Anharmonic contribution to the stabilization of Mg(OH)2 from first principles.

Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P3[combining macron] (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P3[combining macron]m1 (No. 164, point group -3m) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH)2, in particular for the P3[combining macron]m1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P3[combining macron]m1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P3[combining macron]m1 structure towards P3[combining macron]. In this paper, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.

Fe/Ni core/shell nanowires and nanorods: a combined first-principles and atomistic simulation study.

In recent years, construction and characterization of core-shell structures have attracted great attention because of their unique functional properties and their integration into technological devices. However, some aspects of their basic physics still remain to be explored. In this study, we report on an extensive hierarchical multiscale modeling methodology applied to Fe-Ni core/shell nanostructures of technological interest. As a first step, supported on a first-principles study, we develop a methodology to compute primordial but unprecedented parameters such as the exchange coupling and the equilibrium bond distances at the interface, namely JFe-Ni = 35.48 meV and d = 2.5 Å. This methodology can be used for computing fundamental parameters in mixed systems by knowing the parameters in the bulk samples, and the so-obtained results can be used in higher size scale simulations. As a proof, the results obtained are used as input parameters for atomistic simulations on Fe-Ni samples made out of a Fe core surrounded by a Ni shell whose external diameter varies finely in the range 60-110 nm. The inner diameter and height are fixed to be 40 and 50 nm, respectively. We address the structural, electronic, static magnetic and hysteresis properties of the Fe-Ni core/shell cylindrical nanostructures in different size ranges. These nanostructures reveal different magnetic properties with novel complex states, which are studied in detail.

First-principles study of pressure-induced structural phase transitions in MnF2.

In this work we report a complete structural and magnetic characterization of crystalline MnF2 under pressure obtained using first principle calculations. Density functional theory was used as the theoretical framework, within the generalized gradient approximation plus the Hubbard formalism (GGA+U) necessary to describe the strong correlations present in this material. The vibrational, the magnetic exchange couplings and the structural characterization of MnF2 in the rutile ground state structure and potential high pressure phases are reported. The quasiharmonic approximation has been used to obtain the free energy, which at the same time is used to evaluate the different structural transitions at 300 K. Based on previous theoretical and experimental studies on AF2 compounds, ten different structural candidates were considered for the high pressure regime, which led us to propose a path for the MnF2 structural transitions under pressure. As experimental pressure settings can lead to non-hydrostatic conditions, we consider hydrostatic and non-hydrostatic strains in our calculations. According to our results we found the following sequence for the pressure-driven structural phase transition in MnF2: rutile (P42/mnm) → α-PbO2-type (Pbcn) → dist. HP PdF2-type (Pbca) → dist. fluorite (I4/mmm) → cotunnite (Pnma). This structural path is correlated with other phase transitions reported on other metal rutile fluorides. In particular, we found that our proposed structural phase transition sequence offers an explanation of the different paths observed in the literature by taking into account the role of the hydrostatic conditions. In order to get a deep understanding of the modifications of MnF2 under pressure, we have analyzed the pressure evolution of the structural, vibrational, electronic, and magnetic properties for rutile and for each of the high pressure phases.

Atomic and molecular oxygen adsorbed on (111) transition metal surfaces: Cu and Ni.

Density functional theory is used to investigate the reaction of oxygen with clean copper and nickel [111]-surfaces. We study several alternative adsorption sites for atomic and molecular oxygen on both surfaces. The minimal energy geometries and adsorption energies are in good agreement with previous theoretical studies and experimental data. From all considered adsorption sites, we found a new O2 molecular precursor with two possible dissociation paths on the Cu(111) surface. Cross barrier energies for the molecular oxygen dissociation have been calculated by using the climbing image nudge elastic band method, and direct comparison with experimental results is performed. Finally, the structural changes and adsorption energies of oxygen adsorbed on surface when there is a vacancy nearby the adsorption site are also considered.

First-principles study of Mn3 adsorbed on Au(111) and Cu(111) surfaces.

A theoretical study of the Mn trimer adsorbed on the noble metal surfaces Au(111) and Cu(111) is reported. The calculations were performed using first-principles methods within the density functional theory and the generalized gradient approximation in the collinear and non-collinear magnetic phases. The system was modeled by considering a surface unit cell of 25 atoms to improve the trimer's isolation on the surface. We evaluated the trimer as a linear chain and forming triangular structures. The triangular trimer can be adsorbed in two possible configurations, above an empty surface triangle site (Δ) or on a triangle with a surface atom at the center in a hexagonal structure (H). The difference is the coordination of the Mn with surface atoms. We studied the antiferromagnetic (AF), ferromagnetic (FM), and non-collinear (NC) magnetic cases. As a result, the lowest energy configuration on both metals is the AFΔ configuration, which has an isosceles triangle shape. In comparison, the NC and the FM configurations adopt an equilateral geometry. The same trend was observed for the H configurations, but they are less bonded. The results are supported by calculating the spin-polarized electronic structure and the electronic charge transfer. Finally, we computed the energy barriers that inhibit the transformation of the linear chain to a delta Mn trimer on both substrates.

High-pressure characterization of multifunctional CrVO4.

The structural stability and physical properties of CrVO4 under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

First-principles study of Ni adatom migration on graphene with vacancies.

A theoretical study based on first-principles calculations about the interaction and diffusion of Ni atoms on pristine graphene and graphene with a single vacancy is presented. In the first case, we explored the structural changes due to the adsorption of Ni on graphene and the effects on the electronic structure. In the case of graphene with a vacancy, we analyzed the impact of the adsorbed Ni atom on the distortion of the graphene structure and how it depends on the distance from the graphene defect. In the analysis, we observed the changes in the electron localization function and the charge density. By knowing the interaction map of Ni with graphene, and the structural changes of the network, we performed energy barrier calculations within the climbing image nudged elastic band methodology to study the nickel diffusion. Finally, we explored how the vacancy and structural distortions affect the minimum energy paths and the saddle points for nickel moving away, around, and towards the vacancy.

[Assessment of a health program for the non-insured population]. / Evaluación de programa de salud para población no asegurada.

OBJECTIVE: Present the results of the evaluation of a program for the non-insured population of the four poorest states of the country implemented by the Ministry of Health of Mexico between 1991 and 1995. METHODS: The effects of the program were evaluated in three areas: i) increase in health services coverage; ii) delivery of personal health services, and iii) changes in health conditions of the target population. The extension of coverage was measured by the increase in potential access due to the construction of health infrastructure projects and the use of additional health manpower, mainly primary health care workers. For the evaluation of the impact of the program on the delivery of services, three surveys were developed: one for service utilization, another one for accessibility, and a third for quality of care. The impact on health conditions was evaluated by changes in health indicators of children under five and women of reproductive age. RESULTS AND CONCLUSIONS: The Program had a positive impact on coverage, accessibility and quality of services. Its impact on health conditions was also positive. However, these last changes cannot be attributed only to the program, but to the sum of several concurrent activities.

High-pressure structural behaviour of HoVO4: combined XRD experiments and ab initio calculations.

We report a high-pressure experimental and theoretical investigation of the structural properties of zircon-type HoVO4. Angle-dispersive x-ray diffraction measurements were carried out under quasi-hydrostatic and partial non-hydrostatic conditions up to 28 and 23.7 GPa, respectively. In the first case, an irreversible phase transition is found at 8.2 GPa. In the second case, the onset of the transition is detected at 4.5 GPa, a second (reversible) transition is found at 20.4 GPa, and a partial decomposition of HoVO4 was observed. The structures of the different phases have been assigned and their equations of state (EOS) determined. Experimental results have also been compared to theoretical calculations which fully agree with quasi-hydrostatic experiments. Theory also suggests the possibility of another phase transition at 32 GPa; i.e. beyond the pressure limit covered by present experiments. Furthermore, calculations show that deviatoric stresses could trigger the transition found at 20.4 GPa under non-hydrostatic conditions. The reliability of the present experimental and theoretical results is supported by the consistency between the values yielded for transition pressures and EOS parameters by the two methods.

[Hypothesis of the compression of morbidity: an example of theoretical development in epidemiology]. / La hipótesis de la compresión de la morbilidad: un ejemplo de desarrollo teórico en epidemiología.

In this paper we review a hypothesis put forth in 1980 by James Fries under the name of the compression of morbidity, as an example of theoretical development in the field of epidemiology. Initially we discuss some of the essential features of scientific theories and make a historical account of the most important causal models that epidemiology has embraced. Then, the basis for proposing that a compression of the morbidity is taking place are described focusing on its advantages and limitations when it is analyzed from the point of view of structuralism. Finally, we conclude that, to the extent that they are correct, models of this kind have the potential of describing and predicting the evolution of human health; hence, their systematic development entails an extraordinary importance for public health.