Ab initio and shell model studies of structural, thermoelastic and vibrational properties of SnO2 under pressure.

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure P(c). A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11-C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B(1g) mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.

Ab initio study of the elastic properties of single and polycrystal TiO(2), ZrO(2) and HfO(2) in the cotunnite structure.

In this work, we study theoretically the elastic properties of the orthorhombic (Pnma) high-pressure phase of IV-B group oxides: titania, zirconia and hafnia. By means of the self-consistent SIESTA code, pseudopotentials, density functional theory in the LDA and GGA approximations, the total energies, hydrostatic pressures and stress tensor components are calculated. From the stress-strain relationships, in the linear regime, the elastic constants C(ij) are determined. Derived elastic constants, such as bulk, Young's and shear modulus, Poisson coefficient and brittle/ductile behavior are estimated with the polycrystalline approach, using Voigt-Reuss-Hill theories. We have found that C(11), C(22) and C(33) elastic constants of hafnia and zirconia show increased strength with respect to the experimental values of the normal phase, P 2(1)/c. A similar situation applies to titania if these constants are compared with its normal phase, rutile. However, shear elastic constants C(44), C(55) and C(66) are similar to the values found in the normal phase. This fact increases the compound anisotropy as well as its ductile behavior. The dependence of unit-cell volumes under hydrostatic pressures is also analyzed. P-V data, fitted to third-order Birch-Murnaghan equations of state, provide the bulk modulus B(0) and its pressure derivatives B'(0). In this case, LDA estimations show good agreement with respect to recent measured bulk moduli of ZrO(2) and HfO(2). Thermo-acoustic properties, e.g. the propagation speed of transverse, longitudinal elastic waves together with associated Debye temperatures, are also estimated.

Ab initio localized basis set study of structural parameters and elastic properties of HfO2 polymorphs.

The SIESTA approach based on pseudopotentials and a localized basis set is used to calculate the electronic, elastic and equilibrium properties of P 21/c, Pbca, Pnma, Fm3m, P42nmc and Pa3 phases of HfO2. Using separable Troullier-Martins norm-conserving pseudopotentials which include partial core corrections for Hf, we tested important physical properties as a function of the basis set size, grid size and cut-off ratio of the pseudo-atomic orbitals (PAOs). We found that calculations in this oxide with the LDA approach and using a minimal basis set (simple zeta, SZ) improve calculated phase transition pressures with respect to the double-zeta basis set and LDA (DZ-LDA), and show similar accuracy to that determined with the PPPW and GGA approach. Still, the equilibrium volumes and structural properties calculated with SZ-LDA compare better with experiments than the GGA approach. The bandgaps and elastic and structural properties calculated with DZ-LDA are accurate in agreement with previous state of the art ab initio calculations and experimental evidence and cannot be improved with a polarized basis set. These calculated properties show low sensitivity to the PAO localization parameter range between 40 and 100 meV. However, this is not true for the relative energy, which improves upon decrease of the mentioned parameter. We found a non-linear behaviour in the lattice parameters with pressure in the P 21/c phase, showing a discontinuity of the derivative of the a lattice parameter with respect to external pressure, as found in experiments. The common enthalpy values calculated with the minimal basis set give pressure transitions of 3.3 and 10.8 GPa for [Formula: see text] and [Formula: see text], respectively, in accordance with different high pressure experimental values.

Repercusión hemodinámica e hidroelectrolítica del ayuno de Ramadán en escolares adolescentes / Haemodynamic and hydrosalinum repercussion on adolescents who practise Ramadan fast

Justificación: el Ramadán se caracteriza por un período de ayuno acompañado de restricción hídrica durante el día, cambiando la ingesta de sólidos y líquidos a la noche durante un mes lunar. El propósito de este estudio ha sido valorar en un grupo de escolares adolescentes, la influencia del ayuno del Ramadán en el equilibrio hidrosalino. Metodología: para ello se seleccionaron de forma aleatoria un grupo de adolescentes escolarizados en centros multiconfesionales de la Ciudad Autónoma de Ceuta, que tras ser convenientemente informados aceptaron participar voluntariamente en el estudio. Se han analizado parámetros fisiológicos como peso, talla, frecuencia cardíaca, presión arterial, electrolitos en plasma y orina, osmolaridad, volumen plasmático y concentraciones hormonales de renina, aldosterona y vasopresina o ADH. Dichos parámetros se determinaron antes (A), durante (B y C) y después (D) del Ramadán. Resultados: los valores de aldosterona disminuyeron en la primera semana del Ramadán (B) y se elevaron considerablemente en las siguientes (A: 410,6ñ47,65; B: 278,2ñ27,60; C: 528,12ñ50,37 y D: 578,12ñ78,24 pg/mL), mientras que la ADH no se modificó significativamente en la primera semana de ayuno (B), disminuyendo en las posteriores (A: 9,1ñ0,78; B:8,9ñ0,79; C: 5,65ñ0,08 y D: 6,22ñ0,32 pg/mL). Los niveles plasmáticos de Na+ y K+ y sus fracciones de excreción mostraron variaciones coherentes con las de dichas hormonas. Conclusiones: la hiperhidratación debida a la ingesta excesiva de agua antes de salir el sol, provoca una hemodilución con disminución de la osmolaridad y mantenimiento del volumen plasmático y presión arterial (PA) a expensas de la inhibición de vasopresina o ADH y de Renina. El aumento en los niveles de K+ plasmáticos, sería responsable de la secreción significativamente elevada de aldosterona (p<0,05), observada el día 21de ayuno (C) y del aumento concomitante de la fracción de excreción de K+ (p<0,05) (AU)

Influencia del estilo de vida en el perfil lipídico / Influence of life style in the lipidic profile

En este trabajo se ha analizado la relación entre el estilo de vida y el perfil lipídico, como factor de riesgo para la salud. Se han seleccionado 15 sujetos deportistas “senior” sanos, atletas especialistas en carrera de fondo, con una media de edad de 51 años, 69 Kg como peso medio y 1,69m como estatura media. Su participación en el estudio fue voluntaria, junto a la de otros 12 sujetos sedentarios y de similares características socioculturales, que configuraron el grupo control, con una media de edad de 49 años, un peso medio de 83 Kg y una talla de 1,72 m. Se ha utilizado el cuestionario de estilos de vida de Heyward sobre hábitos relacionados con la bebida, tabaco, ejercicio físico y dieta, así como la determinación de los niveles de lípidos en sangre (TG, Colesterol total, HDL, LDL, VLDL, y cociente LDL/ HDL Se han comparado los niveles basales de lípidos en ambos grupos con relación al estilo de vida. Los resultados de nuestro trabajo muestran hábitos menos saludables en el grupo de sujetos sedentarios, con mayor ingesta de alcohol y tabaco y significativamente menor realización de ejercicio físico que en el grupo de atletas. El perfil lipídico muestra valores inferiores de colesterol total, triglicéridos, LDL,VLDL, LDL/HDL y mayores de HDL en el grupo de atletas cuyo consumo de alcohol es menor, no consumen tabaco y entrenan de forma regular. El ejercicio físico en nuestro estudio se constituye como el hábito de mayor relevancia en la configuración de unas características antropométricas y un perfil lipídico óptimos para la promoción de la salud y desarrollo de estilos de vida saludables (AU)