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.

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.

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.

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.

Chemical Interactions and Spin Structure in (O2)4: Implications for the ε-O2 Phase.

The chemical interactions and spin structure of (O2)4 in its ground singlet state are analyzed by means of Quantum Chemical Topology descriptors. The energetic contributions of the Interacting Quantum Atoms approach are used to obtain information about the class of interactions displayed along the dissociation path of (O2)4. The exchange-correlation contribution to the binding energy is non-negligible for the O2-O2 interactions at intermolecular distances close to those found for the pressure induced ε phase of solid (O2) and this strengthening of the intermolecular bonding is built up from a simultaneous weakening of the intramolecular bond. This result is of interest in connection with the observed softening of the IR vibron frequency in the lower pressure range of the ε phase. The spin structure in the real space along the dissociation process is interpreted with the help of the so-called electron number distribution functions. At large distances, the four triplet O2 molecules are arranged in a way consistent with an antiferromagnetic structure, whereas at short distances, a significant spin redistribution is driven by the exchange process and it involves a propensity toward a null magnetic moment per molecule. Such probability behavior can be related with the magnetic evolution of solid oxygen across the δ → ε phase transition. Additional calculations of (O2)4 excited states support the conclusion that the relative stabilization and magnetic features of the ground singlet state are due to the onset of the new intermolecular bonds, and not to an exclusive modification of the electronic character within the O2 molecules.

Bases for Understanding Polymerization under Pressure: The Practical Case of CO2.

We present a novel quantitative strategy for monitoring chemical bonding transformations in solids from the topology of their electronic structure. Developed in the context of the electron localization function formalism, it provides an unambiguous characterization of long-range interactions and bond formation. Charge flux between electron localization regions is found to hold the key for identifying the nature of the interaction between the chemically meaningful entities in the solid (valence shells, lone pairs, molecules, etc.). Because of the wide range of interesting properties that high pressure induces in molecular solids, we illustrate the potentialities of our strategy to unveil controversial questions involved in the bond reorganization along the polymerization of CO2. Our study confirms that the topology of the bonding network in the pseudopolymeric phases points toward the incipient formation of the new bonds in the higher pressure polymers. This transformation is identified as a synchronic weakening of the intramolecular (C==O) double bond and the birth of a new intermolecular C--O bond controlled by the oxygen lone pairs. Overall, the relationship that this type of analysis establishes between different polymorphs of the phase diagram could be further exploited for the prediction of the coordination of high pressure phases, opening new avenues for experimental synthesis and structure indexation.

Computation of Local and Global Properties of the Electron Localization Function Topology in Crystals.

We present a novel computational procedure, general, automated, and robust, for the analysis of local and global properties of the electron localization function (ELF) in crystalline solids. Our algorithm successfully faces the two main shortcomings of the ELF analysis in crystals: (i) the automated identification and characterization of the ELF induced topology in periodic systems, which is impeded by the great number and concentration of critical points in crystalline cells, and (ii) the localization of the zero flux surfaces and subsequent integration of basins, whose difficulty is due to the diverse (in many occasions very flat or very steep) ELF profiles connecting the set of critical points. Application of the new code to representative crystals exhibiting different bonding patterns is carried out in order to show the performance of the algorithm and the conceptual possibilities offered by the complete characterization of the ELF topology in solids.

A Quantum Chemical Interpretation of Compressibility in Solids.

The ability of the electron localization function to perform a partition of the unit cell volume of crystalline solids into well-defined, disjoint, and space-filling regions enables us to decompose the bulk compressibility into local contributions with a full chemical meaning. This partition has been applied to a set of prototype crystals of the chemical elements of the first three periods of the periodic table, and the equations of state for core, valence, bond, and lone electron pairs have been obtained. Solids are unequivocally classified into two groups according to their response to hydrostatic pressure. Those with sharing electrons (metals and covalent crystals) obey a simple relationship between the average valence electron density and the zero pressure bulk modulus. The stiffness of the closed-shell systems (molecular and ionic solids) is rationalized resorting to the Pauli principle. Overall, the results clearly correlate with chemical intuition: periodic trends are revealed, cores are almost incompressible and do not contribute appreciably to the macroscopic compressibility, and lone pair basins are rather easier to compress than bond basins.

How electron localization function quantifies and pictures chemical changes in a solid: the B3-->B1 pressure induced phase transition in BeO.

Advantages of the analysis of the topology of the electron localization function (ELF) in the characterization of the chemical bonding in solids are illustrated in the study of the zinc blende --> rock salt transformation in BeO. The 4-fold to 6-fold coordination change is described as a two-step process: first, a catastrophic-like emergence of two new Be-O bonds reveals the onset of the rock salt structure, and second, the new interactions gradationally evolve to achieve the bonding network of the high-pressure phase. The increase in coordination, the volume collapse and the enhancement in the bulk modulus across the transition pathway are qualitatively and quantitatively traced back to the oxygen's valence shell. Although several ELF indexes point toward the expected greater bond polarity in the B3 than in the B1 structure, it can be concluded that there is no substantial modification in the nature of the crystal interactions induced by the phase transformation.

Influence of antiretroviral treatment on quality of life in seropositive inmates.

The aim of the study is to evaluate the influence of antiretroviral treatment on health-related quality of life (HRQOL) of three groups of HIV-positive inmates: those who are taking antiretroviral treatment, those who are not on treatment as it has not yet been indicated, and those who refuse to take treatment even though it has been recommended. A cross-sectional study was conducted on 585 HIV+ inmates in three prisons. The response variable was HRQOL. Independent variables were: sociodemographic variables, psychosocial and drug-related variables. Two multivariate linear regression models were constructed in order to determine the HRQOL, physical health score (PHS) and mental health score (MHS), for each of the three groups identified, using patients who refused treatment as the reference category. Patients who refused therapy had a lower MHS compared with patients in whom treatment was not indicated (P = 0.038). With regard to PHS, patients refusing therapy had a lower score than patients who were not indicated therapy (P = 0.005), and than patients receiving therapy (P = 0.010).

[Patients who refuse antiretroviral treatment in prison]. / Pacientes que rehúsan el tratamiento antirretroviral en el medio penitenciario.

INTRODUCTION: Current studies of HIV+ patients in the prison population have been carried out without considering differences that might exist between patients who accept retroviral treatment and those who do not. One possible reason for this may be the difficulty in gaining access to patients who refuse antiretroviral treatment. However, the prison environment makes it possible to locate and study this type of patient, who up till now has not been the subject of study. The aim of this article is to describe the clinical and psychosocial state of HIV+ inmates who refuse ARVT and compare this data with patients receiving treatment and others for whom treatment has not been indicated. METHODS: Cross-sectional study using 585 HIV+ inmates in three prisons in Andalusia from May to June 2004. Refusal, acceptance and non-indication of ARVT treatment was the grouping variable used. The independent variables were socio-demographic, psychosocial, clinical and other variables relating to the prison environment. RESULTS: 16.8% of patients refused ARVT, while 56.3% were receiving treatment and another 26.8 were not indicated for any medication. Amongst the patients that refused ARVT there was a greater prevalence of HIV co-infection, higher inprison consumption of opiates and methadone treatment, more cases pending and higher rates of recidivism. CONCLUSIONS: these results highlight the existence of a group with unique characteristics that is accessible thanks to the special conditions within the prison environment. It is a group that chooses not to follow therapeutic indications and which represents a risk factor not only for its own health, but also for the community at large.

Leiomioma cutáneo zosteriforme. Tratamiento satisfactorio con doxazosina oral / Zosteriform cutaneous leiomyoma. Satisfactory treatment with oral doxazosine

Describimos el caso de un paciente varón de 50 años de edad que presentaba leiomioma cutáneo zosteriforme de región facial izquierda, intensamente doloroso, y que obtuvo mejoría clínica espectacular tras la administración diaria de 4 mg de doxazosina oral. Este tratamiento fue bien tolerado y no presentó ningún efecto adverso asociado. En la literatura médica de habla inglesa se han publicado sólo dos casos tratados con doxazosina que demostraron excelente resultado terapéutico

We report a 50-year-old man that presented a zosteriform cutaneous leiomyoma in the left facial region, intensely painful, that showed great improvement after the administration of a daily dose of 4 mg of oral doxasozine. The therapy was well tolerated and did not present any associated adverse effect. In the English medical literature only two cases successfully treated with doxasozine have been reported

[Zosteriform cutaneous leiomyoma. Satisfactory treatment with oral doxazosin]. / Leiomioma cutáneo zosteriforme. Tratamiento satisfactorio con doxazosina oral.

We report a 50-year-old man that presented a zosteriform cutaneous leiomyoma in the left facial region, intensely painful, that showed great improvement after the administration of a daily dose of 4 mg of oral doxasozin. The therapy was well tolerated and did not present any associated adverse effect. In the English medical literature only two cases successfully treated with doxasozin have been reported.

Síndrome de Bazex (acroqueratosis paraneoplásica) / Bazex syndrome (acrokeratosis paraneoplastica)

El síndrome de Bazex es una dermatosis paraneoplásica caracterizada por lesiones acrales psoriasiformes que se asocia a una neoplasia subyacente. Presentamos el caso de un paciente varón de 72 años, con lesiones queratósicas, simétricas, en los pabellones auriculares, las manos y los pies, compatibles con síndrome de Bazex. En el estudio que se le realizó, se detectó una recidiva de carcinoma epidermoide de paladar blando y carcinoma de esófago poco diferenciado

Bazex syndrome is a paraneoplastic dermatosis characterized by acral psoriasiform lesions associated with an underlying neoplasm. We present the case of a 72 year old male patient with keratosic, symmetric lesions in the auricular external ears, hands and feet, consistent with Bazex Syndrome. The study conducted detected recurrent squamous cell carcinoma of soft palate and undifferentiated esophageal carcinoma

Infección cutánea por Microsporum nanum / Skin infection due to Microsporum nanum

Microsporum nanum es un hongo zoófilo capaz de producir enfermedad en humanos y animales, aunque en humanos se han descrito muy pocos casos. Presentamos un paciente de 35 años que mostraba una lesión pruriginosa en antebrazo izquierdo que se diagnosticó de Tinea corporis y en cuyo cultivo para hongos se aisló Microsporum nanum

Microsporun namum is a zoofilic fungi capable of causing disease in humans and animals, although it has rarely been described in humans. We present a case of a 35 year old patient. He had an itchy lesion on the left forearm and was diagnosed of Tinea corporis: Microsporum nanum was isolated in the culture for fungi

[Progress of clinical parameters amongst prison inmates receiving antiretroviral treatment]. / Evolución de los parámetros clínicos en reclusos en tratamiento antirretroviral.

INTRODUCTION: Among HIV positive patients the CD4 lymphocyte count, especially the viral load, are the best predictors for progress to full blown AIDS or death. OBJECTIVE: To analyze the factors associated with progress of the CD4 lymphocyte count and viral load in prison inmates in antiretroviral treatment. METHODS: A fixed cohort study was conducted with HIV positive inmates receiving antiretroviral therapy in three Spanish prisons. Adherence to antiretroviral treatment was assessed with the SMAQ questionnaire. To analyze the progress of CD4 and viral load clinical parameters, two fixed effect multilevel linear regression models were utilised. RESULTS: 10% of the sample were women, 42% referred for anxiety or symptoms of depression in the final week, and 46.6% reported having social support inside the prison. CD4 and viral load clinical parameter means were 2.48 and 2.89 respectively, and 38.6% had an undetectable viral load. A negative correlation between viral load and CD4 lymphocyte count (p<0.001) was found. Those inmates who did not present psychological morbidity showed a significant reduction in plasma viral load (p=0.017). CONCLUSIONS: The results of this study show the relevance of psychosocial factors in the immune system.

Evolución de los parámetros clínicos en reclusos en tratamiento antirretroviral / Progress of clinical parameters amongst prison inmates receiving antiretroviral treatment

Introducción: Entre los pacientes infectados por el VIH el recuento linfocitario y fundamentalmente la carga viral, sonlos mejores predictores al estado definitorio de SIDA o muerte. Objetivo: analizar los factores asociados a la evolución delrecuento de linfocitos CD4 y carga viral en reclusos en tratamiento con antirretrovirales. Métodos: Se realizó un estudio decohorte fija a reclusos VIH positivos en tratamiento con antirretrovirales de tres prisiones españolas. La adherencia al tratamientoantirretroviral se midió a través del cuestionario SMAQ. Para analizar la evolución de los parámetros clínicos de CD4y carga viral se realizaron dos modelos de regresión lineal multinivel de efectos fijos. Resultados: El 10% eran mujeres, el42% refirió padecer ansiedad o depresión en la última semana y el 46,6% refirió tener apoyo social dentro de la prisión. Encuanto a la media de los parámetros clínicos de log10 CD4 y log10 carga viral fueron de 2,48 y 2,89 respectivamente, teniendoel 38,6% carga viral indetectable. Se encontró una relación inversa entre la carga viral y el recuento de linfocitos CD4(p<0,001). En cuanto a la carga viral plasmática aquellos reclusos sin morbilidad psíquica mostraron una reducción significativa (p=0,017) de la misma. Conclusiones: se pone de manifiesto la importancia de los factores psicosociales en el sistema inmunitario (AU)

Introduction: Among HIV positive patients the CD4 lymphocyte count, especially the viral load, are the best predictorsfor progress to full blown AIDS or death. Objective: To analyze the factors associated with progress of the CD4lymphocyte count and viral load in prison inmates in antiretroviral treatment. Methods: A fixed cohort study was conductedwith HIV positive inmates receiving antiretroviral therapy in three Spanish prisons. Adherence to antiretroviral treatmentwas assessed with the SMAQ questionnaire. To analyze the progress of CD4 and viral load clinical parameters, two fixedeffect multilevel linear regression models were utilised. Results: 10% of the sample were women, 42% referred for anxietyor symptoms of depression in the final week, and 46.6% reported having social support inside the prison. CD4 andviral load clinical parameter means were 2.48 and 2.89 respectively, and 38.6% had an undetectable viral load. A negative correlationbetween viral load and CD4 lymphocyte count (p<0.001) was found. Those inmates who did not present psychologicalmorbidity showed a significant reduction in plasma viral load (p=0.017). Conclusions: The results of this study showthe relevance of psychosocial factors in the immune system (AU)