Nature of metal-nonmetal transition in metal-ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure

Applying semi-analytical models of nonideal plasma, we evaluate the behavior of the metallic phase in metal-ammonia solutions (MAS). This behavior is mainly controlled by the degenerate electron gas, which remains stable down to 5 MPM due to high solvent polarizability and strong dielectric screening of solvated ions. Comparing the behavior of the metallic state with those of localized solvated electrons, we have estimated the miscibility gap $\Delta n$ for various alkali metals and found $\Delta n$(Na)$> \Delta n($K$)$. It is rather narrow in Rb-NH$_3$ and does not occur in Cs-NH$_3$ solutions, which is in full agreement with the experiments. The case of Li is discussed separately. The difference calculated in the excess free energies of the metallic and nonmetallic phases is in the order of $k_BT$, yielding a thermally fluctuating mixed state at intermediate metal concentrations. It results in a continuous metal-nonmetal (MNM) transition above the consolute point $T_c$ and a phase separation below $T_c$. We propose a criterion for the MNM transition which may be attributed to the line of the maximum of compressibility above $T_c$. This line crosses the spinodal one at the critical temperature. Finally, we assert that a new electronic phase similar to microemulsion should also arise between the spinodal and the binodal lines.Comment: 22 pages, 10 figure

Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations

Using a theory of polarizable fluids, we extend a variational treatment of an excess electron to the many-electron case corresponding to finite metal concentrations in metal-ammonia solutions (MAS). We evaluate dielectric, optical, and thermodynamical properties of MAS at low metal concentrations. Our semi-analytical calculations based on a mean-spherical approximation correlate well with the experimental data on the concentration and the temperature dependencies of the dielectric constant and the optical absorption spectrum. The properties are found to be mainly determined by the induced dipolar interactions between localized solvated electrons, which result in the two main effects: the dispersion attractions between the electrons and a sharp increase in the static dielectric constant of the solution. The first effect provides a classical phase separation for the light alkali metal solutes (Li, Na, K) below a critical temperature. The second effect leads to a dielectric instability, i.e., polarization catastrophe, which is the onset of metallization. The locus of the calculated critical concentrations is in a good agreement with the experimental phase diagram of Na-NH3 solutions. The proposed mechanism of the metal-nonmetal transition is quite general and may occur in systems involving self-trapped quantum quasiparticles.Comment: 13 figures, 42 page

Herzfeld instability versus Mott transition in metal-ammonia solutions

Although most metal-insulator transitions in doped insulators are generally viewed as Mott transitions, some systems seem to deviate from this scenario. Alkali metal-ammonia solutions are a brilliant example of that. They reveal a phase separation in the range of metal concentrations where a metal-insulator transition occurs. Using a mean spherical approximation for quantum polarizable fluids, we argue that the origin of the metal-insulator transition in such a system is likely similar to that proposed by Herzfeld a long time ago, namely, due to fluctuations of solvated electrons. We also show how the phase separation may appear: the Herzfeld instability of the insulator occurs at a concentration for which the metallic phase is also unstable. As a consequence, the Mott transition cannot occur at low temperatures. The proposed scenario may provide a new insight into the metal-insulator transition in condensed-matter physics.Comment: 9 pages, 4 figure

Non-metal-to-metal transition driven by van der Waals forces in an interacting polaronic gas

International audienceUsing path integrals and the theory of polarizable fluids, we develop a model treating non-degenerate interacting Fröhlich polarons at low densities and temperatures. Starting from the dilute regime, we show that at strong electron-phonon coupling, the collective properties of polarons are mainly governed by the London dispersion forces, i.e. induced dipole-dipole van der Waals interactions. At a critical density, these forces provoke a non-metal-to-metal transition by means of a polarization catastrophe and a mechanical instability, which results in a polaron dissociation

Comment on "Model of saturated lithium ammonia as a single-component liquid metal"

International audienceWe demonstrate in this Comment that the theory of simple metals applied to the saturated Li-NH3 solution in the titled paper [U. Pinsook and S. Hannongbua, J. Chem. Phys.124, 074702 (2006)] should account for the peculiarities of the solution, namely, the high solvent polarizability and different energy scales for ion-ion and electron-electron interactions. Calculations not taking into account these peculiarities contradict the experimental phase diagram of the Li-NH3 solution

Reference interaction site model study of self-aggregating cyanine dyes

Using the reference interaction site model and supramolecular approach, we modeled the aggregation of thea-monomethinecyanine dyes in water. Various modifications of the hypernetted-closure expression for the excess free energy have been studied. We found that the partial wave approximation with semiempirical corrections for excluded volume and hydrogen bonding effects provides estimations of the binding and dimerization energies of the aggregates, which are in agreement with available experimental data. The hydrated H-dimers are obtained to be more stable than the hydrated J-dimers. However, the complexes consisting from more than four monomers change their arrangement while self-assembling in water and form ladderlike structures. We propose a model explaining this structural transition

Improved estimates for hydration free energy obtained by the reference interaction site model

We propose to improve the existing free energy expressions obtained within the framework of the reference interaction site model (RISM) combined with the hypernetted closure. The proposed expression is based on the partial wave expression [S. Ten-no, J. Chem. Phys. 115 (2001) 3724] but includes semiempirical corrections to account properly for excluded volume and hydrogen bonding effects. Testing several free energy expressions for various polar and hydrophobic solutes, we have found that such empirical parameterization of the partial wave expression can provide accurate estimates of hydration energies for different hydrophobic and polar solutes. The proposed correction allows one to reduce the discrepancy between the experimental and the calculated data down to 0.7 kcal/mol. (C) 2007 Elsevier B.V. All rights. reserved

An operational Haar wavelet method for solving fractional Volterra integral equations

A Haar wavelet operational matrix is applied to fractional integration, which has not been undertaken before. The Haar wavelet approximating method is used to reduce the fractional Volterra and Abel integral equations to a system of algebraic equations. A global error bound is estimated and some numerical examples with smooth, nonsmooth, and singular solutions are considered to demonstrate the validity and applicability of the developed method

Preprint no.: 4 2010A CAS Wavelet Method for Solving Nonlinear Fredholm Integro- Differential Equations of Fractional Order

In this paper we present a computational method for solving a class of nonlinear Fredholm integro- differential equations of fractional order which is based on CAS (Cosine And Sine) wavelets. The CAS wavelet operational matrix of fractional integration is derived and used to transform the equation to a system of algebraic equations. some examples are included to demonstrate the validity and applicability of the technique