New double perovskites,LaBaTaNi1−xCoxO6: Structural,dielectric and magnetic studies

Double perovskites of the type LaBaNi_1?xCo_xTaO₆ (0 ≤ x ≤ 1) have been synthesized by solid state method. The compounds crystallize in the tetragonal space group, I4/m. Rietveld refinement has been carried out to determine the phase purity and to study the cation ordering. X-ray photoelectron spectroscopy confirms Co(II) in all compositions. The end members in LaBaNi_1?xCo_xTaO₆ show high dielectric constant values. Antiferromagnetic ordering has been observed for all the compositions and the ordering temperature in LaBaNi_1?xCo_xTaO₆ gradually decreases with increase in Co doping, which has been attributed to the decrease in covalence of Co/Ni?O bonds.     

Density-temperature-softness scaling of the dynamics of glass-forming soft-sphere liquids

We employ the principle of dynamic equivalence between soft-sphere and hard-sphere fluids [Phys. Rev. E 68, 011405 (2003)] to describe the interplay of the effects of varying the density n, the temperature T, and the softness (characterized by a softness parameter ν(-1)) on the dynamics of glass-forming soft-sphere liquids in terms of simple scaling rules. The main prediction is the existence of a dynamic universality class associated with the hard-sphere fluid, constituted by the soft-sphere systems whose dynamic parameters depend on n, T, and ν only through the reduced density n*≡nσ(HS)(T*,ν). A number of scaling properties observed in recent experiments and simulations involving glass-forming fluids with repulsive short-range interactions are found to be a direct manifestation of this general dynamic equivalence principle.     

Equilibrium structure of the multi-component screened charged hard-sphere fluid

The generalized mean spherical approximation of the structural properties of the binary charge-symmetric fluid of screened charged hard-spheres of the same diameter, i.e., the screened restricted primitive model, is extended to include binary charge-asymmetric and multi-component fluids. Molecular dynamics simulation data are generated to assess the accuracy of the corresponding theoretical predictions.     

On a class of extended mean spherical approximations

The extended mean spherical approximation of Martina B, and del Rio is solved variationally for the restricted primitive model of 1—1 and 2—2 electrolyte solutions. Thermodynamic properties are calculated and compared to Monte Carlo results for the same model. A modification of the extended mean spherical approximation is presented that is more internally self-consistent and is in better agreement with Monte Carlo data.     

Study of two large-dimension Murillo's paintings by means of macro X-ray fluorescence imaging,point X-ray fluorescence analysis,and stratigraphic studies

During the recent restoration of two large-dimension paintings by Murillo, entitled “Miracle of the loaves and fishes” and “Moses and the water from the rock of Horeb,” several studies about the materials employed for their execution were carried out. Macro X-ray fluorescence scanning technique was performed on both works complemented by point X-ray fluorescence and stratigraphic analyses, in order to characterize the different components of the paintings (i.e., ground layer, pigments, and binders). The results allowed us to better elucidate Murillo painting technique and his creative process. Ground layer was prepared in the usual fashion of the artist, using earths, calcium carbonate, iron oxide pigments, and white lead. The polichromy is composed of lead white mixed with various pigments, depending on the colour tone to be obtained in the pictorial composition: Red and yellow ochres, vermilion, azurite, smalt, and lead-tin yellow were the main pigments identified during our study. Finally, the presence of previous conservation treatments was evidenced by detecting pigments (zinc white, Prussian blue, etc.) not coeval with the artist palette.     

Sum rules for Sαβ(k,ω) for two diffusing species

Starting from the Smoluchowski equation without hydrodynamic interactions for two species of spherical diffusing particles, sum rules are derived here for the first three moments of S_αβ(k,ω), i.e., for the initial value of the first, second and third time-derivatives of F_αβ(k, t) (the time-dependent correlations between the fluctuations in the local concentration of diffusing particles of species α and β). These sum rules are written in terms of the potential of interaction u_αβ(r) between the diffusing particles and the two- and three-particles distribution functions. This derivation is motivated by its potential use in the study of counterion effects on the diffusion of highly charged colloidal particles. Thus, we propose to approximate the memory function involved in the time evolution equation for F_αβ(k, t by a two-parameter model, with its (k-dependent) parameters being determined by the sum rules derived here. This procedure, along with Kirkwood's superposition approximation, reduces the dynamical problem to the knowledge of the radial distribution functions g_αβ(r).     

Spatial correlations in nonequilibrium systems: The effect of diffusion

We show how the ideas of the fluctuation-dissipation theory can be used to calculate spatial correlations in interacting systems away from equilibrium. The only spatially dependent dissipative process considered is diffusion, and spatial correlations are generated by the nonlocal spatial dependence of the chemical potential. The results are the lowest order in a hierarchy of generalized hydrodynamic type calculations applicable to nonequilibrium systems. We derive equations for the density correlation functions at stationary state supported by diffusive fluxes and show that they have the local equilibrium form. The static correlation function is obtained from the fluctuation-dissipation theorem, which we show to be equivalent to the Ornstein-Zernike integral equation. At equilibrium we demonstrate that the dynamical structure factor obtained by these methods includes the expected wave-vector dependent diffusion constant. Finally we derive a necessary and sufficient condition for local equilibrium to obtain at a stationary state and show by explicit calculation that chemical processes can give rise to significant nonequilibrium correlations.     

Determination of the REE content,geological age,and absorbed alpha dose of allanite mineral from Palawan,Philippines

Allanite is a common accessory mineral in igneous rocks that contains significant amounts of rare-earth elements (REEs), thorium (Th), and uranium (U). The presence of Th and U in the allanite exposes it to radiation resulting in radiation damage in its crystal structure and further leads to metamictization. Hence, allanite can be used as a natural analogue to assess the long-term radiation effects in materials for high-level nuclear waste disposal. It provides information on the effect of α-decay on the crystal structure including the stability and integrity of the material. In this study, the absorbed α-dose of allanite from Ombo, Palawan, Philippines, was estimated from the Th and U content and geological age of the mineral using X-ray fluorescence and gamma-ray spectrometry. The amount of Th and REE was measured to be around 1.37 and ~30.4 wt.%, respectively. Radiometric dating using gamma-ray spectrometry estimated the age of the allanite mineral to be around 24–35 million years. Having obtained the amount of radioactivity and the time of exposure, the corresponding absorbed α-dose was estimated to be at 2.84 × 10¹⁴ α-decays/mg. Results suggest that the allanite mineral studied has not accumulated significant radiation damage to cause amorphization and still exhibit a crystalline structure. This study may provide data on the properties of allanite or silicate matrices as part of the ongoing studies on silicate minerals as natural analogues.