V. Marquina

Synthesis by molten salt method of the AFeO3 system (A=La, Gd) and its structural, vibrational and internal hyperfine magnetic field characterization

Abstract Polycrystalline samples of LaFeO3 and GdFeO3 were synthesized by the molten salt method. Some properties and the quality of the resulting compounds were investigated. The crystal structure and purity of the samples were determined through X-ray diffraction and Rietveld analysis. The vibrational properties were characterized by Raman and IR spectroscopy. Mossbauer spectroscopy was used to determine the ionic state of the Fe ions and the internal hyperfine magnetic fields. Considerable reduction of the heat treatment (temperature and time) for the reaction to take place was achieved without detriment of the quality of the compounds.

On the performance of Usain Bolt in the 100?m sprint

Many university texts on mechanics consider the effect of air drag force, using the slowing down of a parachute as an example. Very few discuss what happens when the drag force is proportional to both u and u 2 . In this paper we deal with a real problem to illustrate the effect of both terms on the speed of a runner: a theoretical model of the world-record 100 m sprint of Usain Bolt during the 2009 World Championships in Berlin is developed, assuming a drag force proportional to u and to u 2 . The resulting equation of motion is solved and fitted to the experimental data obtained from the International Association of Athletics Federations, which recorded Bolt’s position with a laser velocity guard device. It is worth noting that our model works only for short sprints. (Some figures may appear in colour only in the online journal)

NANOCRYSTALLIZATION STUDIES IN CO-RICH AMORPHOUS-ALLOYS

Abstract Nanocrystallization from Vitrovac® amorphous ribbon was studied by Mossbauer spectroscopy, transmission electron microscopy and differential scanning calorimetry. Results are interpreted in terms of a crystallization of Co, Co 2 B, and Co 3 B, leading to an increase in Fe content in the residual amorphous phase, which in turn results in an increase in hyperfine field and Curie temperature.

Structural transitions in a TiNiFe shape memory alloy

Results for resistance versus temperature, differential scanning calorimetry (DSC), Moessbauer spectroscopy, and magnetic susceptibility measurements in a Ti[sub 50]Ni[sub 47]Fe[sub 3] sample are presented. The resistance versus temperature curve shows a sharp increase at 245K and then rises slowly until a maximum is attained at [approx]150K. The DSC curve also exhibits a peak at the former temperature. The room temperature Moessbauer spectrum consists of a single line. As the temperature is lowered to 270K, a small asymmetric quadrupole doublet appears. The isomer shift and quadrupole splitting values of this doublet increase with decreasing temperature until almost constant values are achieved at temperatures [approx]100K. The magnetic susceptibility curve shows a sharp decrease that begins at 255K and then an unexpected increase at 90K. The authors relate these observations with a charge density wave-driven premartensitic transition in this system, and with the final martensitic phase.

A jumping cylinder on an inclined plane

The problem of a cylinder of mass m and radius r, with its centre of mass out of the cylinder’s axis, rolling on an inclined plane that makes an angle α with respect to the horizontal, is analysed. The equation of motion is partially solved to obtain the site where the cylinder loses contact with the inclined plane (jumps). Several simplifications are made: the analysed system consists of an homogeneous disc with a one-dimensional straight line mass parallel to the disc axis at a distance y < r of the centre of the cylinder. To compare our results with experimental data, we use a styrofoam cylinder to which a long brass rod is embedded parallel to the disc axis at a distance y < r from it, so the centre of mass lies at a distance d from the centre of the cylinder. Then the disc rolls without slipping on a long wooden ramp inclined at 15 ◦ ,3 0 ◦ and 45 ◦ with respect to the horizontal. To determine the jumping site, the movements are recorded with a high-speed video camera (Casio EX ZR100) at 240 and 480 frames per second. The experimental results agree well with the theoretical predictions.

Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa2(Cu1−xFex)4O8+δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid Tc reduction as iron concentration increases.

Systematic Mössbauer study of YBa2Cu3−xFe x Oδ and PrBa2Cu3−xFe x Oδ as a function of temperature

We study the similitudes and differences between YBa2Cu3−xFexOδ and PrBa2Cu3−xFexOδ systems, using Mössbauer spectroscopy. Both systems, withz=0.04, were studied at several temperatures. The spectra of PrBa2Cu3−xFexOδ showed four asymmetric quadrupole doublets with apparently different line widths, which were fitted with four symmetric superimposed quadrupole doublets with the same line width, but assuming a small (∼4–6 kOe) residual magnetic field in the Fe sites, which are mainly the Cu(1) sites of the 1–2–3 structure. On the other hand, the PrBa2Cu3−xFexOδ spectra showed only three asymmetric quadrupole doublets which can be fitted with three superimposed symmetric doublets and a residual magnetic field. In this case, the Fe atoms also occupy the Cu(1) sites of the 1–2–3 structure. The temperature variations of the Mössbauer spectra and their parameters seem to indicate that, in the Pr-system, a structural change takes place between 12 K and 40 K.

Interactions of calcium with the external surfaces of fullerenes and endofullerenes doped with radioactive sodium iodide

AbstractWe report first-principles calculations carried out to analyze the adsorption of calcium on the outer surface of the fullerene C60, yielding [C60 + mCa]. Geometric optimization (GO) and molecular dynamics (MD) simulation were performed using the plane-wave pseudopotential method within the framework of density functional theory (DFT) and time-dependent DFT (TD-DFT) to investigate the configurations, the associated energies in the ground state, and the stabilities of fullerenes and endofullerenes doped with radioactive sodium iodide when they interact with calcium atoms on the outer fullerene surface (i.e., [nNa131I@C60 + mCa]). The reason for investigating these calcium-functionalized (endo)fullerene systems was to gauge their potential stability when used as vectors to deliver radioiodine to cancerous tissue in the human body. In the simulations, we found that the geometric limit on the number of calcium atoms that can be physisorbed on the outer surface of an empty fullerene while maintaining its structural stability is 28 calcium atoms, which also takes into account the proportional expansion of the fullerene as the number of absorbed calcium atoms increases. However, the stability of a fullerene system during calcium adsorption also strongly depends on whether any atoms or molecules are being encapsulated by the fullerene, as these encapsulated atoms/molecules can also interact with the fullerene and influence its stability. A Mulliken electronegativity analysis revealed that, when atoms inside and/or outside the fullerene donate charge (electrons) to the fullerene, the fullerene expands. The excess charge on the carbon atoms of the fullerene weakens some of the carbon–carbon bonds, potentially causing them to break, in which case the fullerene loses its ability to encapsulate molecules and releases them. Graphical AbstractDFT simulation of a endo fullerene doped with radioactive sodium iodide interacting with 28 calcium atoms in a geometric arrangement

Effect of Fe substitution in the structure and superconducting properties of the (Y0.8Pr0.2)Ba2Cu4−xFexO8 system

Abstract The effects of iron substitutions on the superconducting and structural properties of (Y 0.8 Pr 0.2 )Ba 2 Cu 4− x Fe x O 8 were studied. As iron concentration increases, the rate at which T c diminishes with x is the same as for the YBa 2 Cu 4− x Fe x O 8 system, but faster than in the YBa 2 Cu 3− x Fe x O 7− δ system. The Mossbauer spectra of the different samples, together with Rietveld refinements of their corresponding X-ray spectra, indicate that the iron atoms occupy preferably the Cu(1) sites of the double (CuO) 2 chains in fivefold coordination; that is, due to their tri-valency, the iron atoms attract extra oxygen atoms, named O(5), that place themselves along the a -axis in between two adjacent planes of the double (CuO) 2 chains. As x is increased, the Cu(1)–O(4) bond length decreases, whereas the Cu(2)–O(4) one increases. Also, a reduction of the occupation factors of the oxygen atoms in the CuO 2 planes is observed. Some of these changes may be due to the presence of O(5) atoms, which in turn affect the charge-transfer mechanism between the (CuO) 2 double chains and the CuO 2 planes where superconductivity takes place, and could be responsible of the rate at which T c decreases.

Suppression of Tc in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system

In this paper, the effects produced by iron substitutions in the (Y0.9Ca0.1)Ba2Cu4−xFexO8 system on the superconducting and structural properties are studied. The Rietveld-fit of the crystal structure and Mossbauer spectroscopy results of (Y0.9Ca0.1)Ba2Cu4−xFexO8 samples indicate that the iron atoms occupy the Cu(1) sites of the (Cu–O)2 double chains in fivefold coordination at low iron concentrations. Besides at high iron concentrations the iron atoms occupy the Cu(1) sites of single Cu–O chains and Cu(2) sites in the CuO2 planes of the (Y0.9Ca0.1)Ba2Cu4O8 phase with structural defects. Simultaneously, as iron concentration increases, a faster decrease of Tc is observed in this material compared with the YBa2Cu3−xFexO7−δ system. According to the charge-transfer model proposed for YBa2Cu4O8 under pressure, the decrease in the Cu(1)–O(4) bond length in parallel to the increase in the Cu(2)–O(4) bond length may affect the charge-transfer mechanism leading to the suppression of Tc.