Bubbles enable volumetric negative compressibility in metastable elastocapillary systems.

Although coveted in applications, few materials expand when subject to compression or contract under decompression, i.e., exhibit negative compressibility. A key step to achieve such counterintuitive behaviour is the destabilisations of (meta)stable equilibria of the constituents. Here, we propose a simple strategy to obtain negative compressibility exploiting capillary forces both to precompress the elastic material and to release such precompression by a threshold phenomenon - the reversible formation of a bubble in a hydrophobic flexible cavity. We demonstrate that the solid part of such metastable elastocapillary systems displays negative compressibility across different scales: hydrophobic microporous materials, proteins, and millimetre-sized laminae. This concept is applicable to fields such as porous materials, biomolecules, sensors and may be easily extended to create unexpected material susceptibilities.

Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks.

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

In situ studies of reversible solid-gas reactions of ethylene responsive silver pyrazolates.

Solid-gas reactions and in situ powder X-ray diffraction investigations of trinuclear silver complexes {[3,4,5-(CF3)3Pz]Ag}3 and {[4-Br-3,5-(CF3)2Pz]Ag}3 supported by highly fluorinated pyrazolates reveal that they undergo intricate ethylene-triggered structural transformations in the solid-state producing dinuclear silver-ethylene adducts. Despite the complexity, the chemistry is reversible producing precursor trimers with the loss of ethylene. Less reactive {[3,5-(CF3)2Pz]Ag}3 under ethylene pressure and low-temperature conditions stops at an unusual silver-ethylene complex in the trinuclear state, which could serve as a model for intermediates likely present in more common trimer-dimer reorganizations described above. Complete structural data of three novel silver-ethylene complexes are presented together with a thorough computational analysis of the mechanism.

Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN2 and CaHfN2.

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN2) and calcium hafnium nitride (CaHfN2), by solid state metathesis reactions between Ca3N2 and MCl4 (M = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca3N2 + MCl4 → CaMN2 + 2 CaCl2, reactions prepared this way result in Ca-poor materials (CaxM2-xN2, x < 1). A small excess of Ca3N2 (ca. 20 mol %) is needed to yield stoichiometric CaMN2, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr3+ intermediates early in the reaction pathway, and the excess Ca3N2 is needed to reoxidize Zr3+ intermediates back to the Zr4+ oxidation state of CaZrN2. Analysis of computationally derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN2. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.

Effect of Crystallite Size on the Flexibility and Negative Compressibility of Hydrophobic Metal-Organic Frameworks.

Flexible nanoporous materials are of great interest for applications in many fields such as sensors, catalysis, material separation, and energy storage. Of these, metal-organic frameworks (MOFs) are the most explored thus far. However, tuning their flexibility for a particular application remains challenging. In this work, we explore the effect of the exogenous property of crystallite size on the flexibility of the ZIF-8 MOF. By subjecting hydrophobic ZIF-8 to hydrostatic compression with water, the flexibility of its empty framework and the giant negative compressibility it experiences during water intrusion were recorded via in operando synchrotron irradiation. It was observed that as the crystallite size is reduced to the nanoscale, both flexibility and the negative compressibility of the framework are reduced by ∼25% and ∼15%, respectively. These results pave the way for exogenous tuning of flexibility in MOFs without altering their chemistries.

Rare Earth Nitrate Hybrid Double Perovskites [Me4N]2[MLn(NO3)6] (M = Na-Cs; Ln = La-Gd, ex. Pm).

An exploration of the synthetic and structural phase space of rare earth hybrid double perovskites A2B'BX6 (A = organocation, B' = M+, B = M3+, X = molecular bridging anion) that include X = NO3- and B' = alkali metal is reported, complementing earlier studies of the [Me4N]2[KB(NO3)6] (B = Am, Cm, La-Nd, Sm-Lu, Y) (Me4N = (CH3)4N+) compounds. In the present efforts, the synthetic phase space of these systems is explored by varying the identity of the alkali metal ion at the B'-site. Herein, we report three new series of the form [Me4N]2[B'B(NO3)6] (B = La-Nd, Sm-Gd; B' = Na, Rb, Cs). The early members of the Na-series crystallize in the trigonal space group R3̅ from La to Nd where a phase transition occurs in the phase between 273 and 300 K, going from R3̅ to the high-symmetry, cubic space group Fm3̅m. The preceding trigonal members of the Na-series also undergo phase transitions to cubic symmetry at temperatures above 300 K, establishing a decreasing trend in the phase-transition temperature. The remainder of the Na-series, as well as the Rb- and Cs-series, all crystallize in Fm3̅m at 300 K. The temperature-dependent phase behavior of the synthesized phases is studied via variable-temperature spectroscopic methods and high-resolution powder X-ray diffractometry. All phases were characterized via single-crystal and powder X-ray diffraction and Fourier transform infrared (FT-IR) and Raman spectroscopic methods. These results demonstrate the versatility of the perovskite structure type to include rare earth ions, nitrate ions, and a suite of alkali metal ions and serve as a foundation for the design of functional rare earth hybrid double perovskite materials such as those possessing useful multiferroic, optical, and magnetic properties.

Bimetallic Zeolitic Imidazole Frameworks for Improved Stability and Performance of Intrusion-Extrusion Energy Applications.

Hydrophobic flexible zeolitic imidazole frameworks (ZIFs) represent reference microporous materials in the area of mechanical energy storage, conversion, and dissipation via non-wetting liquid intrusion-extrusion cycle. However, some of them exhibit drawbacks such as lack of stability, high intrusion pressure, or low intrusion volume that make them non-ideal materials to consider as candidates for real applications. In this work, we face these limitations by exploiting the hybrid ZIF concept. Concretely, a bimetallic SOD-like ZIF consisting of Co and Zn ions was synthesized and compared with Co-ZIF (ZIF-67) and Zn-ZIF (ZIF-8) showing for the first time that the hybrid ZIF combines the good stability of ZIF-8 with the higher water intrusion volume of ZIF-67. Moreover, it is shown that the hybrid-ZIF approach can be used to tune the intrusion/extrusion pressure, which is crucial for technological applications.

Mechanism of Water Intrusion into Flexible ZIF-8: Liquid Is Not Vapor.

Zeolitic Imidazolate Frameworks (ZIF) find application in storage and dissipation of mechanical energy. Their distinctive properties linked to their (sub)nanometer size and hydrophobicity allow for water intrusion only under high hydrostatic pressure. Here we focus on the popular ZIF-8 material investigating the intrusion mechanism in its nanoscale cages, which is the key to its rational exploitation in target applications. In this work, we used a joint experimental/theoretical approach combining in operando synchrotron experiments during high-pressure intrusion experiments, molecular dynamics simulations, and stochastic models to reveal that water intrusion into ZIF-8 occurs by a cascade filling of connected cages rather than a condensation process as previously assumed. The reported results allowed us to establish structure/function relations in this prototypical microporous material, representing an important step to devise design rules to synthesize porous media.

Colossal barocaloric effects with ultralow hysteresis in two-dimensional metal-halide perovskites.

Pressure-induced thermal changes in solids-barocaloric effects-can be used to drive cooling cycles that offer a promising alternative to traditional vapor-compression technologies. Efficient barocaloric cooling requires materials that undergo reversible phase transitions with large entropy changes, high sensitivity to hydrostatic pressure, and minimal hysteresis, the combination of which has been challenging to achieve in existing barocaloric materials. Here, we report a new mechanism for achieving colossal barocaloric effects that leverages the large volume and conformational entropy changes of hydrocarbon order-disorder transitions within the organic bilayers of select two-dimensional metal-halide perovskites. Significantly, we show how the confined nature of these order-disorder phase transitions and the synthetic tunability of layered perovskites can be leveraged to reduce phase transition hysteresis through careful control over the inorganic-organic interface. The combination of ultralow hysteresis and high pressure sensitivity leads to colossal reversible isothermal entropy changes (>200 J kg-1 K-1) at record-low pressures (<300 bar).

Cation reordering instead of phase transitions: Origins and implications of contrasting lithiation mechanisms in 1D ζ- and 2D α-V2O5.

Substantial improvements in cycle life, rate performance, accessible voltage, and reversible capacity are required to realize the promise of Li-ion batteries in full measure. Here, we have examined insertion electrodes of the same composition (V2O5) prepared according to the same electrode specifications and comprising particles with similar dimensions and geometries that differ only in terms of their atomic connectivity and crystal structure, specifically two-dimensional (2D) layered α-V2O5 that crystallizes in an orthorhombic space group and one-dimensional (1D) tunnel-structured ζ-V2O5 crystallized in a monoclinic space group. By using particles of similar dimensions, we have disentangled the role of specific structural motifs and atomistic diffusion pathways in affecting electrochemical performance by mapping the dynamical evolution of lithiation-induced structural modifications using ex situ scanning transmission X-ray microscopy, operando synchrotron X-ray diffraction measurements, and phase-field modeling. We find the operation of sharply divergent mechanisms to accommodate increasing concentrations of Li-ions: a series of distortive phase transformations that result in puckering and expansion of interlayer spacing in layered α-V2O5, as compared with cation reordering along interstitial sites in tunnel-structured ζ-V2O5 By alleviating distortive phase transformations, the ζ-V2O5 cathode shows reduced voltage hysteresis, increased Li-ion diffusivity, alleviation of stress gradients, and improved capacity retention. The findings demonstrate that alternative lithiation mechanisms can be accessed in metastable compounds by dint of their reconfigured atomic connectivity and can unlock substantially improved electrochemical performance not accessible in the thermodynamically stable phase.

The Molecular Path Approaching the Active Site in Catalytic Metal-Organic Frameworks.

How molecules approach, bind at, and release from catalytic sites is key to heterogeneous catalysis, including for emerging metal-organic framework (MOF)-based catalysts. We use in situ synchrotron X-ray scattering analysis to evaluate the dominant binding sites for reagent and product molecules in the vicinity of catalytic Ni-oxo clusters in NU-1000 with different surface functionalization under conditions approaching those used in catalysis. The locations of the reagent and product molecules within the pores can be linked to the activity for ethylene hydrogenation. For the most active catalyst, ethylene reagent molecules bind close to the catalytic clusters, but only at temperatures approaching experimentally observed onset of catalysis. The ethane product molecules favor a different binding location suggesting that the product is readily released from the active site. An unusual guest-dependence of the framework negative thermal expansion is documented. We hypothesize that reagent and product binding sites reflect the pathway through the MOF to the active site and can be used to identify key factors that impact the catalytic activity.

A Molecular Compound for Highly Selective Purification of Ethylene.

Purification of C2 H4 from an C2 H4 /C2 H6 mixture is one of the most challenging separation processes, which is achieved mainly through energy-intensive, cryogenic distillation in industry. Sustainable, non-distillation methods are highly desired as alternatives. We discovered that the fluorinated bis(pyrazolyl)borate ligand supported copper(I) complex {[(CF3 )2 Bp]Cu}3 has features very desirable in an olefin-paraffin separation material. It binds ethylene exclusively over ethane generating [(CF3 )2 Bp]Cu(C2 H4 ). This molecular compound exhibits extremely high and record ideal adsorbed solution theory (IAST) C2 H4 /C2 H6 gas separation selectivity, affording high purity (>99.5 %) ethylene that can be readily desorbed from separation columns. In-situ PXRD provides a "live" picture of the reversible conversion between [(CF3 )2 Bp]Cu(C2 H4 ) and the ethylene-free sorbent in the solid-state, driven by the presence or removal of C2 H4 . Molecular structures of trinuclear {[(CF3 )2 Bp]Cu}3 and mononuclear [(CF3 )2 Bp]Cu(C2 H4 ) are also presented.

Ligand-Directed Conformational Control over Porphyrinic Zirconium Metal-Organic Frameworks for Size-Selective Catalysis.

Zirconium-based metal-organic frameworks (Zr-MOFs) have aroused enormous interest owing to their superior stability, flexible structures, and intriguing functions. Precise control over their crystalline structures, including topological structures, porosity, composition, and conformation, constitutes an important challenge to realize the tailor-made functionalization. In this work, we developed a new Zr-MOF (PCN-625) with a csq topological net, which is similar to that of the well-known PCN-222 and NU-1000. However, the significant difference lies in the conformation of porphyrin rings, which are vertical to the pore surfaces rather than in parallel. The resulting PCN-625 exhibits two types of one-dimensional channels with concrete diameters of 2.03 and 0.43 nm. Furthermore, the vertical porphyrins together with shrunken pore sizes could limit the accessibility of substrates to active centers in the framework. On the basis of the structural characteristics, PCN-625(Fe) can be utilized as an efficient heterogeneous catalyst for the size-selective [4 + 2] hetero-Diels-Alder cycloaddition reaction. Due to its high chemical stability, this catalyst can be repeatedly used over six times. This work demonstrates that Zr-MOFs can serve as tailor-made scaffolds with enhanced flexibility for target-oriented functions.

New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides.

Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K-Ni-S compounds. It is also possible to access several new phases, including a new polytope of ß-K2Ni3S4, as well as low-valence KNi4S2 and K4Ni9S11. KNi4S2 is a two-dimensional low-valence nickel-rich sulfide, and ß-K2Ni3S4 has a hexagonal lattice. Moreover, using KNi4S2 as a template, we obtained a new layered binary Ni2S by topotactic deintercalation of K. The new binary Ni2S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi4S2 and Ni2S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi4S2 and Ni2S, corresponding to the flat-bands with large Ni-dx2-y2 character.

Design and Characterization of Metal Nanoparticle Infiltrated Mesoporous Metal-Organic Frameworks.

The infiltration of palladium and platinum nanoparticles (NPs) into the mesoporous metal-organic framework (MOF) CYCU-3 through chemical vapor infiltration (CVI) and incipient wetness infiltration (IWI) processes was systematically explored as a means to design novel NP@MOF composite materials for potential hydrogen storage applications. We employed a traditional CVI process and a new ″green″ IWI process using methanol for precursor infiltration and reduction under mild conditions. Transmission electron microscopy-based direct imaging techniques combined with synchrotron-based powder diffraction (SPD), energy-dispersive X-ray spectroscopy, and physisorption analysis reveal that the resulting NP@MOF composites combine key NP and MOF properties. Room temperature hydrogen adsorption capacities of 0.95 and 0.20 mmol/g at 1 bar and 2.9 and 1.8 mmol/g at 100 bar are found for CVI and IWI samples, respectively. Hydrogen spillover and/or physisorption are proposed as the dominating adsorption mechanisms depending on the NP infiltration method. Mechanistic insights were obtained through the crystallographic means using SPD-based difference envelope density analysis, providing previously underexplored details on NP@MOF preparations. Consequently, important host-guest correlations influencing the global hydrogen adsorption properties are discussed, and they demonstrate that employing MOFs as platforms for NPs is an alternative approach to the development of versatile materials for improving current hydrogen storage technologies.

New Insights into the High-Performance Black Phosphorus Anode for Lithium-Ion Batteries.

Black phosphorus (BP) is a promising anode material in lithium-ion batteries (LIBs) owing to its high electrical conductivity and capacity. However, the huge volume change of BP during cycling induces rapid capacity fading. In addition, the unclear electrochemical mechanism of BP hinders the development of rational designs and preparation of high-performance BP-based anodes. Here, a high-performance nanostructured BP-graphite-carbon nanotubes composite (BP/G/CNTs) synthesized using ball-milling method is reported. The BP/G/CNTs anode delivers a high initial capacity of 1375 mA h g-1 at 0.15 A g-1 and maintains 1031.7 mA h g-1 after 450 cycles. Excellent high-rate performance is demonstrated with a capacity of 508.1 mA h g-1 after 3000 cycles at 2 A g-1 . Moreover, for the first time, direct evidence is provided experimentally to present the electrochemical mechanism of BP anodes with three-step lithiation and delithiation using ex situ X-ray diffraction (XRD), ex situ X-ray absorption spectroscopy (XAS), ex situ X-ray emission spectroscopy, operando XRD, and operando XAS, which reveal the formation of Li3 P7 , LiP, and Li3 P. Furthermore, the study indicates an open-circuit relaxation effect of the electrode with ex situ and operando XAS analyses.

High Thermopower in a Zn-Based 3D Semiconductive Metal-Organic Framework.

Conductive metal-organic frameworks (c-MOFs) have drawn increasing attention for their outstanding performance in energy-related applications. However, the majority of reported c-MOFs are based on 2D structures. Synthetic strategies for 3D c-MOFs are under-explored, leaving unrealized functionality in both their structures and properties. Herein we report Zn-HAB, a 3D c-MOF comprised of hexaaminobenzene and Zn(II). Zn-HAB is shown to have microporosity with a band gap of approximately 1.68 eV, resulting in a moderate conductivity of 0.86 mS cm-1 and a high Seebeck coefficient of 200 µV K-1 at 300 K. The power factor of 3.44 nW m-1 K-2 constitutes the first report of the thermoelectric properties of an intrinsically conductive 3D MOF.

Peritectic phase transition of benzene and acetonitrile into a cocrystal relevant to Titan, Saturn's moon.

Benzene and acetonitrile are two of the most commonly used solvents found in almost every chemical laboratory. Titan is one other place in the solar system that has large amounts of these compounds. On Titan, organic molecules are produced in the atmosphere and carried to the surface where they can mineralize. Here, we report the phase diagram of mixtures of acetonitrile and benzene, and provide an account of the structure and composition of the phases. To mimic the environment on Titan more accurately, we tested the stability of the structure under liquid ethane. The results provide new insights into the structure and stability of potential extraterrestrial minerals. In light of Dragonfly, NASA's upcoming mission to Titan, revisiting the fundamental chemistry of the smallest molecules with modern methods and techniques can have significant contributions to this epochal mission and can open new research directions in chemistry.

Overcoming Fundamental Limitations in Adsorbent Design: Alkene Adsorption by Non-porous Copper(I) Complexes.

Purifying alkenes from alkanes requires cryogenic distillation. This consumes energy equivalent to countries of ca. 5 million people. Replacing distillation with adsorption processes would significantly increase energy efficiency. Trade-offs between kinetics, selectivity, capacity, and heat of adsorption have prevented production of an optimal adsorbent. We report adsorbents that overcome these trade-offs. [Cu-Br]3 and [Cu-H]3 are air-stable trinuclear complexes that undergo reversible solid-state inter-molecular rearrangements to produce dinuclear [Cu-Br⋅(alkene)]2 and [Cu-H⋅(alkene)]2 . The reversible solid-state rearrangement, confirmed in situ using powder X-ray diffraction, allows adsorbent design trade-offs to be overcome, coupling low heat of adsorption (-10 to -17 kJ mol-1alkene ), high alkene:alkane selectivity (47; 29), and uptake capacity (>2.5 molalkene mol-1Cu3 ). Most remarkably, [Cu-H]3 displays fast uptake and regenerates capacity within 10 minutes.