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.

A mechanism for aragonite to post-aragonite transition in MCO3 (M = Ca, Sr and Ba) carbonates: evidence of a hidden metastable polymorph.

To advance in the understanding of the Earth's carbon cycle, it is necessary to determine thermodynamic boundaries and kinetic barriers associated with the pressure-induced polymorphic sequence of alkaline-earth carbonates. Following a symmetry-based strategy within the martensitic approximation, we propose a two-step mechanism mediated by a hexagonal P63/mmc structure for the aragonite to post-aragonite transformation in the MCO3 (M = Ca, Sr, Ba) crystal family. The calculated transition pressures and activation energies, from ∼7 to 42 GPa and ∼0.3 to 0.6 eV, respectively, are low enough to allow this transformation to occur under mantle conditions. Our analysis reveals that the intermediate hexagonal structure is the early one proposed by Holl et al., Phys. Chem. Miner., 2000, 27, 467-473 for high pressure BaCO3, and later considered as metastable. Phonon calculations inform that this P63/mmc structure is in fact unstable at zero pressure. Remarkably, our molecular dynamics calculations showed that this instability smoothly leads to a dynamically stable P63mc structure, which we confirm is actually the phase observed by Holl et al. This finding allows us to reconcile previous controversial data and contributes to clarifying the role of carbonates in the Earth's interior.

Pressure and temperature stability boundaries of cubic SiC polymorphs: a first-principles investigation.

A better understanding of the effects of temperature and pressure on the wide gap SiC semiconductor is necessary for both (i) an improvement of the performance of this compound in a variety of technological applications and (ii) a clarification of controversial issues related to the stability of its cubic polymorphs at high pressure and high temperature. Bearing in mind this double demand, we perform first-principles calculations of the phonon band structures, vibrational density of states, and thermal and mode Grüneisen parameters of the zinc blende (B3) and rock-salt (B1) cubic polymorphs of 3C-SiC covering pressures and temperatures up to 120 GPa and 3000 K, respectively. Under a martensitic description of the B3-B1 transformation, we found that the large hysteresis pressure range observed at room temperature (35-100 GPa) disappears at around 1100 K. The calculated Clapeyron slope of this transformation is slightly negative, with average values of -2.9 MPa K-1 in the 0-3000 K interval and -3.7 MPa K-1 at 2000 K. We also study the decomposition reaction of the two cubic polymorphs into their elemental constituents (C and Si), finding a decreasing (increasing) decomposition temperature for the B3 (B1) phase as the pressure is applied. All these results are sustained by good agreement with other recently reported experimental and theoretical thermodynamic data that have also been evaluated under our quasi-harmonic approximation framework.

Pressure-Driven Metallization in Hafnium Diselenide.

The quest for new transition metal dichalcogenides (TMDs) with outstanding electronic properties operating under ambient conditions draws us to investigate the 1T-HfSe2 polytype under hydrostatic pressure. Diamond anvil cell (DAC) devices coupled to in situ synchrotron X-ray, Raman, and optical (VIS-NIR) absorption experiments along with density functional theory (DFT)-based calculations prove that (i) bulk 1T-HfSe2 exhibits strong structural and vibrational anisotropies, being the interlayer direction especially sensitive to pressure changes, (ii) the indirect gap of 1T-HfSe2 tends to vanish by a -0.1 eV/GPa pressure rate, slightly faster than MoS2 or WS2, (iii) the onset of the metallic behavior appears at Pmet ∼10 GPa, which is to date the lowest pressure among common TMDs, and finally, (iv) the electronic transition is explained by the bulk modulus B0-Pmet correlation, along with the pressure coefficient of the band gap, in terms of the electronic overlap between chalcogenide p-type and metal d-type orbitals. Overall, our findings identify 1T-HfSe2 as a new efficient TMD material with potential multipurpose technological applications.

Controlling the off-center positions of anions through thermodynamics and kinetics in flexible perovskite-like materials.

Due to the network flexibility of their BX3 sub-lattice, a manifold of polymorphs with potential multiferroic applications can be found in perovskite-like ABX3 materials under different pressure and temperature conditions. The potential energy surface of these compounds usually presents equivalent off-center positions of anions connected by low energetic barriers. This feature facilitates a competition between the thermodynamic and kinetic control of the transitions from low to high symmetry structures, and explains the relationship between the rich polymorphism and network flexibility. In the rhombohedral phase of iron trifluoride, our first-principles electronic structure and phonon calculations reveal the factors that determine which of the two scenarios dominates the transition. At the experimentally reported rhombohedral-cubic transition temperature, the calculated fluorine displacements are fast enough to overcome forward and backward a barrier of less than 30 kJ mol-1, leading to an average structure with cubic symmetry. In addition, lattice strain effects observed in epitaxial growth and nanocrystallite experiments involving BX3 compounds are successfully mimicked by computing the phase stability of FeF3 under negative pressures. We predict a transition pressure at -1.8 GPa with a relative volume change around 5%, consistent with a first-order transition from the rhombohedral to the cubic structure. Overall, our study illustrates how, by strain tuning, either a thermodynamic or a kinetic pathway can be selected for this transformation.

Temperature and pressure-induced strains in anhydrous iron trifluoride polymorphs.

Various structural configurations of iron trifluoride appear at the nanoscale and macroscopic size, either in the amorphous or crystalline state. The specific atomic organization in these structures crucially alters the performance of FeF3 as an effective cathode in Li-ion batteries. Our detailed first-principles computational simulations examine the structural strains induced by temperature and stress on the four anhydrous polymorphs observed so far in FeF3 at ambient pressure. A wealth of data covering previous experimental results on their equilibrium structures and extending their characterization with new static and isothermal equations of state is provided. We inform on how porous apertures associated with the six-octahedra rings of the HTB and pyrochlore phases are modified under compressive and expansive strains. A quasi-auxetic behavior at low pressures for the ground state rhombohedral phase is detected, which is in concordance with its anomalous structural anisotropy. In contrast with the effect of temperature, this structure undergoes under negative pressure phase transitions to the other three polymorphs, indicating potential conditions where low-density FeF3 could show a better performance in technological applications.

Highs and Lows of Bond Lengths: Is There Any Limit?

Two distinct points on the potential energy curve (PEC) of a pairwise interaction, the zero-energy crossing point and the point where the stretching force constant vanishes, allow us to anticipate the range of possible distances between two atoms in diatomic, molecular moieties and crystalline systems. We show that these bond-stability boundaries are unambiguously defined and correlate with topological descriptors of electron-density-based scalar fields, and can be calculated using generic PECs. Chemical databases and quantum-mechanical calculations are used to analyze a full set of diatomic bonds of atoms from the s-p main block. Emphasis is placed on the effect of substituents in C-C covalent bonds, concluding that distances shorter than 1.14 Šor longer than 2.0 Šare unlikely to be achieved, in agreement with ultra-high-pressure data and transition-state distances, respectively. Presumed exceptions are used to place our model in the correct framework and to formulate a conjecture for chained interactions, which offers an explanation for the multimodal histogram of O-H distances reported for hundreds of chemical systems.

Generalized Stress-Redox Equivalence: A Chemical Link between Pressure and Electronegativity in Inorganic Crystals.

The crystal structure of many inorganic compounds can be understood as a metallic matrix playing the role of a host lattice in which the nonmetallic atomic constituents are located, the Anions in Metallic Matrices (AMM) model stated. The power and utility of this model lie in its capacity to anticipate the actual positions of the guest atoms in inorganic crystals using only the information known from the metal lattice structure. As a pertinent test-bed for the AMM model, we choose a set of common metallic phases along with other nonconventional or more complex structures (face-centered cubic (fcc) and simple cubic Ca, CsCl-type BaSn, hP4-K, and fcc-Na) and perform density functional theory electronic structure calculations. Our topological analysis of the chemical pressure (CP) scalar field, easily derived from these standard first-principles electronic computations, reveals that CP minima appear just at the precise positions of the nonmetallic elements in typical inorganic crystals presenting the above metallic subarrays: CaF2, rock-salt and CsCl-type phases of CaX (X = O, S, Se, Te), BaSnO3, K2S, and NaX (X = F, Cl, Br, I). A theoretical basis for this correlation is provided by exploring the equivalence between hydrostatic pressure and the oxidation (or reduction) effect induced by the nonmetallic element on the metal structure. Indeed, our CP analysis leads us to propose a generalized stress-redox equivalence that is able to account for the two main observed phenomena in solid inorganic compounds upon crystal formation: (i) the expansion or contraction experienced by the metal structure after hosting the nonmetallic element while its topology is maintained and (ii) the increasing or decreasing of the effective charge associated with the anions in inorganic compounds with respect to the charge already present in the interstices of the metal network. We demonstrate that a rational explanation of this rich behavior is provided by means of Pearson-Parr's electronegativity equalization principle.

Disclosing the behavior under hydrostatic pressure of rhombohedral MgIn2Se4 by means of first-principles calculations.

AM2X4 crystalline materials display important technological electronic, optical and magnetic properties that are sensitive to general stress effects. In this paper, the behavior under hydrostatic pressure of the ambient condition rhombohedral phase of MgIn2Se4 is investigated in detail for the first time. We carried out first-principles calculations within the density functional theory framework aimed at determining the pressure-induced polymorphic sequence of this selenide. To accurately evaluate transition pressures at room temperature, thermal corrections have been included after the computation of phonon dispersion curves in potential candidate phases, namely the initial rhombohedral R3[combining macron]m one, inverse and direct spinels, LiTiO2-type and defective I4[combining macron] structures. Only the transition from the R3[combining macron]m to the inverse spinel phase was found to fulfill the thermodynamic and mechanical stability criteria. The direct spinel could appear as metastable if kinetic effects hinder the above transition. Additionally, electronic band structures and chemical bonding properties were analyzed from the outcome of our quantum-mechanical solutions reporting band gap values and ionicity and noncovalent interaction indexes. It is shown that the investigated compound keeps behaving as a semiconductor, loses its van der Waals interactions, and becomes more covalent as hydrostatic pressure is applied.

Chemical pressure-chemical knowledge: squeezing bonds and lone pairs within the valence shell electron pair repulsion model.

The valence shell electron pair repulsion (VSEPR) model is a demanding testbed for modern chemical bonding formalisms. The challenge consists in providing reliable quantum mechanical interpretations of how chemical concepts such as bonds, lone pairs, electronegativity, or hypervalence influence (or modulate) molecular geometries. Several schemes have been developed thus far to visualize and characterize these effects; however, to the best of our knowledge, no scheme has yet incorporated the analysis of the premises derived from the ligand close-packing (LCP) extension of the VSEPR model. Within the LCP framework, the activity of the lone pairs of the central atom and ligand-ligand repulsions constitute the two key features necessary to explain certain controversial molecular geometries that do not conform to the VSEPR rules. Considering the dynamical picture obtained when electron local forces at different nuclear configurations are evaluated from first-principles calculations, we investigate the chemical pressure distributions in a variety of molecular systems, namely, electron-deficient molecules (BeH2, BH3, BF3), several AX3 series (A: N, P, As; X: H, F, Cl), SO2, ethylene, SF4, ClF3, XeF2, and nonequilibrium configurations of water and ammonia. Our chemical pressure maps clearly reveal space regions that are totally consistent with the molecular and electronic geometries predicted by VSEPR and provide a quantitative correlation between the lone pair activity of the central atom and electronegativity of ligands, which are in agreement with the LCP model. Moreover, the analysis of the kinetic and potential energy contributions to the chemical pressure allows us to provide simple explanations on the connection between ligand electronegativity and electrophilic/nucleophilic character of the molecules, with interesting implications in their potential reactivity. NH3, NF3, SO2, BF3, and the inversion barrier of AX3 molecules are selected to illustrate our findings.

Early evolutionary divergence between papillary and anaplastic thyroid cancers.

Background: Papillary thyroid cancer (PTC) is the most common thyroid carcinoma and exhibits an almost uniformly good prognosis, while anaplastic thyroid cancer (ATC) is less frequent and is one of the most aggressive cancers usually resistant to conventional treatment. Current hypothesis posits that ATC derives from PTC through the progressive acquisition of a discrete number of genomic alterations and implies that the mutational landscape of ATC resembles that of PTC. However, the clinical behaviour of ATC and PTC is radically different. We decided to address the disconnection between the clinical behaviour of ATC and PTC and the proposed model of the progressive development of ATC from PTC. Patients and methods: We carried out exome sequencing of DNA from 14 ATC specimens including three cases of concomitant ATC and PTC as well as their corresponding normal DNA from 14 patients. The sequencing results were validated using droplet digital PCR. We carried out immunohistochemistry and immunofluorescence studies of the concomitant ATC and PTC cases. In addition, we integrated our sequencing results with the existing TCGA data. Results: Most of the somatic mutations identified in the ATC component differed from the ones in PTC in the cases of concomitant ATC and PTC. The trunks of the phylogenetic trees representing the somatic mutations were short with long branches. In one case of concomitant PTC and ATC specimens, we observed an infiltration of PTC cells within the ATC component. Moreover, we integrated our results with data obtained from TCGA and observed that the most frequent mutations found in ATC presented high cancer cell fraction values and were significantly different from the PTC ones. Conclusion: ATC diverge from PTC early in tumour development and both tumour types evolve independently. Our work allows the understanding of the relationship between ATC and PTC facilitating the clinical management of these malignancies.

Hysteresis and bonding reconstruction in the pressure-induced B3-B1 phase transition of 3C-SiC.

The determination of kinetic factors affecting phase metastability is crucial for the design of materials out of the ambient conditions. At a given temperature, the kinetic barrier associated with the reconstruction of the bonding network of a pressure-induced phase transition can be only overcome at pressures where the available vibrational energy of the system is equal or higher than the corresponding activation energy. Our work demonstrates that these pressures provide boundaries to hysteresis cycles that can be evaluated following a three-step computational strategy: (i) total energy electronic structure calculations, (ii) determination of vibrational contributions by means of a simple Debye model, and (iii) description of the energetic profile along the transition path in the framework of the martensitic approximation. In the 3C-SiC polytype, our results reveal that the high pressure rock-salt (B1) structure can not be quenched on release of pressure unless temperature is close to 0 K. The B1 phase transforms back to the low-pressure zinc blende (B3) polymorph at 300 K if pressure is below 30 GPa, in very good agreement with experimental observations. These results are supported by a full characterization of the B3-B1 energetic transition profile in terms of the chemical changes of the bonding network topologically analysed with the electron localization function.

Computer simulations of 3C-SiC under hydrostatic and non-hydrostatic stresses.

The response of 3C-SiC to hydrostatic pressure and to several uni- and bi-axial stress conditions is thoroughly investigated using first principles calculations. A topological interpretation of the chemical bonding reveals that the so-called non-covalent interactions enhance only at high pressure while the nature of the covalent Si-C bonding network keeps essentially with the same pattern. The calculated low compressibility agrees well with experimental values and is in concordance with the high structural stability of this polymorph under hydrostatic pressure. Under uniaxial [001] stress, the c/a ratio shows a noticeable drop inducing a closure of the band gap and the emergence of a metallic state around 40 GPa. This behavior correlates with a plateau of the electron localization function exhibiting a roughly constant and non-negligible value surrounding CSi4 and SiC4 covalent bonded units.

Structure and bonding in crystalline cesium uranyl tetrachloride under pressure.

A thorough investigation of pressure effects on the structural properties of crystalline cesium uranyl chloride was performed by means of first-principles calculations within the density functional theory framework. Total energies, equilibrium geometries and vibrational frequencies were computed at selected pressures up to 50 GPa. Zero pressure results present good agreement with available experimental and theoretical data. Our calculated equation of state parameters reveal that Cs2UO2Cl4 is a high compressible material, similar to other ionic compounds with cesium cations, and displays a structural anisotropic behavior guided by the uranyl moiety. An unexpected variation of the U-O bond length, dUO, is detected as pressure is applied. It leads to a dUO-stretching frequency relationship that cannot be described by the traditional Badger's rule. Interestingly enough, it can be explained in terms of a change in the main factor controlling dUO. At low pressure, the charge transferred to the uranyl cation induces an increase of the bond length and a red shift of the stretching frequencies, whereas it is the mechanical effect of the applied pressure above 10 GPa that is the dominant factor that leads to a shortening of dUO and a blue shift of the stretching frequencies.

[Gastric tube esophagoplasty using the retroesternal route in children]. / Sustitución esofágica con estómago tubulizado retroesternal en la infancia.

INTRODUCTION: Esophageal replacement is a surgical alternative once native esophagus can't be preserved. Different organs and routes for the replacement have been described, being the retroesternal route the least used. The aim is to present our results using gastric tube esophagoplasty with a retroesternal approach. PATIENTS AND METHODS: We performed a retrospective and descriptive study of 11 patients operated from 2000 to 2015. Median age at surgery was 2.2 years (5 months-9 years) and median weight was 11.2 kg (7.8-21). A gastric tube esophagoplasty using the retroesternal route, forced pyloric dilatation and end-to-side esophago-gastric cervical anastomosis were performed. RESULTS: Ten esophagus replacements had long-gap esophageal atresia and one, severe esophagus caustication secondary to button battery ingestion. No intraoperatory complications were observed. Three patients developed anastomosis leak. Two cases developed anastomotic stenosis managed with endoscopic dilatation in 2 and 4 occasions, respectively. Four patients showed occasional dumping syndrome and are asymptomatic after medical treatment. With a median follow up of 6.3 years (0.2-14.8), all our patients are alive and complete oral diet has been established in all of them. CONCLUSIONS: Gastric tube esophagoplasty using the retroesternal route is a suitable technique in order to reestablish gastrointestinal continuity once native esophagus can't be preserved. In our experience is a safe option, related to few complications.

INTRODUCCION: La sustitución esofágica es una de las opciones quirúrgicas en pacientes en los que no es posible la preservación del esófago. Existen diferentes técnicas según el órgano ascendido y la vía de ascenso, siendo la vía retroesternal la menos empleada. Se describen los resultados con el uso de estómago tubulizado retroesternal. PACIENTES Y METODOS: Estudio descriptivo retrospectivo de una serie de 11 pacientes intervenidos entre los años 2000 y 2015, con una edad media en el momento de la intervención de 2,2 años (5 meses-9 años) y un peso de 11,2 kg (7,8-21 kg). Se realizó gastroplastia tubulizada con dilatación forzada de píloro, ascenso gástrico por vía retroesternal y anastomosis esófago-gástrica cervical término-lateral. RESULTADOS: Diez sustituciones se realizaron en pacientes con atresia de esófago long-gap y una, tras una causticación esofágica por pila de botón. No hubo ninguna complicación intraoperatoria. En tres pacientes hubo fuga anastomótica. En dos pacientes se produjo estenosis que precisó dilataciones en 2 y en 4 ocasiones, respectivamente. Cuatro pacientes presentaron síndrome dumping ocasional que se resolvió con tratamiento médico. Con un seguimiento medio de 6,3 años (0,2-14,8), ningún paciente ha fallecido y en todos se ha logrado la nutrición oral completa. CONCLUSIONES: La gastroplastia tubulizada retroesternal es una técnica eficaz para restablecer la continuidad gastrointestinal en aquellos pacientes en los que no es posible preservar el esófago. Puede ser una opción segura y con escasas complicaciones.

The role of isomerization in the kinetics of self-assembly: p-terphenyl-m-dicarbonitrile on the Ag(111) surface.

Using a toolkit of theoretical techniques comprising ab initio density functional theory calculations, the nudged elastic band method and kinetic Monte Carlo (KMC) modeling, we investigate in great detail how para-terphenyl-meta-dicarbonitrile (pTmDC) molecules diffuse and isomerize to self-assemble on the Ag(111) surface. We show that molecules "walk" on the surface via a pivoting mechanism moving each of its two "legs" one at a time. We then identify a peculiar "under-side" isomerization mechanism capable of changing the molecules chirality, and demonstrate that it is fundamental in understanding the growth of hydrogen bonding assembles of ribbons, linkers, clusters and brickwall islands on the Ag(111) surface, as observed in recent scanning tunneling microscopy experiments (ChemPhysChem, 2010, 11, 1446). The discovered underlying atomistic mechanism of self-assembly may be behind the growth of other hydrogen bonding structures of chiral molecules on metal surfaces.

The curious case of proton migration under pressure in the malonic acid and 4,4'-bipyridine cocrystal.

In the search for new active pharmaceutical ingredients, the precise control of the chemistry of cocrystals becomes essential. One crucial step within this chemistry is proton migration between cocrystal coformers to form a salt, usually anticipated by the empirical ΔpKa rule. Due to the effective role it plays in modifying intermolecular distances and interactions, pressure adds a new dimension to the ΔpKa rule. Still, this variable has been scarcely applied to induce proton-transfer reactions within these systems. In our study, high-pressure X-ray diffraction and Raman spectroscopy experiments, supported by DFT calculations, reveal modifications to the protonation states of the 4,4'-bipyridine (BIPY) and malonic acid (MA) cocrystal (BIPYMA) that allow the conversion of the cocrystal phase into ionic salt polymorphs. On compression, neutral BIPYMA and monoprotonated (BIPYH+MA-) species coexist up to 3.1 GPa, where a phase transition to a structure of P21/c symmetry occurs, induced by a double proton-transfer reaction forming BIPYH22+MA2-. The low-pressure C2/c phase is recovered at 2.4 GPa on decompression, leading to a 0.7 GPa hysteresis pressure range. This is one of a few studies on proton transfer in multicomponent crystals that shows how susceptible the interconversion between differently charged species is to even slight pressure changes, and how the proton transfer can be a triggering factor leading to changes in the crystal symmetry. These new data, coupled with information from previous reports on proton-transfer reactions between coformers, extend the applicability of the ΔpKa rule incorporating the pressure required to induce salt formation.