Jahn-Teller distortion in the SnH4+ ion

The extension of the Jahn-Teller distortion of the SnH₄⁺ structure is estimated from ab initio SCF CI calculations using pseudopotential atomic approximation with and without relativistic correction. These calculations show the lowest minima of SnH₄⁺ to have C_2v symmetry and are very close to the C_3v minima. The calculated photoelectron spectrum of SnH₄ is in good agreement with the experimental values.     

Effect of the temperature on the synthesis of (K,Na)NbO<Subscript>3</Subscript>-modified nanoparticles by a solid state reaction route

(Na_0.5K_0.5)NbO₃ (KNN)-modified nano-particulated powders, based on variations of sodium–potassium niobate, were synthesised by solid state reaction from carbonate starting materials. The nanoparticles were attained by an optimization of the raw materials particle size and particle refinement of the carbonates during their decomposition. Particle sizes between 50 and 200 nm have been obtained as a function of calcination (decomposition) temperature. The obtained powders showed a co-existence between a tetragonal phase and an orthorhombic phase. The optimization of the raw materials particle size and the particle refinement of the carbonates during their decomposition play a key role in the formation of the KNN-modified nano particles. The developed method is well suited for the production of KNN-modified nano powders at low cost for mass production.     

Single facility location on a network under mill and delivered pricing

^** E-mail: pelegrin{at}um.es Firms normally use either a mill price or a delivered pricepolicy, depending on market conditions (type of good, transportationway, customers location, costs, etc). In this paper, the problemof selecting the best location for an entering firm in competitionwith some pre-existing firms, under each price policy, is studiedon a network for the first time. With mill pricing, an equilibriumin price rarely exists and it is assumed that all competingfirms set a common mill price for all customers. With deliveredpricing, there exists a Nash equilibrium in price and it isassumed that the equilibrium price in each area is offered tothe customers in that area. In both cases, we consider thatcustomers buy from the cheapest facility and the same rulesare used for tie breaking in the lowest cost. While the profitmaximization problem for the entering firm always has optimalsolutions under mill pricing, this problem might not have anoptimal solution under delivered pricing. We show some discretizationresults and give procedures to find the full set of optimal,or -optimal, solutions to the problem under the two price policies.A comparison of results with the two price policies is givenby using an illustrative example.     

Structural characterization and oxidative dehydrogenation activity of V2O5/Ce(x)Zr(1-x)O2/SiO2 catalysts

The thermal stability of a nanosized Ce(x)Zr(1-x)O2 solid solution on a silica surface and the dispersion behavior of V2O5 over Ce(x)Zr(1-x)O2/SiO2 have been investigated using XRD, Raman spectroscopy, XPS, HREM, and BET surface area techniques. Oxidative dehydrogenation of ethylbenzene to styrene was performed as a test reaction to assess the usefulness of the VOx/Ce(x)Zr(1-x)O2/SiO2 catalyst. Ce(x)Zr(1-x)O2/SiO2 (1:1:2 mol ratio based on oxides) was synthesized through a soft-chemical route from ultrahigh dilute solutions by adopting a deposition coprecipitation technique. A theoretical monolayer equivalent to 10 wt % V2O5 was impregnated over the calcined Ce(x)Zr(1-x)O2/SiO2 sample (773 K) by an aqueous wet impregnation technique. The prepared V2O5/Ce(x)Zr(1-x)O2/SiO2 sample was subjected to thermal treatments from 773 to 1073 K. The XRD measurements indicate the presence of cubic Ce0.75Zr0.25O2 in the case of Ce(x)Zr(1-x)O2/SiO2, while cubic Ce0.5Zr0.5O2 and tetragonal Ce0.16Zr0.84O2 in the case of V2O5/Ce(x)Zr(1-x)O2/SiO2 when calcined at various temperatures. Dispersed vanadium oxide induces more incorporation of zirconium into the ceria lattice, thereby decreasing its lattice size and also accelerating the crystallization of Ce-Zr-O solid solutions at higher calcination temperatures. Further, it interacts selectively with the ceria portion of the composite oxide to form CeVO4. The RS measurements provide good evidence about the dispersed form of vanadium oxide and the CeVO4 compound. The HREM studies show the presence of small Ce-Zr-oxide particles of approximately 5 nm size over the surface of amorphous silica and corroborate with the results obtained from other techniques. The catalytic activity studies reveal the ability of vanadium oxide supported on Ce(x)Zr(1-x)O2/SiO2 to efficiently catalyze the ODH of ethylbenzene at normal atmospheric pressure. The remarkable ability of Ce(x)Zr(1-x)O2 to prevent the deactivation of supported vanadium oxide leading to stable activity in the time-on-stream experiments and high selectivity to styrene are other important observations.     

Nb2O5 "pathway effect" on hydrogen sorption in Mg

In the present work we investigate the hydrogen sorption mechanism in a MgH(2)/Nb(2)O(5) composite and analyze why Nb(2)O(5) could strongly improve hydrogen sorption kinetics in magnesium. Hereby we make use of the fact that Nb(2)O(5) nanoparticles are able to reduce the milling time significantly with the achievement of excellent sorption kinetics, and can so exclude effects occurring at long-term milling that make difficult the study of the mechanism. On the basis of extensive chemical, crystalline, and microstructural characterization of the MgH(2)/Nb(2)O(5) nanopowder system, a "pathway model" is proposed, which explains the kinetic hydrogen sorption improvement by a formation of pathways of niobium oxide species with lower oxidation state that facilitate the hydrogen transport into the sample. This mechanism is shown to be supported by additional oxidation experiments, which indicate increased oxygen diffusion through these pathways.     

The Dimension of the Space of Harmonic 2-Spheres in the 6-Sphere

Using the twistorial approach and some previous results, weprove the conjecture that the dimension of the moduli spaceof harmonic maps of area 4d from the 2-sphere to the 2n-sphereis 2d + n² for the particular case n = 3. 2000 Mathematics SubjectClassification 53A10 (primary), 53C42 (secondary).     

Numerical resolution of an exact heat conduction model with a delay term

In this paper we analyze, from the numerical point of view, a dynamic thermoelastic problem. Here, the so-called exact heat conduction model with a delay term is used to obtain the heat evolution. Thus, the thermomechanical problem is written as a coupled system of partial differential equations, and its variational formulation leads to a system written in terms of the velocity and the temperature fields. An existence and uniqueness result is recalled. Then, fully discrete approximations are introduced by using the classical finite element method to approximate the spatial variable and the implicit Euler scheme to discretize the time derivatives. A priori error estimates are proved, from which the linear convergence of the algorithm could be derived under suitable additional regularity conditions. Finally, a two-dimensional numerical example is solved to show the accuracy of the approximation and the decay of the discrete energy.     

Exploring the Link between Reformed Teaching Practices and Pupil Learning in Elementary School Mathematics

This study examined the classroom practices of beginning elementary school teachers' instruction of mathematics and how it connected to their pupils' learning. The Reformed Teaching Observation Protocol (RTOP) was used to measure the extent to which beginning teachers used reformed teaching practices. As a measure of pupil learning, we utilized assessment scores specific to the mathematics unit observed and correlated them with teachers' RTOP scores. We found that beginning teachers who implemented reformed teaching practices tended to have pupils who scored higher on the district mathematics test with a statistically significant correlation of 0.56 (p < .05). Implications of these findings and others are discussed in terms of using the RTOP to improve practice at the elementary school level and for future school‐based research.     

Anti-stokes laser cooling in bulk erbium-doped materials

We report the first observation of anti-Stokes laser-induced cooling in the Er3+:KPb2Cl5 crystal and in the Er3+:CNBZn (CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.     

An outranking-based fuzzy logic model for collaborative group preferences

Some ideas coming from Multi Criteria Decision Aid are here extended to group decision. We present a logical model in order to reflect the degree of truth of the following predicate “group G considers that option x is at least as good as y.” The proposal considers the strength of the majority using its number and the intensity of its preference, and it also acknowledges the importance of the minorities of a certain numerical significance that manifest an intense disagreement with the predicate of the outranking. The effects of the “majority dictatorship” are restricted. Since it considers simultaneously the strength of the majority, the importance of unhappy minorities, and the intensity of the preference/opposition, this model exhibits desirable qualities of the classic methods by Condorcet and Borda. This model can be used to solve problems regarding selection, ranking, classification, and sorting. Various examples are given, which show the quality of the solutions that were obtained.