Exploring the Role of Ligand Connectivity in MOFs Mechanical Stability: The Case of MIL-100(Cr).

The key parameters governing the mechanical stability of highly porous materials such as metal-organic frameworks (MOFs) are yet to be clearly understood. This study focuses on the role of the linker connectivity by investigating the mechanical stability of MIL-100(Cr), a mesoporous MOF with a hierarchical structure and a tritopic linker, and comparing it to MIL-101(Cr) having instead a ditopic linker. Using synchrotron X-ray diffraction and infrared spectroscopy, we investigate the high-pressure behavior of MIL-100(Cr) with both solid and fluid pressure transmitting media (PTM). In the case of a solid medium, MIL-100(Cr) undergoes amorphization at about 0.6 GPa, while silicone oil as a PTM delays amorphization until 12 GPa due to the fluid penetration into the pores. Both of these values are considerably higher than those of MIL-101(Cr). MIL-100(Cr) also exhibits a bulk modulus almost ten times larger than that of MIL-101(Cr). This set of results coherently proves the superior stability of MIL-100(Cr) under compression. We ascribe this to the higher connectivity of the organic linker in MIL-100(Cr), which enhances its interconnection between the metal nodes. These findings shed light on the importance of linker connectivity in the mechanical stability of MOFs, a relevant contribution to the quest for designing more robust MOFs.

Halogen molecular modifications at high pressure: the case of iodine.

Metallization and dissociation are key transformations in diatomic molecules at high densities particularly significant for modeling giant planets. Using X-ray absorption spectroscopy and atomistic modeling, we demonstrate that in halogens, the formation of a connected molecular structure takes place at pressures well below metallization. Here we show that the iodine diatomic molecule first elongates by ∼0.007 Å up to a critical pressure of Pc ∼ 7 GPa, developing bonds between molecules. Then its length continuously decreases with pressure up to 15-20 GPa. Universal trends in halogens are shown and allow us to predict for chlorine a pressure of 42 ± 8 GPa for molecular bond-length reversal. Our findings contribute to tackling the molecule invariability paradigm in diatomic molecular phases at high pressures and may be generalized to other abundant diatomic molecules in the universe, including hydrogen.

Mesoporous Metal-Organic Framework MIL-101 at High Pressure.

The chromium terephthalate MIL-101 is a mesoporous metal-organic framework (MOF) with unprecedented adsorption capacities due to the presence of giant pores. The application of an external pressure can effectively modify the open structure of MOFs and its interaction with guest molecules. In this work, we study MIL-101 under pressure by synchrotron X-ray diffraction and infrared (IR) spectroscopy with several pressure transmitting media (PTM). Our experimental results clearly show that when a solid medium as NaCl is employed, an irreversible amorphization of the empty structure occurs at about 0.4 GPa. Using a fluid PTM, as Nujol or high-viscosity silicone oil, results in a slight lattice expansion and a strong modification of the peak frequency and shape of the MOF hydroxyl vibration below 0.1 GPa. Moreover, the framework stability is enhanced under pressure with the amorphization onset shifted to about 7 GPa. This coherent set of results points out the insertion of the fluid inside the MIL-101 pores. Above 7 GPa, concomitantly to the nucleation of the amorphous phase, we observe a peculiar medium-dependent lattice expansion. The behavior of the OH stretching vibrations under pressure is profoundly affected by the presence of the guest fluid, showing that OH bonds are sensitive vibrational probes of the host-guest interactions. The present study demonstrates that even a polydimethylsiloxane silicone oil, although highly viscous, can be effectively inserted into the MIL-101 pores at a pressure below 0.2 GPa. High pressure can thus promote the incorporation of large polymers in mesoporous MOFs.

Atypical reversed pressure-induced phase transformation in Ge nanowires.

Phase transformations of Ge under compression/decompression cycle at room temperature were studied in a diamond anvil cell (DAC) using in situ synchrotron x-ray diffraction, Raman spectroscopy and near infrared absorption techniques. Upon compression similar behavior is observed in nanowires and in bulk although a higher stability is observed in nanowires. The cubic-diamond phase (Ge-3C), the most energetically favorable phase, transforms into the ß-tin metallic phase at high pressure and the reverse Ge-ß-tin to Ge-3C transformation is generally inhibited by kinetics when pressure is released. While the transformation in Ge bulk leads mostly to Ge-ST12 phase, the loading/unloading cycle of Ge nanowires in DAC leads back to Ge-3C, exhibiting unprecedented size effects. A comprehensive characterization of the final states is described.

Mechanism of pressure induced amorphization of SnI4: A combined x-ray diffraction-x-ray absorption spectroscopy study.

We have studied the amorphization process of SnI4 up to 26.8 GPa with unprecedented experimental details by combining Sn and I K-edge x-ray absorption spectroscopy and powder x-ray diffraction. Standard and reverse Monte Carlo extended x-ray absorption fine structure (EXAFS) refinements confirm that the penta atomic SnI4 structural unit tetrahedron is a fundamental structural unit that appears preserved through the crystalline phase-I to crystalline phase-II transition that has been previously reported between 7 GPa and 10 GPa. Up to now, unexploited iodine EXAFS reveals to be extremely informative and confirms the progressive formation of iodine-iodine short bonds close to 2.85 Å. A coordination number increase of Sn in the crystalline phase-II region appears to be excluded, while the deformation of the tetrahedral units proceeds through a flattening that keeps the average I-Sn-I angle close to 109.5°. Moreover, we put in evidence the impact of pressure on the Sn near edge structure under competing geometrical and electronic effects.

Pressure-Induced Spin-Crossover Features at Variable Temperature Revealed by In Situ Synchrotron Powder X-ray Diffraction.

An accurate high-pressure X-ray diffraction investigation, at various temperatures, on a powder of a spin-crossover (SCO) complex has allowed the rare deconvolution of the structural features of the high-spin and low-spin phases. As a result, the pressure dependence of the structural parameters of the high-spin and low-spin phases can be discussed independently in the pressure domain where both phases co-exist within the powder. Consequently, crucial unprecedented information is given, such as the variation of bulk moduli with temperature, similar here in amplitude for both spin phases, the temperature-dependence of the pressure-induced SCO abruptness, the temperature dependence of the pressure at which SCO occurs, and arguments for a possible piezo-hysteresis. Performed on the molecular complex [Fe(PM-PeA)2 (NCSe)2 ] (PM-PeA=N-(2'-pyridylmethylene)-4-(phenylethynyl) aniline), this study reveals a pressure-induced SCO at 0.16 GPa and demonstrates that, when increasing temperature, the pressure of transition increases linearly, the abruptness of the pressure-induced SCO strongly increases, and the bulk moduli decrease.

A thermodynamically consistent phase diagram of a trimorphic pharmaceutical, l-tyrosine ethyl ester, based on limited experimental data.

Crystalline polymorphs possess different physical properties, and phase changes between those polymorphs may affect the properties of engineered materials such as drugs. This is very well illustrated by the large effort that is put into the capability to predict phase behaviour of pharmaceuticals to avoid the unexpected appearance of different crystal forms. Much progress has been made, but one of the remaining challenges is (the accuracy in) the prediction of phase behaviour as a function of temperature. Obviously, predictions should at a certain point be verified against experimental data; however, it may not always be easy to elucidate the phase behaviour of a given compound experimentally, because thermodynamically and kinetically controlled phenomena occur in a convoluted fashion in experimental data. The present paper discusses the trimorphism of l-tyrosine ethyl ester as an example case of how experimental data in combination with the thermodynamic tenets lead to a consistent phase diagram, which can be used as the basis for pharmaceutical formulations and for comparison with polymorph predictions by computer. The positions of the two-phase equilibria I-II, I-III, and I-L have been obtained experimentally. Using the Clapeyron equation and the alternation rule, it has been shown how the positions of the other equilibria II-L, III-L, and II-III can be deduced in combination with the stability rankings of the phases and the phase equilibria. The experimental data have been obtained by synchrotron X-ray diffraction, Raman spectroscopy, and thermal analysis as a function of pressure and temperature. Furthermore, laboratory X-ray diffraction as a function of temperature and differential scanning calorimetry have been used. At room temperature, form II is the most stable phase, which remains stable with increasing pressure, as it possesses the smallest specific volume. Form I becomes stable above 33 °C (306 K), but with increasing pressure it turns into form III. On thermodynamic grounds, form III is expected to have a stable domain at very low temperatures.

Unraveling the mechanical behaviour of hydrazine borane (NH2-NH2-BH3).

Crystalline B-N-H compounds of low molecular weight have been intensively investigated over the past two decades owing to their promises for chemical hydrogen storage. Hydrazine borane NH2-NH2-BH3 is one of the most recent examples of this family of materials. In the present work, we explored its structural behaviour under mechanical stimulus by synchrotron high pressure X-ray diffraction. It has been evidenced that hydrazine borane shows a gradual pressure-induced decrease of its unit cell dimension and the process is reversible when the applied pressure is released. The compressibility of this material was established to be relatively low (high bulk modulus) and highly anisotropic. As revealed by molecular simulations based on Density Functional Theory calculations, the mechanical behaviour of NH2-NH2-BH3 was correlated to the pressure-induced changes of its crystal structure in terms of intra- and intermolecular bond lengths and angles parameters.

Energy resolution of the CdTe-XPAD detector: calibration and potential for Laue diffraction measurements on protein crystals.

The XPAD3S-CdTe, a CdTe photon-counting pixel array detector, has been used to measure the energy and the intensity of the white-beam diffraction from a lysozyme crystal. A method was developed to calibrate the detector in terms of energy, allowing incident photon energy measurement to high resolution (approximately 140 eV), opening up new possibilities in energy-resolved X-ray diffraction. In order to demonstrate this, Laue diffraction experiments were performed on the bending-magnet beamline METROLOGIE at Synchrotron SOLEIL. The X-ray energy spectra of diffracted spots were deduced from the indexed Laue patterns collected with an imaging-plate detector and then measured with both the XPAD3S-CdTe and the XPAD3S-Si, a silicon photon-counting pixel array detector. The predicted and measured energy of selected diffraction spots are in good agreement, demonstrating the reliability of the calibration method. These results open up the way to direct unit-cell parameter determination and the measurement of high-quality Laue data even at low resolution. Based on the success of these measurements, potential applications in X-ray diffraction opened up by this type of technology are discussed.

Room-temperature photoinduced electron transfer in a Prussian blue analogue under hydrostatic pressure.

In from the cold: The Co(III)Fe(II) state of a CoFe Prussian blue analogue undergoes a Co(III)-Fe(II) →(Co(II)-Fe(III))* electron transfer at room temperature when irradiated by visible light (532 nm; see scheme). This property was confirmed using energy-dispersive X-ray absorption spectroscopy at the Co and Fe K-edges of the piezo-induced Co(III)Fe(II) state.

Compressibility of structural modulation waves in the chain compounds BaCoX2O7 (X = As, P): a powder study.

BaCoX2O7 (X = As, P) are built on magnetic 1D units in which strong aperiodic undulations originate from incommensurate structural modulations with large atomic displacive amplitudes perpendicular to the chain directions, resulting in very unique multiferroic properties. High-pressure structural and vibrational properties of both compounds have been investigated by synchrotron X-ray powder diffraction and Raman spectroscopy at room temperature and combined with density functional calculations. A structural phase transition is observed at 1.8 GPa and 6.8 GPa in BaCoAs2O7 and BaCoP2O7, respectively. Sharp jumps are observed in their unit-cell volumes and in Raman mode frequencies, thus confirming the first-order nature of their phase transition. These transitions involve the disappearance of the modulation from the ambient-pressure polymorph with clear spectroscopic fingerprints, such as reduction of the number of Raman modes and change of shape on some peaks. The relation between the evolution of the Raman modes along with the structure are presented and supported by density functional theory structural relaxations.

On the Stiffness of Gold at the Nanoscale.

The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V0 = 67.65(3) Å3, K0 = 170(3)GPa, at zero pressure.

Energy-dispersive X-ray absorption spectroscopy at LNLS: investigation on strongly correlated metal oxides.

An energy-dispersive X-ray absorption spectroscopy beamline mainly dedicated to X-ray magnetic circular dichroism (XMCD) and material science under extreme conditions has been implemented in a bending-magnet port at the Brazilian Synchrotron Light Laboratory. Here the beamline technical characteristics are described, including the most important aspects of the mechanics, optical elements and detection set-up. The beamline performance is then illustrated through two case studies on strongly correlated transition metal oxides: an XMCD insight into the modifications of the magnetic properties of Cr-doped manganites and the structural deformation in nickel perovskites under high applied pressure.

Mechanical energy storage performance of an aluminum fumarate metal-organic framework.

The aluminum fumarate MOF A520 or MIL-53-FA is revealed to be a promising material for mechanical energy-related applications with performances in terms of work and heat energies which surpass those of any porous solids reported so far. Complementary experimental and computational tools are deployed to finely characterize and understand the pressure-induced structural transition at the origin of these unprecedented levels of performance.

High-pressure study of x-ray diffuse scattering in ferroelectric perovskites.

We present a high-pressure x-ray diffuse scattering study of the ABO3 ferroelectric perovskites BaTiO3 and KNbO3. The well-known diffuse lines are observed in all the phases studied. In KNbO3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO3 linearly decreases to zero at approximately 11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.

Six-fold-coordinated phosphorus by oxygen in AlPO4 quartz homeotype under high pressure.

AlPO4 belongs to the berlinite quartz homeotype family, which has been the subject of intense high-pressure research triggered by the supposed existence of reversible pressure-induced amorphization. X-ray diffraction experiments, complemented with ab initio calculations, demonstrate the existence of two high-pressure crystalline polymorphs and show that AlPO4 shares the same two-stage densification mechanism as silica. In the first step, a compact hexagonal sublattice of oxygen atoms is formed. In the second step, the cations redistribute in the interstices giving rise to a monoclinic distorted CaCl2 phase. The most outstanding feature of the phase is that phosphorous becomes six-fold coordinated by oxygen, adopting a configuration unknown so far in solid-state science. This finding opens possibilities in the high-pressure chemistry of phosphorus. The close relationship of AlPO4 with silica suggests the existence of completely unexplored families of compounds analogous to those of six-fold-coordinated silicates but based on PO6.