Thermodynamic non-additivity in disordered systems with extended phase space

Non-additivity effects in coupled dynamic-stochastic systems are investigated. It is shown that there is a mapping of the replica approach to disordered systems with finite replica indexn on Tsallis non-extensive statistics, if the average thermodynamic entropy of the dynamic subsystem differs from the information entropy for the probability distribution in the stochastic subsystem. The entropic indexq is determined by the entropy difference ΔS. In the case of incomplete information, the entropic indexq=1−n is shown to be related to the degree of lost information.     

Continuum percolation of the four-bonding-site associating fluids

Wertheim’s integral equation theory for associating fluids is reformulated for the study of the connectedness properties of associating hard spheres with four bonding sites. The association interaction is described as a square-well saturable attraction between these sites. The connectedness version of the Ornstein-Zernike (OZ) integral equation is supplemented by the PY-like closure relation and solved analytically within an ideal network approximation in which the network is represented as resulting from the crossing of ideal polymer chains. The pair connectedness functions and the mean cluster size are calculated and discussed. The condition for the percolation transition and the analytical form of the percolation threshold are derived. The connection of the percolation with the gas-liquid phase transition is discussed.     

Negative linear compressibility in confined dilatating systems

The role of a matrix response to a fluid insertion is analyzed in terms of a perturbation theory and Monte Carlo simulations applied to a hard sphere fluid in a slit of fluctuating density-dependent width. It is demonstrated that a coupling of the fluid-slit repulsion, spatial confinement, and the matrix dilatation acts as an effective fluid-fluid attraction, inducing a pseudocritical state with divergent linear compressibility and noncritical density fluctuations. An appropriate combination of the dilatation rate, fluid density, and the slit size leads to the fluid states with negative linear compressibility. It is shown that the switching from positive to negative compressibility is accompanied by an abrupt change in the packing mechanism.     

Structural rearrangement of solid surfaces due to competing adsorbate-substrate interactions

Employing a generalized lattice gas theory and the Brownian dynamics simulation, we show that the competing displacive interaction in an adsorbate may cause a continuous distortive transition in the underlying substrate. The threshold for the transition is determined by the competition of the substrate rigidity and the quasielastic energy induced by the adsorbate. In the presence of a strong pinning and repulsive lateral interaction, the resulting structure appears as a compromise between the square lattice of the substrate and the hexagonal arrangement of the adsorbate. For hexagonal substrate lattices the simulation demonstrates that various adsorbate structures (from honeycomb lattices to quasicrystalline pentagonal configurations) may be observed, depending on the effective radii of interaction. Due to the long-ranged coupling the substrate may acquire a substructure induced by the adsorbate. This paper represents a generalization of the work published in Phys. Rev. Lett. 81, 3904 (1998).     

Behavior of nanosized tungsten oxide bronzes produced by high-temperature electrolysis in model processes of desulfurization of petroleum products

Tungsten oxide bronzes were produced by an original method of high-temperature electrochemical deposition. Their catalytic properties in a model process of desulfurization of petroleum products-benzothiophene peroxidation were studied. The effect of the particle size on the catalytic and catalase activity of samples was shown.     

On a mechanism of low-pressure insertion of chain molecules into crystalline matrices

A microscopic mechanism of low-pressure insertion and separation of chain-like molecules in host matrices is proposed. It is shown that the intramolecular correlations combined to appropriate host activities are responsible for a low-pressure condensation of chain molecules. This allows to recover a fine structure of the isotherms and to explain recent experiments on the insertion of C(2)H(2) and CO(2) guest species into metal-organic microporous materials. We argue that the mechanism should be dominant in low-dimensional host geometries, where the effects related to the chain conformations or reorientations are strongly suppressed and the major factors are the chain connectivity and the segment packing.     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

Constrained equilibrium as a tool for characterization of deformable porous media

A new method for characterizing the deformable porous materials with noncritical adsorption probes is proposed. The mechanism is based on driving the adsorbate through a sequence of constrained equilibrium states with the insertion isotherms forming a pseudocritical point or a van der Waals-type loop. In the framework of a perturbation theory and Monte Carlo simulations we have found a link between the loop parameters and the host morphology. This allows one to characterize porous matrices through analyzing a shift of the pseudocritical point and a shape of the pseudospinodals.