Properties of Ir4 clusters in the gas phase and on oxide substrates

Results of a theoretical study on the properties of Ir4 clusters in the gas–phase and on oxide surfaces are presented. The work is based on density functional theory (DFT) within the generalized gradient approximation (GGA) and ultrasoft pseudopotentials. Properties of a small particle such as Ir4 cluster are entirely determined by its geometry. The already known result that the most stable form of Ir4 in the gas–phase is the square structure which is significantly more stable than the butterfly and tetrahedron is confirmed. This result is in contradiction with experiments which indicate that the oxide supported Ir4 adopts a tetrahedral configuration. It is shown in this thesis that the chemical environment has a strong influence on the relative stability of Ir4 clusters. On MgO(100) surface, the square isomer remains the most stable Ir4 structure, well separated in energy from the other two. Moreover, the tetrahedron is heavily distorted by the interaction with the surface oxygen. Presence of point defects (neutral and charged O vacancies) affects the energy ordering making tetrahedron and square very close in energy, but the structural distortion of the tetrahedron is even bigger and the predicted data do not correspond to experiments. On TiO2(110) the tetrahedron and square structures become degenerate and the butterfly becomes the least stable isomer. Moreover, structural distortions are very small, in agreement with experimental data. It is shown that the TiO2 surface influences the relative stability of the three isomers through a particularly strong electrostatic field. Interactions of Ir4 with H, C and O atoms as well as with CO molecules have been studied. Adsorption of a single C atom strongly influences the relative stability of the three isomers. Upon C adsorption, the butterfly becomes the most stable gas–phase isomer while on both surfaces the tetrahedron is the most probable structure. Adsorption of a single H or O atom does not produce the same effect. The interaction with CO molecules is also important given the experimental procedure used for producing supported Ir4 clusters. It is shown that on MgO(100), CO dissociation is as probable as the competing process CO desorption justifying the presence of carbon adatoms on Ir4 clusters which brings theoretical predictions in better agreement with experimental data

Contribution to Accident Prediction Models Development for Rural Two-Lane Roads in Serbia

Over the last three decades numerous research efforts have been conducted worldwide to determine the relationship between traffic accidents and traffic and road characteristics. So far, the mentioned studies have not been carried out in Serbia and in the region. This paper represents one of the first attempts to develop accident prediction models in Serbia. The paper provides a comprehensive literature review, describes procedures for collection and analysis of the traffic accident data, as well as the methodology used to develop the accident prediction models. The paper presents models obtained by both univariate and multivariate regression analyses. The obtained results are compared to the results of other studies and comparisons are discussed. Finally, the paper presents conclusions and important points for future research. The results of this research can find theoretical as well as practical application

Harnessing non-stoichiometry and disorder in thermoelectric materials

Thermoelectric materials require an exquisite balancing of thermal and electronic transport properties. Core to achieving such a balance in thermoelectric materials is the pursuit of non-stoichiometric compositions. Non-stoichiometry serves to control the charge carrier concentration, alter the electronic structure, control electron and phonon scattering, and produce anomalies in the phononic structure. As such, the optimized material is far removed from the original parent compound. Looking to the future, a deeper understanding of non-stoichiometry and its impact on electronic and phononic transport is critical to designing the next generation of thermoelectric materials. To convey the importance of non-stoichiometry in thermoelectric materials, we will begin with two classic case examples that highlight how non-stoichiometry profoundly alters transport in thermoelectric materials. These include (i) the alteration of the electronic structure through resonant states in PbTe and (ii) alteration to phonon transport via ‘rattling’ modes in skutterudite compounds. With this foundation, we discuss our recent efforts to control transport in pnictide and chalcogenide compounds through a combination of first principles calculations of defect structures, combinatorial growth of alloys, and bulk synthesis. For example, Figure 1 highlights how first principles calculations can offer insight into native defect populations and their impact on electronic structure. Strategies to accelerate discovery in this high dimensional phase space and critical challenges that remain serve to conclude this discussion of thermoelectric materials. Please click Additional Files below to see the full abstract