Simultaneous manipulation of iron(II) spin crossover and LIESST behaviour using pressure, temperature and light.

Spin crossover (SCO) and light-induced excited spin state trapping (LIESST) effects were studied using high pressure X-ray diffraction at cryogenic temperatures on a single crystal of the {[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (FeNb) coordination polymer. The studied compound does not show SCO or LIESST at ambient pressure, but these effects can be enforced by a mechanical stimulus. The obtained results demonstrate the manipulation of the spin state via the appropriate combination of multiple stimuli simultaneously.

Stability, Chemical Bonding, and Electron Lone Pair Localization in AsN at High Pressure by Density Functional Theory Calculations.

The covalent bonding framework of crystalline single-bonded cubic AsN, recently synthesized under high pressure and high temperature conditions in a laser-heated diamond anvil cell, is here studied by means of density functional theory calculations and compared to single crystal X-ray diffraction data. The precise localization of the nonbonding electron lone pairs and the determination of their distances and orientations are related to the presence of characteristic structural motifs and space regions of the unit cell dominated by repulsive electronic interactions, with the relative orientation of the electron lone pairs playing a key role in minimizing the energy of the structure. We find that the vibrational modes associated with the expression of the lone pairs are strongly localized, an observation that may have implications for the thermal conductivity of the compound. The results indicate the thermodynamic stability of the experimentally observed structure of AsN above ∼17 GPa, provide a detailed insight into the nature of the chemical bonding network underlying the formation of this compound, and open new perspectives to the design and high pressure synthesis of new pnictogen-based advanced materials for potential applications of energetic and technological relevance.

High-pressure and high-temperature synthesis of crystalline Sb3 N5.

A chemical reaction between Sb and N2 was induced under high-pressure (32-35 GPa) and high-temperature (1600-2200 K) conditions, generated by a laser heated diamond anvil cell. The reaction product was identified by single crystal synchrotron X-ray diffraction at 35 GPa and room temperature as crystalline antimony nitride with Sb3 N5 stoichiometry and structure belonging to orthorhombic space group Cmc21 . Only Sb-N bonds are present in the covalent bonding framework, with two types of Sb atoms respectively forming SbN6 distorted octahedra and trigonal prisms and three types of N atoms forming NSb4 distorted tetrahedra and NSb3 trigonal pyramids. Taking into account two longer Sb-N distances, the SbN6 trigonal prisms can be depicted as SbN8 square antiprisms and the NSb3 trigonal pyramids as NSb4 distorted tetrahedra. The Sb3 N5 structure can be described as an ordered stacking in the bc plane of bi- layers of SbN6 octahedra alternated to monolayers of SbN6 trigonal prisms (SbN8 square antiprisms). The discovery of Sb3 N5 finally represents the long sought-after experimental evidence for Sb to form a crystalline nitride, providing new insights about fundamental aspects of pnictogens chemistry and opening new perspectives for the high-pressure chemistry of pnictogen nitrides and the synthesis of an entire class of new materials.

Conformation-aggregation interplay in the simplest aliphatic ethers probed under high pressure.

The structures of the simplest symmetric primary ethers [(CnH2n+1)2O, n = 1-3] determined under high pressure revealed their conformational preferences and intermolecular interactions. In three new polymorphs of diethyl ether (C2H5)2O, high pressure promotes intermolecular CH...O contacts and enforces a conversion from the trans-trans conformer present in the α, ß and γ phases to the trans-gauche conformer, which is higher in energy by 6.4 kJ mol-1, in the δ phase. Two new polymorphs of dimethyl ether (CH3)2O display analogous transformations of the CH...O bonds. The crystal structure of di-n-propyl ether (C3H7)2O, determined for the first time, is remarkably stable over the whole pressure range investigated from 1.70 up to 5.30 GPa.

High-Pressure and High-Temperature Chemistry of Phosphorus and Nitrogen: Synthesis and Characterization of α- and γ-P3N5.

The direct chemical reactivity between phosphorus and nitrogen was induced under high-pressure and high-temperature conditions (9.1 GPa and 2000-2500 K), generated by a laser-heated diamond anvil cell and studied by synchrotron X-ray diffraction, Raman spectroscopy, and DFT calculations. α-P3N5 and γ-P3N5 were identified as reaction products. The structural parameters and vibrational frequencies of γ-P3N5 were characterized as a function of pressure during room-temperature compression and decompression to ambient conditions, determining the equation of state of the material up to 32.6 GPa and providing insight about the lattice dynamics of the unit cell during compression, which essentially proceeds through the rotation of the PN5 square pyramids and the distortion of the PN4 tetrahedra. Although the identification of α-P3N5 demonstrates for the first time the direct synthesis of this compound from the elements, its detection in the outer regions of the laser-heated area suggests α-P3N5 as an intermediate step in the progressive nitridation of phosphorus toward the formation of γ-P3N5 with increasing coordination number of P by N from 4 to 5. No evidence of a higher-pressure phase transition was observed, excluding the existence of predicted structures containing octahedrally hexacoordinated P atoms in the investigated pressure range.

Negative linear compressibility at extreme pressure.

Structural studies of chemical elements in extreme pressure conditions often lead to unpredictable and surprising results. At ultra-high pressure Yuan et al. [IUCrJ (2022), 9, 253-260] report a new crystal phase of selenium that exhibits negative linear compressibility.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen.

Chemical reactivity between As and N2 , leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single-crystal synchrotron X-ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P21 3 space group, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non-bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Extending the Stability Field of Polymeric Carbon Dioxide Phase V beyond the Earth's Geotherm.

We present a study on the phase stability of dense carbon dioxide (CO_{2}) at extreme pressure-temperature conditions, up to 6200 K within the pressure range 37±9 to 106±17 GPa. The investigations of high-pressure high-temperature in situ x-ray diffraction patterns recorded from laser-heated CO_{2}, as densified in diamond-anvil cells, consistently reproduced the exclusive formation of polymeric tetragonal CO_{2}-V at any condition achieved in repetitive laser-heating cycles. Using well-considered experimental arrangements, which prevent reactions with metal components of the pressure cells, annealing through laser heating was extended individually up to approximately 40 min per cycle in order to keep track of upcoming instabilities and changes with time. The results clearly exclude any decomposition of CO_{2}-V into the elements as previously suggested. Alterations of the Bragg peak distribution on Debye-Scherrer rings indicate grain coarsening at temperatures >4000 K, giving a glimpse of the possible extension of the stability of the polymeric solid phase.

Crystal Structure and Non-Hydrostatic Stress-Induced Phase Transition of Urotropine Under High Pressure.

High-pressure behavior of hexamethylenetetramine (urotropine) was studied in situ using angle-dispersive single-crystal synchrotron X-ray diffraction (XRD) and Fourier-transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure-transmitting media to study the effect of deviatoric stress on phase transformations. Up to 4 GPa significant damping of molecular librations and atomic thermal motion was observed. A first-order phase transition to a tetragonal structure was observed with an onset at approximately 12.5 GPa and characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non-hydrostatic conditions, induced by deviatoric or uniaxial stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage-like molecules and may be considered a common feature of such systems. DFT computations were performed to model urotropine equation of state and pressure dependence of vibrational modes. The first successful Hirshfeld atom refinements carried out for high-pressure diffraction data are reported. The refinements yielded more realistic C-H bond lengths than the independent atom model even though the high-pressure diffraction data are incomplete.

High pressure synthesis of phosphine from the elements and the discovery of the missing (PH3)2H2 tile.

High pressure reactivity of phosphorus and hydrogen is relevant to fundamental chemistry, energy conversion and storage, and materials science. Here we report the synthesis of (PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3 and H2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T â‰² 1000 K) chemical reaction of black phosphorus and liquid hydrogen, followed by room T compression above 3.5 GPa. Group 15 elements were previously not known to form H2-containing vdW compounds of their molecular hydrides. The observation of (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR, Raman), therefore represents the discovery of a previously missing tile, specifically corresponding to P for pnictogens, in the ability of non-metallic elements to form such compounds. Significant chemical implications encompass reactivity of the elements under extreme conditions, with the observation of the P analogue of the Haber-Bosch reaction for N, fundamental bond theory, and predicted high pressure superconductivity in P-H systems.

Pressure-Induced Polymerization of Polycyclic Arene-Perfluoroarene Cocrystals: Single Crystal X-ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons.

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp3 carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions, such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene cocrystals, naphthalene/octafluoronaphthalene (NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22% before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the pressure-induced breakdown of π-stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating one-dimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit-cell parameters. We show that the polymerization takes place along the arene-perfluoroarene π-stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

The p-sc structure in phosphorus: bringing order to the high pressure phases of group 15 elements.

Black phosphorus was studied by state-of-the-art synchrotron X-ray diffraction in a Diamond Anvil Cell during room temperature compression in the presence of He, H2, N2 and Daphne Oil 7474. The data demonstrate that the existence of the pseudo simple-cubic (p-sc) structure above 10.5 GPa is an intrinsic feature of P independent from the pressure transmitting medium. In the case of He, the pressure evolution of the lattice parameters and unit cell volume of P across the A17, A7 and p-sc structures was obtained and the corresponding EOS derived, providing a deeper insight on the recently reported p-sc structure. The results presented in this letter highlight the key role of the s-p orbital mixing in the formation and stabilization of the p-sc structure up to ∼30 GPa, solving apparent contradictions emerging from previous literature and finally bringing order to the sequence of the high pressure A7 layered structure in group 15 elements.

Crystalline polymeric carbon dioxide stable at megabar pressures.

Carbon dioxide is a widespread simple molecule in the Universe. In spite of its simplicity it has a very complex phase diagram, forming both amorphous and crystalline extended phases above 40 GPa. The stability range and nature of these phases are still debated, especially in view of their possible role within the deep carbon cycle. Here, we report static synchrotron X-ray diffraction and Raman high-pressure experiments in the megabar range providing evidence for the stability of the polymeric phase V at pressure-temperature conditions relevant to the Earth's lowermost mantle. The equation of state has been extended to 120 GPa and, contrary to earlier experimental findings, neither dissociation into diamond and ε-oxygen nor ionization was observed. Severe deviatoric stress and lattice deformation along with preferred orientation are removed on progressive annealing, thus suggesting CO2-V as the stable structure also above one megabar.

Impact of High Pressure on Metallophilic Interactions and Its Consequences for Spectroscopic Properties of a Model Tetranuclear Silver(I)-Copper(I) Complex in the Solid State.

Structure-property relationships were investigated via combined high-pressure spectroscopic and X-ray diffraction techniques for a model luminescent Ag2Cu2L4 (L = 2-diphenylphosphino-3-methylindole) complex in the crystalline state. The experimental results were contributed by theoretical calculations, compared with the previously evaluated light-induced geometrical changes, and discussed in the context of available literature to date. To the best of our knowledge, this is the first study of this kind devoted to a coinage-metal complex for which the argentophilic interactions are crucial. High-pressure X-ray diffraction and optical spectroscopy experiments showed close correspondence between structural changes and optical properties. The unit-cell angles, absorption edges, emission maxima, decay lifetimes and silver-copper bond trends, all change around 2-3 GPa. A blue-shift to red-shift switch when increasing the pressure was observed for both absorption and emission spectra. This is unique behavior when compared to the literature-reported coinage metal systems. It also occurred that the pressure-induced structural changes differ notably from the geometrical distortions observed for the excited state. Interestingly, shortening of the Ag-Ag bond itself does not ensure the red shift of the absorption and emission spectra. All the optical spectroscopy data are suggestive of an important role of defects, likely related to the lack of a hydrostatic pressure transmitting medium, for pressures higher than 3 GPa.

The role of H-bond in the high-pressure chemistry of model molecules.

Pressure is an extraordinary tool to modify direction and strength of intermolecular interactions with important consequences on the chemical stability of molecular materials. The decrease of the distance among nearest neighbour molecules can give rise to reactive configurations reflecting the crystal arrangement and leading to association processes. In this context, the role of the H-bonds is very peculiar because their usual strengthening with rising pressure does not necessarily configure a decrease of the reaction activation energy but, on the contrary, can give rise to an anomalous stability of the system. In spite of this central role, the mechanisms by which a chemical reaction is favoured or prevented by H-bonding under high pressure conditions is a poorly explored field. Here we review a few studies where the chemical behaviour of simple molecular systems under static compression was related to the H-bonding evolution with pressure. These results are able to clarify a wealth of changes of the chemical and physical properties caused by the strengthening with pressure of the H-bonding network and provide additional tools to understand the mechanisms of high-pressure reactivity, a mandatory step to make these synthetic methods of potential interest for applicative purposes.

Interlayer Bond Formation in Black Phosphorus at High Pressure.

Black phosphorus was compressed at room temperature across the A17, A7 and simple-cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X-ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple-cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two-step mechanism for the A7 to simple-cubic phase transition, indicating the existence of an intermediate pseudo simple-cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non-layered simple-cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene-based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

Structure-melting relations in isomeric dibromobenzenes.

1,4-Dibromobenzene melts at a considerably higher temperature than the 1,2- and 1,3-isomers. This melting-point difference is consistent with the molecular symmetry, as described by Carnelley's rule, and with the frequency of Br...Br halogen bonds. The lowest melting point of 1,3-dibromobenzene correlates with its two symmetry-independent molecules, indicating their inability to pack closely. Single crystals of 1,2- and 1,3-dibromobenzene have been grown under isochoric conditions in a diamond-anvil cell and at isobaric conditions in a glass capillary. Their structures have been determined in situ by X-ray diffraction. At 295 K 1,2-dibromobenzene crystallizes at 0.2 GPa as orthorhombic, space group Pbca, Z' = 1, and 1,3-dibromobenzene at 0.3 GPa as orthorhombic, space group P212121, Z' = 2. The same crystal phases are formed at ambient pressure by freezing these liquids below 256.15 and 248.45 K, respectively. The third isomer, 1,4-dibromobenzene, is a solid at room temperature and crystallizes as monoclinic, space group P21/a. Striking relations between the structures and melting points of the corresponding dibromobenzene and dichlorobenzene isomers have been discussed.

Pressure-induced pseudorotation in crystalline pyrrolidine.

Pressure of 1.14(3) GPa leads to the crystallization of a new ß phase of pyrrolidine, where every second molecule pseudorotates to the CH(2)-off envelope conformation, energetically less favored than the NH-off envelope. The ß phase is triclinic, space group P ̅1, but it is isostructural with monoclinic phase α, where all molecules are in the N-off envelope conformation. The energy associated with the pseudorotation constitutes only about 5% of the total energy change. The energetically unfavorable axial N-H position is retained.

Molecular symmetry and isostructural relations in crystal phases of trihalomethanes CHCl3, CHBrCl2, CHBr2Cl, and CHBr3.

Bromodichloromethane (CHBrCl(2)), dibromochloromethane (CHBr(2)Cl), and their parent trihalomethanes, chloroform (CHCl(3)) and bromoform (CHBr(3)), form an intriguing series of isostructural crystal phases, the sequence of which depends on the Br/Cl substitution and thermodynamic conditions. The phase behavior of these compounds has been studied by isobaric calorimetry and isothermal compression, and the crystal structure of CHBrCl(2) has been determined at 0.10 MPa/200 K, 0.73, 1.26, 2.53 GPa (all at 295 K), and that of CHBr(2)Cl at 0.43, 1.24 GPa (all at 295 K). CHBrCl(2) frozen by isobaric cooling at 0.10 MPa crystallizes in space group P1 with Z = 2, while its high-pressure polymorph in space group Pnma (Z = 4) is stable at 295 K from its freezing pressure at 0.48 to at least 2.53 GPa. At the freezing pressure of 0.29 GPa, CHBr(2)Cl crystallizes in space group P6(3), with Z = 2, and at 1.27 GPa, it transforms to the orthorhombic structure, space group Pnma (Z = 4); CHCl(3) has the identical symmetries, but their reverse sequence was observed. A subtle isostructural phase transition has been observed at 0.10 MPa and 214.9 K in CHBr(2)Cl. The relations between isostructural phases, their symmetry, and site occupation factors of halogen atoms observed in the low-temperature and high-pressure phases of trihalomethanes (CHCl(3), CHBrCl(2), CHBr(2)Cl, and CHBr(3)) have been explained by the directional character of electrostatic interactions between the molecules. A gradual ordering of the disordered Br and Cl atoms has been achieved in the compressed crystals, where the narrower volume of the atomic sites correlates with the increased occupancy of the smaller atom (chlorine). The molecular symmetry has been shown to control the molecular aggregation in the crystalline state, consistent with the crystal site-symmetry and the balance of electrostatic matching and dispersion forces between molecules.

Polar symmetry in new high-pressure phases of chloroform and bromoform.

Polar ordering has been induced by pressure in solid chloroform (trichloromethane), CHCl3, and bromoform (tribromomethane), CHBr3, obtained by isochoric and isothermal freezing in a diamond anvil cell. Structures of these new polymorphs have been determined by single-crystal X-ray diffraction, CHCl3 at 0.62 and 0.75 GPa and CHBr3 at 0.20 and 0.35 GPa. Despite different centrosymmetric structures of all low-temperature phases of CHCl3 (space group Pbcn) and CHBr3 (P6(3)/m, P1, and P3), the high-pressure phases are isostructural in space group P6(3). The polar phase of CHBr3 is formed at 295 K, already at the freezing pressure of approximately 0.1 GPa, while CHCl3 transforms from the Pbcn phase into the P6(3) phase between 0.62 and 0.75 GPa. It has been demonstrated that the electrostatic contribution to halogen...halogen and H...halogen interactions in the CHCl3 and CHBr3 molecular crystals is favorable for the polar aggregation and that this effect intensifies with increasing pressure.