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.

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.

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.

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.

El gran imitador del infarto agudo de miocardio / No disponible

La enfermedad de Tako-tsubo o acinesia apical transitoria es una entidad poco frecuente (0,5-1% de todos los pacientes con sospecha de síndrome coronario agudo) que suele aparecer un mujeres mayores de 50 años con pocos factores de riesgo cardiovascular. Se caracteriza por presentarse en forma de dolor torácico retroesternal opresivo acompañado de vegetatismo, alteraciones electrocardiográficas y sólo en el50% de los pacientes se encuentra elevación de las enzimas de daño miocárdico. El electrocardiograma y la ecocardiografía muestran anomalías transitorias que desaparecen después de superado el episodio agudo y el cateterismo precoz suele ser normal. Se trata de una patología de pronóstico benigno, aunque presenta más complicaciones iniciales que el infarto agudo de miocardio (IAM) convencional, pero con mejor pronóstico a corto y medio plazo. El tratamiento se basa en la administración de ansiolíticos, evitar la hipovolemia, el uso de vasodilatadores y el control de la frecuencia cardíaca que debe mantenerse en límites bajos. El uso de bloqueadores beta debe ser cuidadoso (AU)

Tako-tsubo disease or transient apical akinesis is a rare condition (0.5%-1% of all the patients with suspicion of acutecoronary syndrome). It generally appears in women over 50with few cardiovascular risk factors. It is characterized by retrosternal oppressive chest pain accompanied by vegetative symptoms, electrocardiographic alterations and elevation of the enzymes of myocardial injury are only found in 50% of the patients. The electrocardiogram and echocardiography show transient abnormalities that disappear after overcoming the acute episode and the early catetherism is generally normal. The prognosis of this disease is benign, although there are more initial complications than in conventional acute myochardial infarction (AMI), but it has a better short and middle-term prognosis. Treatment is based on the administration of ansiolytics, avoiding hypovolemia, use of vasodilators and monitoring of the heart rate that should be maintained in the lower limits. Use of beta blockers should be done with care (AU)

Novel silicate anion: Si(8)O(22)(12-). Hydrothermal synthesis and X-ray powder structure of three new niobium silicates.

A sodium niobium(V) tetrasilicate, Na(2)H(NbO)Si(4)O(11).1.25H(2)O (1), was synthesized hydrothermally at 190 degrees C from a mixture of silicic acid, NaOH, NbCl(5), and H(2)O(2) and added hydrochloric acid. The successive treatment of 1 with nitric acid yielded HNb(H(2)O)Si(4)O(11).H(2)O (2). Contact of 2 with cesium hydroxide solutions converted it to the partially exchanged Cs(+) phase Cs(0.66)H(0.33)Nb(H(2)O)Si(4)O(11) (3). The three structures were solved from X-ray powder diffraction data. All compounds crystallize in the monoclinic space group P2(1)/m with Z = 2, containing the Si(8)O(22)(12-) anion. This new silicate anion type is related to the Si(4)O(11)(6-) unit, which is present in the amphibole series. The difference between both anion types lies in the chain periodicity: two for amphiboles or four for the new niobium silicates. These niobium silicates have framework structures enclosing tunnels in which the alkali cations reside.