ConspectusNitrides represent a class of chemical compounds of high scientific and technological relevance. Nevertheless, due to the challenging synthetic conditions, essentially related to the stability of the N2 molecule, nitrides have remained largely unexplored compared with the corresponding oxides.The laser-heated diamond anvil cell (LH-DAC), providing access to the GPa pressure range and temperatures as high as several thousands of K, has dramatically changed the rules of the game, unveiling thermodynamic conditions in which N2 becomes unstable and polymerizes into extended crystalline phases. A variety of N-compounds have been indeed synthesized by laser-heating the corresponding elements in a N2 environment under high-pressure (HP) conditions.Nevertheless, only recently group 15 elements heavier than N have been targeted by this method, and excluding α-P3N5 and γ-P3N5, crystalline pnictogen nitrides have remained essentially unknown.Since the discovery of phosphorene, while the quest for 2D materials has raised the interest for group 15 xenes and their N-doping as a key development, the inherent tendency of pnictogens to adopt crystalline layered structures persisting at high pressure has increased focus on binary N-compounds with heavier pnictogens, with implications for fundamental chemistry and potential applications.In this scenario, the discovery of the pseudo simple-cubic (p-sc) structure in the phase diagram of P, and, a few years later, the high-pressure-high-temperature (HP-HT) synthesis of PH3 from the elements and the discovery of the crystalline van der Waals (vdW) compound (PH3)2H2 have further reconnected the HP behavior of P to that of lighter N.These experimental studies, highlighting consistency in the structural and reactive properties of group 15 elements at high pressure, together with additional theoretical and computational insights, have opened new perspectives and motivated further investigations about the existence of crystalline pnictogen nitrides.Indeed, not only α- and γ-P3N5, but also three other crystalline polymorphs of phosphorus nitride (δ-P3N5, PN2, and α'-P3N5), have been synthesized by the direct HP-HT chemical reaction of P and N2 in a LH-DAC.Moving down in group 15, the same method has led to the discovery of the first crystalline nitrides of arsenic (AsN) and antimony (Sb3N5), whose existence has always represented an open question in inorganic chemistry, and to the structural characterization of two crystalline polymorphs of bismuth nitride (BiN).This Account provides an overview of the recent progress in the high-pressure and high-temperature synthesis of crystalline pnictogen nitrides, demonstrating the effective activation of a direct chemistry between N and heavier pnictogens. The presented results mark fundamental advancements in the chemistry of group 15 elements and pioneer the discovery of new advanced pnictogen-based materials of energetic and technological relevance, potentially recoverable under ambient conditions as stable or metastable systems.

The electrochemical 2e- oxygen reduction reaction (ORR) represents a green, cost-effective strategy towards hydrogen peroxide (H2O2) production other than a promising and more sustainable alternative to the currently anthraquinone-based technology. Light-weight hetero-doped carbon networks, particularly oxidized systems containing variables O-functionalities, have been deeply investigated as promising and selective metal-free electrocatalysts for the process. Following previous and positive outcomes from our team on the tailored surface engineering of complex C-nanocarbon networks with phenolic dangling groups as effective O-functionalities engaged for the molecular oxygen activation and its selective (2e-) electroreduction, we propose hereafter a facile, scalable and highly reproducible one-pot protocol for the mild and controlled oxidation of multi-walled carbon nanotubes. The as-prepared materials have shown a phenolic enriched surface and a superior ability to foster the almost chemoselective 2e- ORR process already under low overpotential values.

Black phosphorus (bP) is a crystalline material that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. The knowledge of its pressure-temperature (P-T) phase diagram, and in particular, the slope and location of its melting curve is fundamental for better understanding the synthesis and stability conditions of this important material. Despite several experimental studies, important uncertainties remain in the determination of this melting curve. Here we report accurate melting points measurements, using in situ high-temperature and high-pressure high-resolution synchrotron x-ray diffraction. In particular, we have employed an original and accurate pressure and temperature metrology based on the unique anisotropic P-T response of bP, that we used as sensor for the simultaneous determination of pressure and temperature up to 5 GPa and 1700 K. We confirmed the existence of and located a solid-liquid-liquid triple point at the intersection of the low- and high-pressure melting curves. Finally, we have characterized the irreversibility of the transformation in the low-pressure regime below 1 GPa, as the low-density liquid does not crystallize back to bP but into red phosphorus on temperature quenching.

A chemical reaction between Sb and N 2 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 Sb 3 N 5 stoichiometry and structure belonging to orthorhombic space group Cmc 2 1 . Only Sb−N bonds are present in the covalent bonding framework, with two types of Sb atoms respectively forming SbN 6 distorted octahedra and trigonal prisms and three types of N atoms forming NSb 4 distorted tetrahedra and NSb 3 trigonal pyramids. Taking into account two longer Sb−N distances, the SbN 6 trigonal prisms can be depicted as SbN 8 square antiprisms and the NSb 3 trigonal pyramids as NSb 4 distorted tetrahedra. The Sb 3 N 5 structure can be described as an ordered stacking in the bc plane of bi- layers of SbN 6 octahedra alternated to monolayers of SbN 6 trigonal prisms (SbN 8 square antiprisms). The discovery of Sb 3 N 5 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.

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.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. The knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of the anisotropic thermo-mechanical properties and exfoliation mechanism of these materials. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane thermo-mechanical parameters by subjecting these materials to hydrostatic pressure conditions and collecting a large number of data points in small pressure and temperature increments. Based on the anisotropic behavior of bP, we propose the use of this material as sensor for the simultaneous determination of pressure and temperature in the range 0-5 GPa, 298-1700 K.

While polymeric carbon monoxide (pCO) has been experimentally found to remain amorphous and undecomposed at room temperature up to 50 GPa, the question of whether crystalline counterparts of it can be obtained naturally raises. From different computational studies, it can be inferred that either the crystallization of amorphous pCO (a-pCO) or its decomposition into a mixture of CxOy suboxides (x > y) or carbon and CO2 may occur. In this study, we report experimental investigations of the high temperature (700-4000 K) transformation of a-pCO in the 47-120 GPa pressure range, conducted by x-ray diffraction in laser heated diamond anvil cells. Our results show the formation of no crystalline phases other than CO2 phase V, thus indicating the decomposition of the pristine a-pCO into CO2 and, likely, a mixture of amorphous CxOy suboxides and amorphous carbon hardly detectable at extreme conditions. These results support the theoretical picture of the pCO decomposition. We also show that the pressure-temperature kinetic border for this decomposition is very steep, thus indicating a strongly pressure-dependent kinetic barrier.

Methane is widespread in the Universe, and its occurrence is intimately connected with that of water, often as clathrate hydrate, likely the priority form in which methane is stored in icy moons, water-rich exoplanets but also in the depths of Earth’s oceans. Arrangement and stability range of the crystalline structures, decomposition conditions, and miscibility of the resulting dense fluid mixtures are crucial for modeling the static and dynamic properties of these complex extraterrestrial environments and for identifying possible prebiotic reactive events under transient favorable conditions of pressure, temperature, and irradiation. Here, we report a high-pressure study of methane hydrate up to 4 GPa and 550 K. Ex situ synthesis of crystalline methane hydrate allowed the analysis of homogeneous samples by state of the art Raman and FTIR spectroscopy, accessing information which considerably expands and modifies our knowledge of the crystalline structures, of the decomposition conditions, and of the molten fluid’s characteristics in a wide pressure and temperature range.

Synthetic nitride chemistry is a very rapidly developing field.Many nitrides and nitride-based heterostructures are promising materials that can find application as superhard compounds, wide-bandgap semiconductors etc. Applying different high pressure-high temperature techniques facilitates nitridation and is the key to the synthesis of novel phases directly from elements.Both nitrogen and arsenic belong to group 15 of the periodic table, although they exhibit completely different physical and chemical properties.At normal conditions nitrogen consists of very stable (945 kJ mol

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.

The high pressure transformation of carbon monoxide into an amorphous polymeric material (a-pCO) is known since decades, yet the determination of its local structure is made difficult by the strong photoreactivity of this material along with the high hygroscopicity of the recovered samples. While accurate, nondestructive IR spectroscopy measurements at a few GPa show that a-pCO is a complex solid dominated by the sp(2) hybridization for C and with oxygen forming both single and double bonds with carbon, the potential evolution of this chemical structure at higher pressures is hitherto terra incognita. Here we report a high pressure investigation of pCO up to 56 GPa, at room temperature, based on in situ IR spectroscopy and synchrotron X-ray diffraction (XRD) measurements in diamond anvil cells (DACs). Our study shows that this material remains amorphous up to the maximum investigated pressure, with sp(2) C substantially converting to sp(3) C upon increasing pressure above 12-13 GPa. This occurrence indicates that unsaturated a-pCO modifies with pressure toward an entirely single bonded amorphous material similarly to its theoretically predicted crystalline counterpart.

Chemical reactivity between As and N 2 , 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 P2 1 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.

Black phosphorus (bP) is a crystalline material which can be seen as an ordered stacking of two-dimensional layers, referred to as phosphorene. The knowledge of the linear thermal expansion coefficients (LTECs) of bP is of great interest in the field of 2D materials for a better understanding of the anisotropic thermal properties and exfoliation mechanism of this material. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the LTECs of bP. Here, we report accurate thermal expansion measurements along the three crystallographic axes using in situ high temperature X-ray diffraction. From the progressive reduction of the diffracted intensities with temperature, we monitored the loss of the crystal structure of bP across the investigated temperature range, evidencing two thermal expansion regimes at temperature below and above 706 K. Below 706 K, a strong out-of-plane anisotropy can be observed, while at temperatures above 706 K a larger thermal expansion occurs along the a crystallographic direction. From our data and by taking advantage of ab initio optimization, we propose a detailed anisotropic thermal expansion mechanism of bP, which leads to an inter- and intra-layer destabilization. An interpretation of it, based on the high T perturbation of the stabilizing sp orbital mixing effect, is provided, consistent with the high pressure data.

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 (PH 3 ) 2 H 2 , a crystalline van der Waals (vdW) compound ( I 4 cm ) made of PH 3 and H 2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T ≲ 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 H 2 -containing vdW compounds of their molecular hydrides. The observation of (PH 3 ) 2 H 2 , 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.

A comparison between the high-pressure (0.8 GPa) photoinduced reactivity of black and red phosphorus at ambient temperature in the presence of ammonia has been conducted in diamond anvil cells (DACs), using spectroscopy (IR and Raman) and X-ray diffraction (XRD). Reactivity has been triggered exploiting the two-photon absorption of near-UV radiation by ammonia. The infrared characterization showed a very complex reactivity in the case of red phosphorus, proceeding to a much higher extent with respect to the black allotrope. Furthermore, Raman spectra showed the formation of molecular hydrogen and phosphine besides three different solid products. Whereas one of them is air sensitive, the other two are recoverable at ambient conditions. IR, Raman, and XRD data for the obtained products have been compared to those acquired on known HxPyNz compounds: for one of the two stable products, a fair matching was found with the XRD pattern and the IR spectrum of P3N3(NH2)(6)center dot(NH3)(0.5), whereas for the other one only the functional groups actually involved in the structure could be evinced from accurate Raman mapping of the sample, with no further information about composition or stoichiometry. High density conditions in combination with near-UV laser irradiation were thus proved to be effective in the formation of two stable reaction products featuring new P-N functionalities, both recoverable at ambient pressure. For the first time, a cyclic triphosphazene has been synthesized through the reaction of red phosphorus and ammonia triggered by UV light under moderate high-pressure conditions, possibly opening new perspectives about this topic.

The role of carbonaceous deposits (coke) formed in dehydrogenation catalysis has been extensively investigated over the last few decades mainly with respect to the deactivation of metal-based and metal-free heterogeneous catalysts. Although much less emphasized, coke deposits grown on selected metal oxides have also been described as active and selective phases for alkene dehydrogenation under an oxidative or non-oxidative atmosphere. This work describes the straightforward preparation of "coked" gamma-Al2O3 composites and their catalytic performance in the ethylbenzene (EB) direct dehydrogenation (DDH) to styrene (ST) under steam- and oxygen-free conditions. The study unveils the effective potentiality of a catalytic system already known to the scientific community but never employed for EB DDH under severe conditions, close to those commonly used in industrial plants (600 degrees C, 10 vol % EB/He, GHSV = 3000 h(-1)) Such a simple catalytic system has revealed a significant stability on long-term trials (>= 150 h) and markedly high ST selectivity (>= 97%) along with process rates (lambda up to 16.3 mmol(ST) g(cat)(-1) h(-1)) that are the highest claimed so far for related carbon systems at work in the process. Furthermore, the outlined performance of our composites in DDH is close to that claimed for classical iron-based industrial catalysts operating in the presence of a large amount of steam. gamma-Al2O3 precoking with an aliphatic C-source has shown additional beneficial effects on the ultimate gamma-Al2O3@C performance in DDH. These findings pave the way for the development of cheap and durable dehydrogenation catalysts. They rewrite (in part at least) the role of coke in a challenging heterogeneous process while offering important hints to the comprehension of the reaction mechanism promoted by plain C-sites.

1 LENS, European Laboratory for Non-Linear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino (FI), Italy 2 ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National Research Council of Italy, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy 3 Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della Lastruccia 3, I-50019, Sesto Fiorentino (FI), Italy 4 ESRF, European Synchrotron Radiation Facility, B.P.220, F-38043 Grenoble Cedex, France

ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National Research Coouncil of Italy, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy LENS European Laboratory for Non-Linear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino (FI), Italy Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino (FI), Italy ESRF European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble (France)

An efficient loading technique has been developed for flammable, toxic, or explosive gases which can be condensed at liquid nitrogen temperature and ambient pressure in membrane diamond anvil cells (DACs). This cryogenic technique consists in a deposition of small quantities of the desired gas directly into the sample chamber. The deposition is performed using a capillary that reaches the space between the diamond anvils. The DAC is kept under inert gas overpressure during the whole process, in order to avoid contamination from atmospheric O2, CO2, and H2O. This technique provides significant advantages over standard cryo-loading and gas-loading when the condensation of dangerous samples at liquid nitrogen temperature raises safety concerns because it allows dealing with minimum quantities of condensed gases. The whole procedure is particularly fast and efficient. The "spray-loading" has been successfully used in our laboratory to load several samples including acetylene, ammonia, ethylene, and carbon dioxide/water or red phosphorus/NH3 mixtures.