X-ray Powder Diffraction Experiments Research Articles

Comparative Study of Experiments and Calculations on the Guest Molecules’ Escaping Mechanism of CL-20-Based Host–Guest Energetic Materials

CL-20-based host–guest energetic materials have high application value in the field of energetic materials. The phase-transition inhibition and stabilization mechanisms of CL-20-based host–guest energetic materials were revealed by combining experiments with molecular dynamics (MD) simulation. Based on the crystal structures, the super-cell models of host–guest energetic materials including CL-20/H2O, CL-20/H2O2, CL-20/N2O, and CL-20/CO2 were constructed. The phase transformation behavior and stability sequence of CL-20-based host–guest energetic materials were investigated using MD integrated with in situ powder X-ray diffraction (PXRD) and differential scanning calorimetry-thermogravimetry (DSC-TG) experiments. The escaping abilities of guest molecules from (002), (020), (021), (102), and (111) crystal faces of CL-20/H2O, CL-20/H2O2, CL-20/N2O, and CL-20/CO2 were evaluated using the target-MD method. It is proposed that the guest molecules could escape from the (002) crystal face more easily than the other faces. The escaping behaviors were studied using MD simulations. Based on the analysis of mean-square displacement (MSD), diffusion coefficients, and binding energies at different temperatures, the thermal stability order of the four CL-20-based host–guest materials was CL-20/CO2 > CL-20/N2O > CL-20/H2O2 > CL-20/H2O. This is consistent with experimental observations by in situ XRD and DSC analyses on these host–guest energetic materials. Meanwhile, by studying the escape paths of guest molecules, it was concluded that the escape method of guest molecules from a CL-20 crystal cell is “jumping” diffusion. By studying the MSD of host molecule CL-20 and the radial distribution function between the host molecule and the guest molecule, the structural stabilization mechanism of host–guest energetic materials was revealed. Our findings will help to reveal the solid-phase-transition inhibition mechanism of host–guest energetic materials and promote the development of host–guest explosives as smart materials.

Metalo Hydrogen-Bonded Organic Frameworks Self-Assembled by Charge-Assisted Synthons for Ultrahigh Proton Conduction

Hydrogen-bonded organic frameworks (HOFs) have been well recognized for constructing solid-state proton-conducting materials due to their inherent well-defined H-bonded networks; however, the design and synthesis of stable HOFs showing ultrahigh superprotonic conductivity is still challenging. Herein, we reported a facile and effective synthetic strategy to build high proton-conducting metalo hydrogen-bonded organic–inorganic frameworks (MHOFs) through charge-assisted inorganic and organic synthons. The supramolecular self-assembly of simple metal salts and tetra-carboxylic acids gives rise to two hydrogen-bonded frameworks, [M(H2O)6][H2tcba] (M = Zn2+, Ni2+; H4tcba = 1,2,4,5-benzenetetracarboxylic acid). X-ray crystallography and gas adsorption confirmed the nonporous nature of the HOIFs. Variable-temperature single-crystal and powder X-ray diffraction experiments reveal the thermal and water stability of the supramolecular structures sustained by exclusively charge-assisted hydrogen bonds. The high density of carboxylic acid groups and coordinated water molecules provide ample protons and hydrophilic environments for efficient proton transport; therefore, variable-temperature and -humidity electrochemical impedance spectroscopy revealed that MHOFs are good supramolecular proton conductors with conductivities up to 1.1 × 10–2 and 2.1 × 10–2 S·cm–1 for 1 and 2, respectively, at 80 °C under 97% RH. Our study demonstrates great potential of charge-assisted inorganic and organic synthons in the design and construction of proton-conducting HOIFs.

Thermodynamic analysis and in situ powder X-ray diffraction investigation of the formation and transformation of mineral phases during pressure oxidation of pyrite

Acid oxidative pressure leaching is a cost-effective way for recovering gold from refractory gold ores such as gold-bearing pyrite, yet our knowledge of the dissolution, crystallization, and phase transformation during pressure leaching is still controversial. This is partially due to the lack of thermodynamic data and in situ studies of the leaching process. Here, we combine thermodynamic modelling and in situ powder X-ray diffraction (PXRD) to investigate mineralogical phase changes during oxidation of pyrite. The polyhedron method has been utilised to estimate the standard Gibbs free energy of formation (∆Gfo) and enthalpy of formation (∆Hfo) of 31 sulphate minerals from 25 °C to 300 °C. The calculated thermodynamic data for the sulphate minerals agree well with the literature, with an average relative error of 0.23% for both ∆Gfo and ∆Hfo demonstrating the reliability of the polyhedron method. Some new thermodynamic data for the mineral phases at elevated temperature was calculated and used to construct the Eh-pH diagrams for the Fe-S-H2O system at 225 °C, which were used to predict phase transformations and chemical reactions during oxidation of pyrite. Based on the analysis of thermodynamic diagrams, the formation of basic ferric sulphate (BFS: Fe(OH)SO4) is favoured at stronger acidic and higher solution redox potential conditions, while hematite and FeSO4∙H2O occur at low solution redox potential conditions. These predictions agreed with the results from laboratory-based in situ PXRD experiments, in which the mineralogical phase transformations were studied during acid pressure treatment of pyrite in a capillary. The influence of initial concentration of ferric ions on the change of mineral phase was revealed. The results confirmed the formation of FeSO4∙H2O and Fe(OH)SO4 phases during the heating (24–225 °C) and annealing stages. These phases may passivate pyrite surface and hence slow down further pyrite oxidation but were re-dissolved during the annealing and/or cooling stages, depending on the initial concentration of ferric ions.

Theoretical and 27Al NMR Spectroscopic Investigations of Binary Intermetallic Alkaline-Earth Aluminides.

The binary alkaline-earth aluminides AEAl2 (AE = Ca and Sr) and AEAl4 (AE = Ca-Ba) have been synthesized from the elements and investigated via powder X-ray diffraction experiments. CaAl2 adopts the cubic MgCu2-type structure (Fd3̅m), while SrAl2 crystallizes in the orthorhombic KHg2-type (Imma). LT-CaAl4 crystallizes with the monoclinic CaGa4-type (C2/m), while HT-CaAl4, SrAl4, and BaAl4 adopt the tetragonal BaAl4-type structure (I4/mmm). The close structural relation of the two CaAl4 polymorphs was established using a group-subgroup relation in the Bärnighausen formalism. In addition to the room-temperature and normal pressure phase of SrAl2, a high-pressure/high-temperature phase has been prepared using multianvil techniques, and its structural and spectroscopic parameters were determined. Elemental analysis by inductively coupled plasma mass spectrometry showed that no significant impurities with other elements besides the weighed ones are present and the chemical compositions match the synthesized ones. The title compounds have been furthermore investigated by 27Al solid-state magic angle spinning NMR experiments to validate the crystal structure and to gain information about the influence of the composition on the electron transfer and the NMR characteristics. This has also been investigated from a quantum chemical point of view using Bader charges, while the stabilities of the binary compounds in the three phase diagrams (Ca-Al, Sr-Al and Ba-Al) have been studied by calculations of formation energies per atom.

Anticooperativity and Competition in Some Cocrystals Featuring Iodine-Nitrogen Halogen Bonds.

Phenomena such as anticooperativity and competition among non-covalent bond donors and acceptors are key considerations when exploring the polymorphic and stoichiomorphic landscapes of binary and higher-order cocrystalline architectures. We describe the preparation of four cocrystals of 1,3,5-trifluoro-2,4,6-triiodobenzene with N-heterocyclic compounds, namely acridine, 3-aminopyridine, 4-methylaminopyridine, and 1,2-di(4-pyridyl)ethane. The cocrystals, which are characterized by single-crystal and powder X-ray diffraction experiments, all show moderately strong and directional iodine⋅⋅⋅nitrogen halogen bonds with reduced distance parameters ranging from 0.79 to 0.92 and carbon-iodine⋅⋅⋅nitrogen bond angles ranging from 165.4(3) to 175.31(7)°. The cocrystal comprising 1,3,5-trifluoro-2,4,6-triiodobenzene and acridine provides a relatively rare example where all three halogen bond donor sites form halogen bonds with three acceptor molecules, overcoming an anticooperative effect. This effect manifests itself through the lengthening of non-halogen-bonded C-I bonds, weakening their potential to form halogen bonds. The effect is only observed once two halogen bonds have been formed to 1,3,5-trifluoro-2,4,6-triiodobenzene; one such bond does not appear to be adequate. Among the four cocrystals studied, competition between the pyridyl nitrogen atoms and the amine nitrogen atoms suggests that the former are the preferred halogen bond acceptors. Analysis by Hirshfeld fingerprint plots and 13 C and 19 F magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy provides additional insights into the prevalence of various short contacts in the crystal structures and into the spectral response to halogen-bond-induced cocrystallization.

Formulation Optimization of Solid Dispersion of Candesartan Cilexetil

Context: Antihypertensive effects were achieved by quickly hydrolyzing candesartan cilexetil (CC), an inactive prodrug of candesartan, into active candesartan during absorption in the gastrointestinal tract. Due to its weak water solubility, CC has an inadequate intestinal absorption and a low oral bioavailability. Aim: The goal of this study was to make the medication CC more soluble in water. Materials and Methods: Low viscosity hydroxypropyl methylcellulose (HPMC) E5LV was used to prepare the solid dispersions through spray drying. Results: Study of dissolution, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), differential scanning calorimeter (DSC), and X-ray diffraction characterized the prepared solid dispersions. CC amorphized from its crystallized state, as shown by the findings from the SEM, DSC, and X-ray powder diffraction experiments. Comparing pure CC and solid dispersion, the dissolution rate was higher with the former. The surfactant and wetting property of HPMC E5LV slowed devitrification and had an anti-plasticization impact, increasing the solubility and stability of the solid dispersion. Conclusion: The final results indicated that the CC, a weakly water-soluble medication, dissolved much better in the solid dispersions.

A rigid calcium 2-ethylhexylphosphate one-dimensional polymer: synthesis, structure, thermal behaviour and decomposition chemistry.

The reactivity of two different phosphate esters has been explored with calcium ions. The reaction of calcium oxide with a mixture of mono/bis 2-ethylhexyl phosphate and 1,10-phenanthroline leads to the formation of the 1-D polymer [(phen)Ca(ehpH)2]n (1). On the other hand, the reaction of Ca(OAc)2 with trimethyl phosphate in the presence of 1,10-phenanthroline however yields needle-shaped colourless crystals of [(phen)Ca(OAc)2]n (2). Compounds 1 and 2 have been characterized by various spectroscopic and analytical techniques. The solid-state structures have been determined by single crystal X-ray diffraction studies. The arrangement of rigid and flexible parts in 1 makes it an interesting compound to be studied for its thermal behaviour. DSC experiments performed above and below room temperature reveal thermal transitions in both the ranges, where below room temperature transition is prominent. A combination of variable temperature powder and single crystal X-ray diffraction experiments provides further insights into these transitions. Solid-state bulk thermolysis of 1 at 500 °C leads to an amorphous material contaminated with graphitic carbon which upon further heating to 600 °C produces crystalline α-Ca(PO3)2.

A new family of anti-perovskite oxyhydrides with tetrahedral GaO4 polyanions.

New solid compounds A3-xGaO4H1-y (A = Sr, Ba; x ∼0.15, y ∼0.3), which are the first oxyhydrides containing gallium ions, have been synthesized by high-pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that the series adopts an anti-perovskite structure consisting of hydride-anion-centered HA6 octahedra with tetrahedral GaO4 polyanions, wherein the A- and H-sites show partial defect. Formation energy calculations from the raw materials support that stoichiometric Ba3GaO4H is thermodynamically stable with a wide band gap. Annealing the A = Ba powder under flowing Ar and O2 gas suggests topochemical H- desorption and O2-/H- exchange reactions, respectively.

Solid state NMR characterization of NaNbO3 and LiNbO3 nanoparticles obtained by microwave-assisted combustion

Nanoparticles of LiNbO3 and NaNbO3 were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO3, there are two polimorphs which are stable at room temperature, space groups P21ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state 23Na and 93Nb Nuclear Magnetic Resonance (NMR) techniques. 23Na results reveal the presence of both phases, Pbma and P21ma, for samples prepared using NaNO3 precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P21ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO3 nanoparticles display the usual rhombohedral structure R3c. 7Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li+ ions are found at the surface of the nanoparticles, while bulk Li+ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.

A noise-robust data assimilation method for crystal structure determination using powder diffraction intensity.

Crystal structure prediction for a given chemical composition has long been a challenge in condensed-matter science. We have recently shown that experimental powder x-ray diffraction (XRD) data are helpful in a crystal structure search using simulated annealing, even when they are insufficient for structure determination by themselves [Tsujimoto et al., Phys. Rev. Mater. 2, 053801 (2018)]. In the method, the XRD data are assimilated into the simulation by adding a penalty function to the physical potential energy, where a crystallinity-type penalty function, defined by the difference between experimental and simulated diffraction angles was used. To improve the success rate and noise robustness, we introduce a correlation-coefficient-type penalty function adaptable to XRD data with significant experimental noise. We apply the new penalty function to SiO2 coesite and ɛ-Zn(OH)2 to determine its effectiveness in the data assimilation method.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

In situ powder X-ray diffraction during hydrogen reduction of MoO3 to MoO2

The hydrogen reduction of molybdenum trioxide to molybdenum dioxide is not yet fully understood as evident by continuous scientific interest. Especially the effect of the potassium content on the reduction process has not yet been considered. We prepared several samples of molybdenum trioxide containing varying amounts of potassium by addition of potassium molybdate (K2MoO4). In situ powder X-ray diffraction experiments were then conducted to study the hydrogen reduction of these samples. We especially focused on the influence of the alkali content and on gaining insight into the importance of the intermediary product γ-Mo4O11. During the reduction process, MoO2 is formed from the reduction of MoO3, which then reacts with the starting material to form γ-Mo4O11. With increasing potassium content, the reduction rate is decreased and the fractional content of γ-Mo4O11 built up during the reduction process is increased. As evident from bulk sample reduction, this results in a significant increase in the grain size visualized via scanning electron microscopy. Our investigations once again underline the importance of γ-Mo4O11 on the morphology of the resulting MoO2 powder.

Phase stability of stress-sensitive Ag2CO3 silver carbonate at high pressures and temperatures

Silver carbonate (Ag2CO3) is a material currently used for artificial carbon storage. In this work, we report synchrotron X-ray powder diffraction (XRD) experiments under high pressure and high temperature in combination with density-functional theory (DFT) calculations on silver carbonate up to 13.3 GPa. Two pressure-induced phase transitions were observed at room temperature: at 2.9 GPa to a high-pressure (HP1) phase and at 10.5 GPa to a second high-pressure phase (HP2). The facts that a) the HP2 phase can be indexed with the initial P21/m structure, b) our DFT calculations predict the initial structure is stable in the entire pressure range, and c) the HP2 phase is stable under decompression suggest that the intermediate HP1 phase is a product of the appearance of non-hydrostatic stresses in the sample. The observed structural transformations are associated to a high sensitivity of this compound to non-hydrostatic conditions. The compressibility of Ag2CO3 has also been determined, showing the c axis is the most compressible and that the bulk modulus increases quickly with applied pressure. We attribute both observations to the weak nature of the closed-shell Ag–Ag interactions in this material. The behavior of Ag2CO3 under heating at approximately 3 GPa was also studied. No temperature-induced phase transitions were found at this pressure, and the thermal expansion was determined to be relatively high for a carbonate.

Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2.

Case studies of 1T-TiSe2 and YBa2Cu3O7-δ have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB2 lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB2 in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the Tc of approx. 39 K. The high-resolution single-crystal XRD experiments in combination with multipolar refinements presented here can reproduce these results but show that the observed temperature-dependent density changes are almost entirely due to a decrease of atomic displacement parameters as a natural consequence of a reduced thermal vibration amplitude with decreasing temperature. Our investigations also shed new light on the presence or absence of magnesium vacancies in MgB2 samples─a defect type claimed to control the superconducting properties of the compound. We propose that previous reports on the tendency of MgB2 to form non-stoichiometric Mg1-xB2 phases (1 - x ∼ 0.95) during high-temperature (HT) synthesis might result from the interpretation of XRD data of insufficient resolution and/or usage of inflexible refinement models. Indeed, advanced refinements based on an Extended Hansen-Coppens multipolar model and high-resolution X-ray data, which consider explicitly the contraction of core and valence shells of the magnesium cations, do not provide any significant evidence for the formation of non-stoichiometric Mg1-xB2 phases during HT synthesis.

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

(Keynote) Dynamics in Prussian Blue Analogs-Based Batteries Revealed By X-Ray Techniques

A key factor for the developing of post-lithium batteries is represented by suitable active materials to be used in the positive and the negative electrodes. Host nanostructured have insertion sites, channels and/or interlayer spacings allowing the rapid insertion and extraction of the required ions, occurring generally with little lattice strain, with concomitant reduction/oxidation of an active metal belonging to the host. In this context Prussian Blue Analogs (PBA) offer an interesting host material for a wide variety of cations, which is also characterized by an appropriate electroactive bimetallic network.Dynamic processes occurring in batteries can be studied by operando modality, which provide a realistic representation of the electrochemical reactions occurring at the electrodes, or in ex situ modality, which reflect a given state of charge (SOC) of the electrode material. Monitoring the local structure around the metallic sites as well as the periodic structure can be performed using spectroscopic methods and in particular X-rays. X-ray absorption spectroscopy (XAS) is a synchrotron radiation-based technique that provide both electronic and structural information on a selected atomic species in a sample. General guidelines are presented in order to give a useful guide for a correct interpretation of the EXAFS spectra of this class of compounds, which is characterized by close, but sufficiently separated, discontinuities of the absorption coefficient due to contiguous transition metal K-edges. The dynamic of the ions insertion/release electrochemical reaction can further be investigated by adopting a chemometric approach using a multivariate curve resolution with alternating least squares algorithm (MCR-ALS), with the intent to assess the number of species involved and their evolutions during the electrochemical process. X-ray powder diffraction (XRPD) experiments allow monitoring the periodic structure of a material following the intercalation/release process of the ions involved. The potentiality of the joint XAS-XRD approach in the newly proposed PBA cathodes materials for rechargeable batteries is here highlighted, giving emphases on the copper hexacyanoferrate, copper nitroprusside, and manganese hexacyanoferrate electrodes.1. Conti, S. Zamponi, M. Giorgetti, M. Berrettoni, W.H. Smyrl, Anal. Chem. 82(2010)3629.2. Giorgetti, ISRN Materials Science, 2013, 938625.3. Mullaliu, M. Sougrati, N. Louvain, G. Aquilanti, M. Doublet, L. Stievano and M. Giorgetti, Electrochimica Acta 257(2017)364.4. Mullaliu, G. Aquilanti, L. Stievano, P. Conti, J. R. Plaisier, S. Cristol, and M. Giorgetti, J. Phys. Chem. C 122(2018)15868.5. Mullaliu, P. Conti, G. Aquilanti, J. Plaisier, L. Stievano, and M. Giorgetti, Condens. Matter 3(2018)36.6. Mullaliu, G. Aquilanti, P. Conti, J. R. Plaisier, M. Fehse, L. Stievano, and M. Giorgetti, J. Phys. Chem. C 123(2019)8588.7. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, and M. Giorgetti, Small Methods 4(2020)1900529.8. Mullaliu, G. Aquilanti, P. Conti, M. Giorgetti, and S. Passerini, ChemSusChem 13(2020) 608.9. Mullaliu, M. Gaboardi, J. R. Plaisier, S. Passerini, and M. Giorgetti, ACS Applied Energy Materials 3(2020)5728.10. Li, A. Mullaliu, S. Passerini, and M. Giorgetti, Batteries 7(2021)Art. No. 5.11. Li, R. Sciacca, M. Maisuradze, G. Aquilanti, J. Plaisier, M. Berrettoni, and M. Giorgetti, Electrochimica Acta 400(2021) Art. No. 139414.

1,3-Butadiene on Titan: Crystal Structure, Thermal Expansivity, and Raman Signatures

Titan is one of the most Earth-like objects in the Solar System, hosting a nitrogen-rich atmosphere and an impressive inventory of organics ranging from simple to complex. 1,3-Butadiene has been suggested to be one of the photochemical products that could be present on Titan’s surface. As the simplest conjugated hydrocarbon, it is expected to partake in many chemical processes that influence Titan’s geology and evolution. However, the crystal structure and thermal expansivity of 1,3-butadiene, two of its most fundamental properties, have remained largely unknown for over eight decades, especially at cryogenic conditions often encountered on planetary bodies. In this work, a series of powder X-ray diffraction and micro-Raman experiments have been conducted in order to elucidate the crystal structure and thermal behavior of solid 1,3-butadiene between 85 and 165 K. Specifically, a single crystalline phase (space group P21/c) is observed in the temperature range of study in which the butadiene molecules form layers of zigzag chains that are stabilized by an intricate network of intermolecular C–H···π interactions. This monoclinic structure is shown to exhibit significant anisotropic thermal expansion along the three directions with the majority occurring along the crystallographic b-axis. The calculated bulk density of 1,3-butadiene (0.877 g/cm3 at 150 K) is considerably lower than previously realized. These results help enhance our understanding of the putative molecular solids that make up Titan’s surface and could hold important implications for physical as well as mechanical processes occurring on this icy satellite.

Crystal structure and magnetism of rare earth intermetallic compound Dy0.33Ho0.33Er0.33Ni

Multi-component rare earth intermetallic compound Dy0.33Ho0.33Er0.33Ni has been prepared by arc melting and characterized by powder X-ray diffraction and magnetization experiments. The sample crystallizes in orthorhombic FeB-type structure at 300 K and ferromagnetically orders around 40 K (TC). The TC is close to the average value of the ferromagnetic transition temperatures of DyNi, HoNi and ErNi compounds. Magnetization has been measured as a function of field up to 15 kOe and low field magnetocaloric effect around the magnetic transition temperature is computed.

Perovskite or Not Perovskite? A Deep-Learning Approach to Automatically Identify New Hybrid Perovskites from X-ray Diffraction Patterns.

Determining the crystal structure is a critical step in the discovery of new functional materials. This process is time consuming and requires extensive human expertise in crystallography. Here, a machine-learning-based approach is developed, which allows it to be determined automatically if an unknown material is of perovskite type frompowder X-ray diffraction. After training a deep-learning model on a dataset of known compounds, the structure types of new unknown compounds can be predicted using their experimental powder X-ray diffraction patterns. This strategy is used to distinguish perovskite-type materials in a series of new hybrid lead halides. After validation, this approach is shown to accurately identify perovskites (accuracy of 92% with convolutional neural network). From the identification of the key features of the patterns used to discriminate perovskites versus nonperovskites, crystallographers can learn how to quickly identify low-dimensional perovskites from X-ray diffraction patterns.

Impact of Pore Flexibility in Imine-Linked Covalent Organic Frameworks on Benzene and Cyclohexane Adsorption.

This work focuses on the impact of covalent organic frameworks' (COFs) pore flexibility in the adsorption and separation of benzene and cyclohexane. With this aim, we have selected the imine-linked 3D COFs COF-300 and LZU-111 as examples of flexible and rigid frameworks, respectively. Optimized syntheses at room temperature or in solvothermal conditions enabled us to selectively isolate the narrow-pore form of COF-300 (COF-300-rt) or a mixture of the narrow-pore and a larger-pore form (COF-300-st), respectively, with different textural properties (BET specific surface area = 39 or 1270 m2/g, respectively, from N2 adsorption at 77 K). In the case of LZU-111, only the room temperature route was successful, leading to the known microporous framework. COF-300-rt, COF-300-st, and LZU-111 were studied for benzene and cyclohexane adsorption and separation in static and dynamic conditions. At 298 K and 1 bar, these COFs adsorb more benzene (251, 221, and 214 cm3/g STP, respectively) than cyclohexane (175, 133, and 164 cm3/g STP, respectively). Moreover, the benzene and cyclohexane isotherms of COF-300-rt and COF-300-st are characterized by steps, as expected with a flexible material. Indeed, in situ powder X-ray diffraction experiments on benzene- and cyclohexane-impregnated batches enabled us to trap, for the first time, a sequence of forms of COF-300 with different pore aperture, rationalizing the stepped hysteretic isotherms. Finally, benzene/cyclohexane separation was evaluated using a benzene/cyclohexane 50:50 v/v flow at different temperatures (T = 298, 323, and 348 K): LZU-111 does not selectively retain any of the two components, while COF-300 exhibits stronger benzene-COF interactions also in dynamic conditions.