Nearest-neighbor Interaction Energies Research Articles

Microscopic Phase-Field Simulation of γ′ Precipitation in Ni-Based Binary Alloys Coupled with CALPHAD Method

In the present work, the first (1st) and second (2nd) nearest-neighbor interaction energies are calculated by coupling the microscopic phase-field kinetic model with the calculation of phase diagrams (CALPHAD) method. The morphological evolution of the γ′ precipitate and the variation of its atomic ordering parameter for Ni–X (X = Al, Fe, Mn, Pt, or Si) alloys during aging are studied. The simulation results predict different occupation preferences for solute and solvent atoms in the γ′ phase, i.e., solute atoms are inclined to occupy the corner sites and solvent atoms tend to occupy the face sites. In order to understand the precipitation process of the γ′ phase systematically, the ordering and clustering behaviors of solute atoms are analyzed.

Thin film growth of phase-separating phthalocyanine-fullerene blends: A combined experimental and computational study

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role for the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 K and 400 K. The evolution of roughness was followed by in-situ real-time X-ray reflectivity. Crystal orientation, island densities and morphology were examined after the growth by X-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast formation of distinct islands at a very early stage of growth. The growth of needle-like CuPc crystals loosing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrates that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 K to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies.

Impact of anisotropic interactions on nonequilibrium cluster growth at surfaces

Using event-driven kinetic Monte-Carlo simulations we investigate the early stage of non-equilibrium surface growth in a generic model with anisotropic interactions among the adsorbed particles. Specifically, we consider a two-dimensional lattice model of spherical particles where the interaction anisotropy is characterized by a control parameter $\eta$ measuring the ratio of interaction energy along the two lattice directions. The simplicity of the model allows us to study systematically the effect and interplay between $\eta$, the nearest-neighbor interaction energy $E_{n}$, and the flux rate $F$, on the shapes and the fractal dimension $D_{f}$ of clusters before coalescence. At finite particle flux $F$ we observe the emergence of rod-like and needle-shaped clusters whose aspect ratio $R$ depends on $\eta$, $E_{n}$ and $F$. In the regime of strong interaction anisotropy, the cluster aspect ratio shows power-law scaling as function of particle flux, $R \sim F^{- \alpha}$. Furthermore, the evolution of the cluster length and width also exhibit power-law scaling with universal growth exponents for all considered values of $F$. We identify a critical cluster length $L_{c}$ that marks a transition from one-dimensional to self-similar two-dimensional cluster growth. Moreover, we find that the cluster properties depend markedly on the critical cluster size $i^{*}$ of the isotropically interacting reference system ($\eta = 1$).

Electronic and energetic properties of Ge(110) pentagons

The electronic and energetic properties of the elementary building block, i.e. a five-membered atom ring (pentagon), of the Ge(110) surface was studied by scanning tunneling microscopy and spectroscopy at room temperature. The Ge(110) surface is composed of three types of domains: two ordered domains ((16×2) and c(8×10)) and a disordered domain. The elementary building block of all three domains is a pentagon. Scanning tunneling spectra recorded on the (16×2), c(8×10) and disordered domains are very similar and reveal three well-defined electronic states. Two electronic states are located at 1.1eV and 0.3eV below the Fermi level respectively, whereas the third electronic state is located at 0.4eV above the Fermi level. The electronic states at −0.3eV and 0.4eV can be ascribed to the pentagons, whilst we tentatively assigned the electronic state at −1.1eV to a Ge–Ge back bond or trough state. In addition, we have analyzed the straight 112¯ oriented step edges. From the kink density and kink–kink distance distributions we extracted the nearest neighbor interaction energy between the pentagons, which exhibit a strong preference to occur in twins, as well as the strain relaxation energy along the pentagon-twin chains.

Lattice cluster theory of associating polymers. IV. Phase behavior of telechelic polymer solutions

The newly developed lattice cluster theory (in Paper I) for the thermodynamics of solutions of telechelic polymers is used to examine the phase behavior of these complex fluids when effective polymer-solvent interactions are unfavorable. The telechelics are modeled as linear, fully flexible, polymer chains with mono-functional stickers at the two chain ends, and these chains are assumed to self-assemble upon cooling. Phase separation is generated through the interplay of self-assembly and polymer/solvent interactions that leads to an upper critical solution temperature phase separation. The variations of the boundaries for phase stability and the critical temperature and composition are analyzed in detail as functions of the number M of united atom groups in a telechelic chain and the microscopic nearest neighbor interaction energy ε(s) driving the self-assembly. The coupling between self-assembly and unfavorable polymer/solvent interactions produces a wide variety of nontrivial patterns of phase behavior, including an enhancement of miscibility accompanying the increase of the molar mass of the telechelics under certain circumstances. Special attention is devoted to understanding this unusual trend in miscibility.

Hydrogen - Hydrogen Adsorption Interaction Energy in Zeolites

A general model based on Ono-Kondo lattice theory for hydrogen adsorption, nearest neighbor interaction energy among adsorbate molecules were derived according to thermodynamic principle. A linear form of the above general model was applied to determine the interaction energies among hydrogen molecules inside adsorption layer from the previous experimental data of hydrogen adsorption on A- and X-type zeolites. The results show that the energies of hydrogen-hydrogen interactions inside the adsorption layer are negative values, indicating that the attractions among the adsorbed hydrogen molecules are prominent in the present work. And the influence of hydrogen molecules outside adsorption layer on the adsorbed hydrogen molecule is not important.

Phospholipid Complexation of General Anesthetics: A Wake-Up Call

General anesthesia represents one of the most important advances in the history of medicine. Despite numerous attempts to determine how general anesthetics function at the molecular level, it remains to be established whether signaling proteins or the surrounding lipids serve as their primary targets. In this talk, I will discuss recent studies that we have carried out in which the effects of chloroform on phospholipid-sterol interactions in the liquid-ordered (lo) and the liquid-disordered (ld) phases have been quantified via the nearest-neighbor recognition (NNR) method. Using host membranes made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol, evidence has been obtained for 1/1 complexes being formed from chloroform and DPPC in both phases. At clinically-relevant concentrations (ca. 2 mM), chloroform was found to weaken cholesterol_DPPC interactions in the lo phase by 24.5 ± 5.5 cal/mol of lipid, while no effect could not be detected in the ld phase. Increasing the concentration of chloroform resulted in a convergence of nearest-neighbor interaction energies and the apparent formation of a common “liquid-anesthetic” (la) phase. The implications of these findings with regard to modern lipid theory of anesthesia will be discussed.

Application of the entropy theory of glass formation to poly(α-olefins)

The entropy theory of glass formation, which has previously been developed to describe general classes of polymeric glass-forming liquids, is extended here to model the thermodynamic and dynamic properties of poly(alpha-olefins). By combining this thermodynamic theory with the Adam-Gibbs model (which relates the configurational entropy to the rate of structural relaxation), we provide systematic computations for all four characteristic temperatures (T(A), T(c), T(g), T(0)), governing the position and breadth of the glass transition, and the fragility parameters (D,m) describing the strength of the temperature dependence of the structural relaxation time, where T(A) is the temperature below which the relaxation is non-Arrhenius, T(c) is the crossover or empirical mode-coupling temperature, T(g) is the glass transition temperature, and T(0) is the temperature at which the extrapolated relaxation time diverges. These temperatures and fragility parameters are evaluated as a function of molar mass, pressure, and the length n of the alpha-olefin side chains. The nearest neighbor interaction energy and local chain rigidities are found to strongly influence the four characteristic temperatures and the low temperature fragility. We also observe an "internal plasticization" of the poly(alpha-olefins) wherein the fragility decreases as the number n of "flexible" side group units increases. Our computations provide solid support for a pressure counterpart of the Vogel-Fulcher-Tammann relation. The entropy theory of glass formation predicts systematic changes in fragility with chain stiffness, cohesive energy, polymerization index, and side chain length, and qualitative trends in these parameters are discussed.

Monte Carlo study of multicomponent adsorption on triangular lattices

The adsorption process of interacting binary gas mixtures containing particles A and B on triangular substrates is studied through grand canonical Monte Carlo simulation in the framework of the lattice-gas model. The energies involved in the adsorption process are four: (1) ϵ0, interaction energy between a monomer (type A or B) and a lattice site; (2) wAA, nearest-neighbor interaction energy between two A particles; (3) wAB (=wBA), nearest-neighbor interaction energy between an A particle and a B particle and (4) wBB, nearest-neighbor interaction energy between two B particles. The process is monitored through partial and total isotherms, differential heats of adsorption and energy of the system, which appear as very sensitive to all lateral interactions. We focus on the case of repulsive lateral interactions, where a rich variety of structural orderings are observed in the adlayer, depending on the value of the parameters wAA, wAB and wBB. Results are rationalized through the determination of the phase diagrams characterizing second order phase transitions in the system. A nontrivial interdependence between the partial surface coverage of both species is observed.

Treatment of Dilute Clusters of Methanol and Water by ab Initio Quantum Mechanical Calculations

Large molecular clusters can be considered as intermediate states between gas and condensed phases, and information about them can help us understand condensed phases. In this paper, ab initio quantum mechanical methods have been used to examine clusters formed of methanol and water molecules. The main goal was to obtain information about the intermolecular interactions and the structure of methanol/water clusters at the molecular level. The large clusters (CH(4)O...(H(2)O)(12) and H(2)O...(CH(4)O)(10)) containing one molecule of one component (methanol or water) and many (12, 10) molecules of the other component were considered. Møller-Plesset perturbation theory (MP2) was used in the calculations. Several representative cluster geometries were optimized, and nearest-neighbor interaction energies were calculated for the geometries obtained in the first step. The results of the calculations were compared to the available experimental information regarding the liquid methanol/water mixtures and to the molecular dynamics and Monte Carlo simulations, and good agreement was found. For the CH(4)O...(H(2)O)(12) cluster, it was shown that the molecules of water can be subdivided into two classes: (i) H bonded to the central methanol molecule and (ii) not H bonded to the central methanol molecule. As expected, these two classes exhibited striking energy differences. Although they are located almost the same distance from the carbon atom of the central methanol molecule, they possess very different intermolecular interaction energies with the central molecule. The H bonding constitutes a dominant factor in the hydration of methanol in dilute aqueous solutions. For the H(2)O...(CH(4)O)(10) cluster, it was shown that the central molecule of water has almost three H bonds with the methanol molecules; this result differs from those in the literature that concluded that the average number of H bonds between a central water molecule and methanol molecules in dilute solutions of water in methanol is about two, with the water molecules being incorporated into the chains of methanol. In contrast, the present predictions revealed that the central water molecule is not incorporated into a chain of methanol molecules, but it can be the center of several (2-3) chains of methanol molecules. The molecules of methanol, which are not H bonded to the central water molecule, have characteristics similar to those of the methane molecules around a central water molecule in the H(2)O...(CH(4))(10) cluster. The ab initio quantum mechanical methods employed in this paper have provided detailed information about the H bonds in the clusters investigated. In particular, they provided full information about two types of H bonds between water and methanol molecules (in which the water or the methanol molecule is the proton donor), including information about their energies and lengths. The average numbers of the two types of H bonds in the CH(4)O...(H(2)O)(12) and H(2)O...(CH(4)O)(10) clusters have been calculated. Such information could hardly be obtained with the simulation methods.

Adsorption of gases on SnS 2: trace of order–disorder phase transition of a honeycomb lattice gas

We have made volumetric measurements of adsorption isotherm of Ar, Kr, O 2 and N 2 on a powder sample of tin disulfide SnS 2 in the temperature range of 77–90 K. Ar, Kr and O 2 exhibited pressure isotherms unusually bending at the coverage of 0.3–0.4 monolayer, while N 2 did not. The isotherms are analyzed by a quasi-chemical approximation theory developed for a honeycomb lattice gas with first nearest neighbor exclusion and with a small adsorption potential difference between the two (1 × 1)-1/2 sublattices. The unusual bending of the pressure isotherms is successfully explained as a trace of the order–disorder phase transition of second order that would take place in the honeycomb lattice gas if the two sublattices had the same adsorption potential. The adsorption potential difference is found to be at most of order of magnitude of the second nearest neighbor interaction energy.

On the Lowest Energy Nucleation Path in a Supersaturated Lattice Gas

A lattice gas with non-conserved spin flip dynamics (of both non-Glauber and Glauber types) is considered at T≪T c , the critical temperature. For arbitrary supersaturation, S, a general expression for the inverse of the nucleation rate along the lowest energy path is derived. The exponential part is identical to the one by Neves and Schonmann [Commun. Math. Phys.137:20 (1991)]. The preexponential can be expressed in terms of elliptic theta-functions for small S, and in the limits, respectively, of S≫T/φ or S≪T/φ (−φ being the nearest-neighbor interaction energy), elementary versions of the general expression are further obtained. The preexponential has a smooth component, as well as small-scale modulations which are approximately periodic in the inverse supersaturation. For S≪T/φ, the smooth part is proportional to \(\sqrt S \), in contrast to the zero-T limit where it is linear in S. The latter limit becomes apparent only at extremely low temperatures which are cubic in S.

A Statistical Model for the Cooperative Thermochromic Transition of Polysilanes

A statistical model for the cooperative thermochromic transition of polysilanes is proposed. The basic assumption of this treatment is that the origin of the thermochromism is the conformational transition between two ordered states, i.e., all-trans and helix structures. We have calculated the partition function of a single polysilane chain by assuming that the total conformational energy of the chain can be described by a simple sum of the nearest-neighbor interaction energies and the energies of trans, gauche(+), and gauche(−) structures. A comparison of the theoretical results with the experimental intensity of absorption maxima has enabled us to estimate the free energy of stabilization of the ordered sequences. The calculation suggests that the transition-type thermochromism is observable only when the conformational stabilization energies of the ordered sequences are large.

Desorption and molecular interactions on surfaces: [formula omitted], [formula omitted] and [formula omitted

We present studies of the isothermal desorption of carbon monoxide from the three low index rhodium surfaces using time-resolved electron energy loss spectroscopy (TREELS). We use a quasiequilibrium model and apply lattice gas theory and the transfer matrix technique to model the desorption isotherms. With the further assumption that the nearest neighbor interaction energy between chemisorbed CO molecules is large and repulsive, we extract the second and third neighbor lattice energies for all three surfaces. We compare our results with the ordered CO overlayer structures reported by low electron energy diffraction measurements. We find that the lateral interaction energies on all three surfaces exhibit weak but non-negligible repulsion and attraction. We also examine and discuss the validity of the quasiequilibrium assumption in analyzing desorption measurements. Finally, we compare and discuss the trends in observed CO structures on various transition metal surfaces.

Diffusion of Interacting Lattice Gases on Heterogeneous Surfaces

The Monte Carlo method is used to simulate diffusion of interacting lattice gases on heterogeneous surfaces. The fluctuation and Kubo-Green methods are utilized for determining the tracer and chemical surface diffusion coefficients. Simulations are carried out on a square array of64 x 64 lattice sites applying periodic boundary conditions. Surface heterogeneity is introduced in terms of the bivariate trap model with random topography. Simulations are performed for a trap concentration θ trap = 0.2 and various values of the trap binding energies ∈ 2 , ∈ 1 and the nearest neighbor interaction energy φ NN . For φ NN < 0 (repulsion) the system exhibits c(2 x 2) ordering at half coverage and low temperatures. Due to the low compressibility of the well-ordered c(2 x 2) lattice gas phase, the chemical diffusion coefficient is large under these circumstances. The effects of surface heterogeneity, ∈ 1 ¬= ∈ 2 , are largely pronounced at low total coverages 0 and low temperatures T where most of the adatoms are trapped by the deep traps. At higher coverages the effects are relatively small albeit significant. The existence of deep traps disturbs c(2 x 2)ordering at θ = 0.5 and low temperatures. It is shown that the breakdown ofthe order substantially affects surface diffusion.

Configurational stabilization of mixed-valence states in the star-burst tetranuclear ruthenium complex bridged with acetylene links

A new tetranuclear ruthenium complex in which a metal was bridged radially to three identical metals with acetylene links showed a mixed-valence state in cyclic voltammograms. The peak potential difference for the mixed-valence state was twice the potential difference for a mixed-valence state of the corresponding binuclear complex. This fact suggests stabilization of the mixed-valence state twice by configurational modification of redox centers. However, this is inconsistent with the prediction from the additive pair model of the redox interaction. A quantum chemical model of the redox interaction was presented, in which the potential difference was expressed by the overlap integral S and the nearest neighbor interaction energy between the reduced center and the oxidized one. Application of the theory gave a negative value of S, which demonstrated reasonably the stabilization of the mixed-valence state from a quantum chemical viewpoint.

Step dynamics and equilibrium structure of monoatomic steps on Si(100)-2 x 1.

Monte Carlo simulations of the dynamics for a $B$-type step on the Si(100)-2\ifmmode\times\else\texttimes\fi{}1 surface are reported. The equilibrium structure of the step is considered to be the result of attachment and detachment processes. Nearest-neighbor interaction energies are considered for detachment from steps and a simple relaxation mechanism is used for attachment. This simple model makes it possible to gain insight into the effects of the relaxation process inherent to the system under study. We show that this mechanism plays a crucial role in determining the equilibrium patterns found in Si(100) steps and that effective interaction energies cannot be determined from the frozen low-temperature step profile without taking the dynamics processes into account appropriately.

Interaction energies of 111In perturbed-angular-correlation probes with 3d and 4sp impurities in Ag, Pd, and Rh.

We present systematic ab initio calculations for the nearest-neighbor interaction energies of $^{111}\mathrm{In}$ perturbed-angular-correlation probe atoms with 3d and 4sp impurity atoms (Sc-As, with Z=21\char21{}33) in Ag, Pd, and Rh. The calculations are based on local-spin-density theory and apply the Korringa-Kohn-Rostoker Green's-function method for spherical potentials. The full nonspherical charge density is evaluated to calculate the double-counting contributions to the total energy. The present calculations reproduce very well the chemical trend of the available experimental interaction energies; attractive interaction of In with 3d impurities in Ag and repulsive interaction with 4sp impurities in Ag. For Mn-In pairs in Ag, the detailed comparison between the calculated results with and without spin-polarization energy shows the importance of magnetism for the 3d impurity-probe interaction enegies and is also useful to elucidate the physical mechanism for the magnetic energy anomalies in cohesive, surface, and solution energies of 3d systems. \textcopyright{} 1996 The American Physical Society.

Statistical thermodynamics of polynuclear linear complexes with mixed valence states by means of the correlated walk

Expressions for the equilibrium electrode potential of linear N-nuclear complexes with homoredox centers were derived by the theory of the correlated walk as a function of the molar fraction of the oxidized moiety, the nearest-neighbor interaction energies and N. When the interaction energy was large in two-, three- and four-nuclear complexes, the expressions predicted two, three and four voltammetric peaks respectively owing to the formation of mixed valence states. The intuitive extension that the N-nuclear complex might exhibit N peaks was invalid. There were three peaks for any odd number of N. In contrast, four peaks appeared for any even number of N more than 4. For a polynuclear complex with N → ∞, the number of the peaks was reduced to two, as if the complex might be a binucleus. The log plot for the fraction vs. potential curve at large values of N deviated from a straight line. The averaged inverse slope was ca. 90 mV at 25°C. From the concentration distribution of a predominant species varying with the potential, the deviation of the log plot was ascribed to the coexistence of various isomers with different interaction energies. The difference in the voltammetric peak potential was approximately linear with the interaction energy for any N. Approximate equations for the potential difference for N= 2, 3 and 4 were obtained, and were applied to the experimental data available for polyferrocenes.

Global analysis of phase states in a random-site model.

The global features of the random-site model of C IV atoms for the alloys (A III B V ) 1-x C 2x IV are determined using the pair-correlation approximation in the limit at the absolute zero of temperature. There are four different types of phase-state configurations for all combinations of nearest-neighbor interaction energies. The first type has a site percolation threshold at the critical composition x c of the order-disorder transition. The second type has a critical composition x c =1/6. The phase-state configuration on the boundary line between types 1 and 2 has critical composition x c =3/8