporation and Co site preferences. Magnetic properties of the Pr3Co2+xGe7–ySny series are highlighted with the Sn-substituted Pr3Co2.514(5)Ge6.66(7)Sn0.360(2) orders magnetically near 5.8 K, while the germanide Pr3Co2.3376(5)Ge7.056(7) exhibits magnetic transitions at 5.3 and 9.3 K, arising from the magnetic sublattices with field-dependent magnetization revealing three metamagnetic transitions at 0.46, 0.80, and 2.1 T.