Relationship between synthesis method-crystal structure-melting properties in cocrystals: the case of caffeine-citric acid.

The influence of the crystal synthesis method on the crystallographic structure of caffeine-citric acid cocrystals was analyzed thanks to the synthesis of a new polymorphic form of the cocrystal. In order to compare the new form to the already known forms, the crystal structure of the new cocrystal (C8H10N4O2·C6H8O7) was solved by powder X-ray diffraction thanks to synchrotron experiments. The structure determination was performed using `GALLOP', a recently developed hybrid approach based on a local optimization with a particle swarm optimizer, particularly powerful when applied to the structure resolution of materials of pharmaceutical interest, compared to classical Monte-Carlo simulated annealing. The final structure was obtained through Rietveld refinement, and first-principles density functional theory (DFT) calculations were used to locate the H atoms. The symmetry is triclinic with the space group P\overline{1} and contains one molecule of caffeine and one molecule of citric acid per asymmetric unit. The crystallographic structure of this cocrystal involves different hydrogen-bond associations compared to the already known structures. The analysis of these hydrogen bonds indicates that the cocrystal obtained here is less stable than the cocrystals already identified in the literature. This analysis is confirmed by the determination of the melting point of this cocrystal, which is lower than that of the previously known cocrystals.

Uranium speciation control by uranyl sulfate and phosphate in tailings subject to a Sahelian climate, Cominak, Niger.

Long-term uranium mobility in tailings is an environmental management issue. The present study focuses on two U-enriched layers, surficial and buried 14.5 m, of the tailings pile of Cominak, Niger. The acidic and oxidizing conditions of the tailings pile combined with evapotranspiration cycles related to the Sahelian climate control U speciation. Uraninite, brannerite, and moluranite as well as uranophane are relict U phases. EXAFS spectroscopy, HR-XRD, and SEM/WDS highlight the major role of uranyl sulfate groups in uranium speciation. Uranyl phosphate neoformation in the buried layer (paleolayer) acts as an efficient trap for uranium.

A Novel Porous Ti-Squarate as Efficient Photocatalyst in the Overall Water Splitting Reaction under Simulated Sunlight Irradiation.

A new porous titanium(IV) squarate metal-organic framework (MOF), denoted as IEF-11, having a never reported titanium secondary building unit, is successfully synthesized and fully characterized. IEF-11 not only exhibits a permanent porosity but also an outstanding chemical stability. Further, as a consequence of combining the photoactive Ti(IV) and the electroactive squarate, IEF-11 presents relevant optoelectronic properties, applied here to the photocatalytic overall water splitting reaction. Remarkably, IEF-11 as a photocatalyst is able to produce record H2 amounts for MOF-based materials under simulated sunlight (up to 672 µmol gcatalyst in 22 h) without any activity loss during at least 10 d.

Steric and electronic effects of ligand substitution on redox-active Fe4S4-based coordination polymers.

One of the notable advantages of molecular materials is the ability to precisely tune structure, properties, and function via molecular substitutions. While many studies have demonstrated this principle with classic carboxylate-based coordination polymers, there are comparatively fewer examples where systematic changes to sulfur-based coordination polymers have been investigated. Here we present such a study on 1D coordination chains of redox-active Fe4S4 clusters linked by methylated 1,4-benzene-dithiolates. A series of new Fe4S4-based coordination polymers were synthesized with either 2,5-dimethyl-1,4-benzenedithiol (DMBDT) or 2,3,5,6-tetramethyl-1,4-benzenedithiol (TMBDT). The structures of these compounds have been characterized based on synchrotron X-ray powder diffraction while their chemical and physical properties have been characterized by techniques including X-ray photoelectron spectroscopy, cyclic voltammetry and UV-visible spectroscopy. Methylation results in the general trend of increasing electron-richness in the series, but the tetramethyl version exhibits unexpected properties arising from steric constraints. All these results highlight how substitutions on organic linkers can modulate electronic factors to fine-tune the electronic structures of metal-organic materials.

Site Occupancy Determination in Th2Zn17- and TbCu7-types Sm2Fe17-xCox Compounds using Synchrotron Resonant Diffraction.

Sm2Fe17 compounds are high-performance permanent magnets. Cobalt substitution allows us to further improve their magnetic properties. Depending on the thermal treatment, cobalt-substituted compounds can be synthesized either in the TbCu7 (disordered) or in the Th2Zn17 (ordered) structure type. Rietveld refinement of the number of transition metal dumbbells replacing rare-earth atoms from synchrotron powder diffraction data shows that the TbCu7 disordered structure has the same composition as the ordered one (a transition metal-to-rare earth ratio of 8.5). Then, cobalt site occupancies have been determined in both structures using synchrotron resonant (anomalous) diffraction. Cobalt is found to be absent from the dumbbell sites. The diffraction results are confirmed by Mössbauer spectroscopy.

The Atypical Hysteresis of [Fe(C6 F5 Tp)2 ]: Overlay of Spin-Crossovers and Symmetry-Breaking Phase Transition.

The [FeII (C6 F5 Tp)2 ] spin-crossover complex is an atypical molecular switch, which can be converted upon annealing between two archetypal spin-crossover behaviours: from an extremely gradual spin-crossover to a broad hysteretic spin-transition (of ca. 65 K). The hysteresis shows an uncommon "rounded shape" that is reproducible upon cycling temperature. In depth structural studies reveal a first crystal phase transition, which occurs upon melting and recrystallizing at high temperature. This first irreversible transition is associated with a radical change in the crystal packing. More importantly, the "rounded and broad" hysteretic transition is shown to occur in a non-cooperative SCO system and is associated with the occurrence of a symmetry-breaking phase transition that appears when roughly ca. 50 % of the SCO complexes are switched.

The structure of kaliophilite KAlSiO4, a long-lasting crystallographic problem.

Kaliophilite is a feldspathoid mineral found in two Italian magmatic provinces and represents one of the 12 known phases with composition close to KAlSiO4. Despite its apparently simple formula, the structure of this mineral revealed extremely complex and resisted structure solution for more than a century. Samples from the Vesuvius-Monte Somma and Alban Hills volcanic areas were analyzed through a multi-technique approach, and finally the crystal structure of kaliophilite was solved using 3D electron diffraction and refined against X-ray diffraction data of a twinned crystal. Results were also ascertained by the Rietveld method using synchrotron powder intensities. It was found that kaliophilite crystallizes in space group P3 with unit-cell parameters a = 27.0597 (16), c = 8.5587 (6) Å, V = 5427.3 (7) Å3 and Z = 54. The kaliophilite framework is a variant of the tridymite topology, with alternating SiO4 and AlO4 tetrahedra forming sheets of six-membered rings (63 nets), which are connected along [001] by sharing the apical oxygen atoms. Considering the up (U) and down (D) orientations of the linking vertex, kaliophilite is the first framework that contains three different ring topologies: nine (1-3-5) (UDUDUD) rings, six (1-2-3) (UUUDDD) rings and twelve (1-2-4) (UUDUDD) rings. This results in a relatively open (19.9 tetrahedra nm-3) channel system with multiple connections between the double six-ring cavities. Such a framework requires a surprisingly large unit cell, 27 times larger than the cell of kalsilite, the simplest phase with the same composition. The occurrence of some Na for K substitution (3-10%) may be related to the characteristic structural features of kaliophilite. Micro-twinning, pseudo-symmetries and anisotropic hkl-dependent peak broadening were also detected, and they may account for the elusive character of the kaliophilite crystal structure.

Mechanistic and Thermodynamic Insights into Anion Exchange by Green Rust.

Fougerite is a naturally occurring green rust, that is, a layered double hydroxide (LDH) containing iron (Fe). Fougerite was identified in natural settings such as hydromorphic soils. It is one of the few inorganic materials with large anion adsorption capacity that stems from the presence of isomorphic substitutions of Fe2+ by Fe3+ in its layers. The importance of anion adsorption in the interlayer of LDH has often been highlighted, but we are still missing a mechanistic understanding and a thermodynamic framework to predict the anion uptake by green rust. We combined laboratory and in operando synchrotron X-ray diffraction and scattering experiments with geochemical modeling to contribute to filling this gap. We showed that the overall exchange process in green rusts having nanometer and micrometer sizes can be seen as a simple anion exchange mechanism without dissolution-recrystallization or interstratification processes. A thermodynamic model of ion exchange, based on the Rothmund and Kornfeld convention, made it possible to predict the interlayer composition in a large range of conditions. This multiscale characterization can serve as a starting point for the building of robust and mechanistic geochemical models that will allow predicting the role of green rust on the geochemical cycle of ions, including nutrients, in soils.

A New Lamellar Gold Thiolate Coordination Polymer, [Au(m-SPhCO2H)]n, for the Formation of Luminescent Polymer Composites.

The photoluminescence of gold thiolate clusters brings about many potential applications, but its origin is still elusive because of its complexity. A strategy in understanding the structure-properties relationship is to study closely related neutral gold thiolate coordination polymers (CPs). Here, a new CP is reported, [Au(m-SPhCO2H)]n. Its structure is lamellar with an inorganic layer made of Au-S-Au-S helical chains, similar to the [Au(p-SPhCO2H)]n analog. An in-depth study of its photophysical properties revealed that it is a bright yellow phosphorescent emitter with a band centered at 615 nm and a quantum yield (QY) of 19% at room temperature and in a solid state. More importantly, a comparison to the para-analog, which has a weak emission, displayed a strong effect of the position of the electron withdrawing group (EWG) on the luminescent properties. In addition, [Au(m-SPhCO2H)]n CPs were mixed with organic polymers to generate transparent and flexible luminescent thin films. The ability to tune the emission position with the appropriate contents makes these nontoxic polymer composites promising materials for lighting devices.

A highly conductive nanostructured PEDOT polymer confined into the mesoporous MIL-100(Fe).

Despite the higher efficiency, larger color range and faster stimulus response of polymeric electrochromic materials, their poor cyclability strongly hampers their application in optoelectronics. As an original strategy to stabilize and further nanostructure these polymers, herein an efficient encapsulation and in situ polymerization inside highly porous metal-organic frameworks (MOFs) is reported. In particular, the successful accommodation of poly(3,4-ethylendioxythiophene) (PEDOT) and its partially oxidized polarons inside the mesopores of the nontoxic iron trimesate MIL-100(Fe) is convincingly proved by a large panel of experimental techniques. Remarkably, the polymer-MOF interaction occurring for entrapped PEDOT within the pores (deeply assessed by experimental and simulation methods) might be responsible for the enhanced electrical conductivity of the resulting PEDOT@MIL-100(Fe) composite when compared to the insulating MIL-100(Fe) and the conductive free PEDOT. Furthermore, it was possible to observe the electrochromic properties of the PEDOT@MIL-100(Fe) composite, achieving an improved stability and good cyclability as a consequence of the effective protection by the MOF matrix.

Correction: Modulation of the mechanical energy storage performance of the MIL-47(VIV) metal organic framework by ligand functionalization.

Correction for 'Modulation of the mechanical energy storage performance of the MIL-47(VIV) metal organic framework by ligand functionalization' by Pascal G. Yot et al., Dalton Trans., 2019, DOI: 10.1039/c8dt04214d.

New Lamellar Silver Thiolate Coordination Polymers with Tunable Photoluminescence Energies by Metal Substitution.

The structures of two lamellar silver thiolate coordination polymers [Ag( p-SPhCO2H)] n (1) and [Ag( p-SPhCO2Me)] n (2) are described for the first time. Their inorganic part is composed of distorted Ag3S3 honeycomb networks separated by noninterpenetrated thiolate ligands. The main difference between the two compounds arises from dimeric hydrogen bonds present for the carboxylic acids. Indepth photophysical studies show that the silver thiolates exhibit multiemission properties, implying luminescence thermochromism. More interestingly, the synthesis of a heterometallic lamellar compound, [Ag0.85Cu0.15( p-SPhCO2H)] n (3), allows to obtain mixed metal thiolate coordination polymers and to tune the photophysical properties with the excitation wavelengths from a green vibronic luminescence to a single red emission band.

Modulation of the mechanical energy storage performance of the MIL-47(VIV) metal organic framework by ligand functionalization.

The functionalization of the metal-organic framework MIL-47(VIV) with ligands bearing bulky functional groups (-Br or -CF3) has been envisaged as a possible route to enhance the mechanical energy storage performances of this family of hybrid porous materials. This exploratory work was carried out by coupling advanced experimental techniques (mercury intrusion and X-ray powder diffraction) supported by density functional theory calculations. MIL-47(VIV)-BDC-CF3 was demonstrated to be one of the most promising porous materials for mechanical energy-related applications with performance in terms of work energy which surpasses that of any porous solids reported so far.

Thermodynamic and crystallographic model for anion uptake by hydrated calcium aluminate (AFm): an example of molybdenum.

Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose 93Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO42- in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl-/MoO42- exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl-/OH- exchange processes occur together with Cl-/MoO42- exchange. This competitive effect must be considered when determining the Cl-/MoO42- selectivity constant.

Commissioning of a multi-beamline femtoslicing facility at SOLEIL.

The investigation of ultrafast dynamics, taking place on the few to sub-picosecond time scale, is today a very active research area pursued in a variety of scientific domains. With the recent advent of X-ray free-electron lasers (XFELs), providing very intense X-ray pulses of duration as short as a few femtoseconds, this research field has gained further momentum. As a consequence, the demand for access strongly exceeds the capacity of the very few XFEL facilities existing worldwide. This situation motivates the development of alternative sub-picosecond pulsed X-ray sources among which femtoslicing facilities at synchrotron radiation storage rings are standing out due to their tunability over an extended photon energy range and their high stability. Following the success of the femtoslicing installations at ALS, BESSY-II, SLS and UVSOR, SOLEIL decided to implement a femtoslicing facility. Several challenges were faced, including operation at the highest electron beam energy ever, and achievement of slice separation exclusively with the natural dispersion function of the storage ring. SOLEIL's setup also enables, for the first time, delivering sub-picosecond pulses simultaneously to several beamlines. This last feature enlarges the experimental capabilities of the facility, which covers the soft and hard X-ray photon energy range. In this paper, the commissioning of this original femtoslicing facility is reported. Furthermore, it is shown that the slicing-induced THz signal can be used to derive a quantitative estimate for the degree of energy exchange between the femtosecond infrared laser pulse and the circulating electron bunch.

Selenite Uptake by Ca-Al LDH: A Description of Intercalated Anion Coordination Geometries.

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO32-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO32--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO3, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl2 shows much stronger affinity and larger distribution ratio (Rd ∼ 17800 L kg-1) toward SeO32- than AFm-SO4 (Rd ∼ 705 L kg-1). At stoichiometric SeO32- loading for anion exchange, the newly formed AFm-SeO3 phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO32- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl-, SO42-, MoO42-, and SeO32-). Contrary to AFm-SeO3 with inner-sphere SeO32- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO32- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

Mn2TeO6: a Distorted Inverse Trirutile Structure.

Inverse trirutile Mn2TeO6 was investigated using in situ neutron and X-ray powder diffraction between 700 °C and room temperature. When the temperature was decreased, a structural phase transition was observed around 400 °C, from a tetragonal (P42/mnm) to a monoclinic phase (P21/c), involving a doubling of the cell parameter along b. This complex monoclinic structure has been solved by combining electron, neutron, and synchrotron powder diffraction techniques at room temperature. It can be described as a distorted superstructure of the inverse trirutile structure, in which compressed and elongated MnO6 octahedra alternate with more regular TeO6 octahedra, forming a herringbone-like pattern. Rietveld refinements, carried out with symmetry-adapted modes, show that the structural transition, arguably of Jahn-Teller origin, is driven by a single primary mode.

Structural Study of a Doubly Ordered Perovskite Family NaLnCoWO6 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb): Hybrid Improper Ferroelectricity in Nine New Members.

The compounds of the doubly ordered perovskite family NaLnCoWO6 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) were synthesized by solid-state reaction, nine of which (Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) are new phases prepared under high-temperature and high-pressure conditions. Their structural properties were investigated at room temperature by synchrotron X-ray powder diffraction and neutron powder diffraction. All of them crystallize in monoclinic structures, especially the nine new compounds have the polar space group P21 symmetry, as confirmed by second harmonic generation measurements. The P21 polar structures were decomposed and refined in terms of symmetry modes, demonstrating that the polar mode is induced by two nonpolar modes in a manner of Hybrid Improper Ferroelectricity. The amplitudes of these three major modes all increase with decreasing the Ln cation size. The spontaneous ferroelectric polarization is estimated from the neutron diffraction data of three samples (Ln = Y, Tb, and Ho) and can be as large as ∼20 µC/cm2.

A Flexible Fluorescent Zr Carboxylate Metal-Organic Framework for the Detection of Electron-Rich Molecules in Solution.

A novel Zr(IV) dicarboxylate metal organic framework (MOF) built up from an s-tetrazine derived ligand was prepared. This solid, which exhibits a diamond type network, combines a good stability in water, a structural flexibility, and fluorescence properties thanks to the organic ligand. It is noteworthy that this fluorescence is quenched when exposed to electron-rich molecules in solution, such as amines or phenol, this phenomenon being associated with the adsorption of the quencher, as unambiguously proven by X-ray diffraction (XRD) analysis. Finally, the quenching efficiency is shown to be governed not only by electronic and steric factors but also by the relative polarity of the solvent, the MOF, and the quencher. This work thus suggests that it is possible to develop new MOF-based sensors presenting in a given medium (such as water) highly selective responses.

Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe.

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms-edge sorption, interfacial dissolution-reprecipitation-are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.