Molten Alkali Halides Research Articles

Corresponding states correlation for the thermal conductivity of molten alkali halides

The principle of corresponding states has been applied to the thermal-conductivity data for molten alkali halides which have been obtained by recent forced Rayleigh scattering measurements. The theory, which was developed by Harada et al. for the transport properties of uni-univalent molten salts, is based on the fluctuation-dissipation theorem with the pair interaction between ions composed of core repulsive and Coulombic potentials. Four characteristic parameters specific to each salt have been used to reduce the thermal conductivity and temperature. It has been found that the thermal conductivity of molten alkali halides is adequately correlated by the corresponding-states correlation (λ* ∝ 1/T*) within experimental accuracy. By employing the correlation, the thermal conductivity of molten alkali fluorides, which could not be measured by the forced Rayleigh scattering method, is predicted.

Impedance Spectroscopic Investigation of the Reduction Mechanism of Tantalumhalides During Electrodeposition in Molten Salts

AbstractThe mechanism of electrochemical reduction of TaCl5 dissolved in molten alkalihalides or their mixtures has been investigated by impedance spectroscopy in the frequency range 10 mHz≤ω≤100 kHz at various cathodic and anodic potentials and temperatures up to 1100 K. From a quantitative interpretation of the diffusion controlled impedance various reduction steps are resolved to better than ±10 mV. This is demonstrated here for a few selected examples. Besides this, the most important observation reported here for the first time is that electronic contributions can become dominant. Only with appropriate consideration of the electronic conduction a quantitative description of the impedance spectra over the whole potential range is found.

Ion Transport in Concentrated Electrolyte Solutions

The electrical conductivity and viscosity of concentrated solutions above several moles per liter are investigated by treating electrolyte solution as high-density plasma (strongly coupled plasma), where the Coulomb interaction energy is larger than the thermal kinetic energy. The normalized nondimensional conductivity, which is the electrical conductivity divided by the ion concentration and scaling parameters, is expressed as a function of the coupling constant Γ≡(Coulomb energy)/(thermal energy)=((ze)2/εa)/kT, where ε is the dielectric constant, a is the interionic distance and k is the Boltzmann constant. The normalized nondimensional conductivities of molten alkali halides except for fluoride at various temperatures lie on a single curve as a function of the Coulomb coupling parameter Γ. These molten salts can be good models for high-density plasma in spite of their different ion sizes.

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. III. molten NaI, KI, RbI, and CsI

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. III. molten NaI, KI, RbI, and CsI

Structure of interphases of low melting point salts and protonic hydrates with mercury and gold in relation to double-layer capacitance behaviour

In relation to the unusual potential dependence of the double-layer capacitance C at lead in molten alkali halide salts, observed by Ukshe, Bukun, Leikis and Frumkin, where the relation of C to potential was like that for the diffuse-layer capacitance in dilute aqueous solutions, comparative measurements are presented for the capacitance behaviour of mercury and gold in some low melting point salts and salt hydrates. The systems investigated are LiAlBr 4+NaAlCl 4+KAlCl 4 (3:5:2) and NaCl+AlCl 3 melts, and proton hydrate salts of CF 3SO 3 − (H 3O +, H 5O 2 +, H 7O 5 +, H 9O 4 +). In no case was the unusual behaviour of molten alkali halides at lead exhibited. The results also enable the progressive transition in double-layer capacitance behaviour between a molten salt, H 3O +·CF 3SO 3 −, at 307 K and systems containing solvent molecules to be evaluated and discussed in terms of interphasial structures, double-layer dimensions and local dielectric permittivity. At gold there are systematic and substantial changes in capacitance with speciation of the proton hydrates from H 3O +, through H 9O 4 +, to H + aq. in dilute solution. At mercury, the species-dependence of the capacitance is also significant but is less systematic in its variation.

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. I. Molten LiCl, NaCl, KCl, RbCl, and CsCl

As a series of experimental determinations of the thermal diffusivity of molten alkali halides, this paper describes measurements on five molten alkali metal chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) in the temperature range up to 1440 K by the forced Rayleigh scattering method. K2Cr2O7 is employed as a dye substance to color the transparent molten salts. The accuracy is estimated to be ± 4 to ±11 % depending on the measured salts. In comparison with the present results converted into thermal conductivity, most of the previous experimental data obtained by steady-state methods show larger values, up to about five times, which may be due to the systematic error caused by the presence of convection and radiation. It is found that the thermal conductivity of these series of molten alkali metal chlorides decreases with increasing molecular weight, and their temperature coefficients are weakly negative.

On the formation and nature of a dipolar Frenkel excitonic insulator

Recently, experiments on expanded fluid mercury, and simulations of alkali metals in liquid ammonia and in molten alkali halides, have motivated interest in the possibility of a Frenkel excitonic insulator (EI) phase. In this paper, the authors discuss several aspects of a model for describing such a phase. They choose a model Hamiltonian which is the asymptotically exact low-density form for a system of atoms each possessing an sp3 basis, and present two methods of analysis. The pairing theory centres on the broadening of the S to P atomic transition into exciton bands, which, as is known, may lead to the formation of a Frenkel EI phase. They analyse two corrections to the traditional exciton picture: (i) double-excitation processes, which are responsible for the van der Waals stabilization energy, are shown to halve the density predicted for the transition to an EI phase; (ii) correct incorporation of non-boson statistics for the exciton operators is argued to drive the transition from first order to second order. The authors also analyse the model Hamiltonian via a Hartree approximation, which proves to be the more tractable method, and further allows an explicit description of the EI phase itself. The validity of the Hartree approximation is justified by comparison with the pairing theory.

Electron localization in molten mixtures of alkali halides. Quantum path integral simulation

A combination of various computer simulation techniques is proposed as a tool for the theoretical studies of electron localization in condensed media. The emerging three-stage algorithm with (1) the classical Molecular Dynamics as the initializing part, (2) the Path Integral simulation as the central part and (3) the variational calculations of the absorption spectrum of trapped electron, allows us to calculate the excess electron properties without use of empirical or adjustable parameters. The algorithm was applied to the study of electron localization in mixtures of molten alkali halides. The comparison with the pulse radiolysis experiments is presented for a range of mixture compositions.

Electrorefining of Titanium Metal in Molten Alkali Halide Mixtures

Electrorefining of Titanium Metal in Molten Alkali Halide Mixtures

Specific Conductance and Free‐Ion Behavior of Component Ions in Molten Alkali Halides

Empirical plots of the specific electrical conductance of the molten alkali halides for a common vs. an effective size parameter are found to form straight lines through the origin. Such empirical relations could be explained by the stationary solution of the classical Newtonian equations for A+ and X−ions under the assumption that they move against a frictional force in the direction of an applied electric field.

Recent developments in the theory of ionic melts

A brief review is given of the present state of the art in the theory and numerical simulation of ionic melts, and in particular of molten alkali halides. Some recent developments concerning the theoretical evaluation of the static pair structure, the phase diagram and the microscopic dynamics in these melts are discussed. The importance of ionic polarizabilities is stressed. Some perspectives for future work are pointed out in the conclusion.

Invariants of spherical harmonics as order parameters in liquids: comparison of the structure of a molten and glassy alkali halide

The second order invariants of spherical harmonics for the geometrical characterization of clusters in disordered systems is applied for the comparison of computer simulated configurations of molten and glassy rubidium bromide. The latter has been obtained by the Stillinger method leading to the “inherent” liquid structure. It is shown that the rotational invariants of spherical harmonics characterize the inherent structure even when vibrations are not damped at all. Angular correlation between the positions of closest neighbours proved to be identical in liquid and glassy state, whereas radial pair correlation functions show considerable difference.

Electrical conductivity, self-diffusion, and volume expansion of alkali halides at the melting point

In an earlier paper, Heisenberg's uncertainty principle was invoked at the melting point T m of crystalline solids to provide fundamental justification for Lindemann's melting law and to compute diffusion coefficients of several alkali halides. The uncertainty principle defines breakdown of Debye zone boundary (ZB) phonons as valid collective excitations when phonon energies and line widths due to anharmonicity become comparable at T m. Upon breakdown, random, high-frequency single-particle motion or “partial decoupling” of crystal ions sets in. Lifetimes of these single-particle ZB motions are determined from the “minimum-uncertainty product” inequality by assuming that it becomes an equality at T m for ZB phonons. The present paper addresses improved formulation of that work and extended application to ionic electrical conductivities of 18 molten alkali halides at T m. It is shown that use of the Debye model produces an approximate lower bound to the mean free time, not the unconstrained direct estimate previouslu implied. This feature is generally reflected in results for ionic conductivities and alkali halide diffusion coefficients for which comparison experimental data were found. However, in spite of this lower-bound formulation and the simple nature of the computation, the results compare favorably with experiment. A model of random single-particle harmonic motion superimposed on the lower-frequency collective motion is proposed to account for volume expansion accompanying the partial decoupling for hard-sphere ions. Experimental comparisons for 15 alkali halides show the decoupling volume change to account largely for the total volume change of melting (in the hard-sphere approximation), yielding a closer agreement with experiment than recent calculations aimed explicitly at the total volume change.

Determination of oxygen in alkali halide salts by proton activation and radiochemical separation

A lanthanum fluoride precipitation method for the separation of18F produced from the18O(p,n)18F reaction in alkali halide salts is described. This radiochemical separation method minimizes interferences from other positron emitters produced by proton bombardment and makes the accurate determination of ppm-level18O in complex alkali halide systems feasible. The interference from the19F(p,d)18F reaction is eliminated by keeping the proton energy less than 8.2 MeV. Applications of this technique to studies of dissolved oxide species in molten alkali halide salts are discussed.

Vacancies in molten salts: a characteristic feature of the structure

The pair distribution function of and bond orientational order around vacancies in simulated hard-sphere fluids and molten alkali halides have been calculated. Two different definitions were used in identifying vacancies: by the geometrical definition a cavity is a vacancy when it is large enough to incorporate a particle, while by the energy definition a cavity is a vacancy when it is suitable for incorporating a phantom particle under attractive interactions with real particles. It was shown that the concept of vacancies as 'lacking' particles is physically meaningful not only in solids but in dense liquids as well. The geometrical features of the nearest-neighbour surroundings of vacancies proved to be similar to those of real particles. Comparison of molten and glassy RbBr indicated that (i) there is a somewhat better ordering to first- and second-neighbour pairs of vacancies in the glassy state than in the liquid state and (ii) the bond orientational orders are almost the same in the liquid and glassy states.

Hydrolysis of molten alkali chlorides, bromides and iodides

An electrochemical technique has been developed for studying equilibrium of the hydrolytic reactions of molten alkali chlorides, bromides, and iodides with exception of lithium salts. It consists of measuring the emf of the cells with gaseous electrodes ▪ this emf being connected with the equilibrium constant by the equation: ▪ Here X = Cl, Br or I, M = Na, K, Rb or Cs, P H 2 , P X 2O are pressures of the indicated gases on the electrodes, a MOH and Δ G 0 H X are activity of the hydroxides in melts under investigation and standard Gibbs energy of reactions 1 2 H 2(g)+ 1 2 X 2(g)⇌H X(g). As the hydrolysis results in forming diluted solutions of MOH in molten MX, one may use the conditional equilibrium constant of reactions H 2O(g) + MX(melt)⇌ MOH(melt)+H X(g) ▪ with a MX being equal to 1. Here N MOH and f* MOH are the mole fraction and limiting value of the activity coefficient of MOH in the melts at N MOH → 0. The equilibrium hydrolytic constant has been determined as a function of temperature. Expressions are obtained for its temperature dependence, this increases as temperature rises and the ionic potential of alkali cations decreases in the series from Na + to Cs +. Other things being equal, the hydrolysability of molten alkali halides diminishes in going from chlorides to iodides. The solutions of MOH in molten MX are shown to be close to a perfect mixture.

Sound absorption in molten alkali chlorides, bromides, iodides and their mixtures

The absorption coefficient α and velocity U of sound with the frequenceis f = 5, 15, 25 and 35 MHz in molten alkali chlorides, bromides, iodides and mixtures of NaCl + CsCl, NaBr + CsBr and NaI + CsI are measured as a function of temperature by the pulse method. α/ f 2 and U are found to be independent of f. The equations are derived for their temperature dependence. Linear relations of sound absorption coefficient to the molar volume of the melts are established. Their volume (second) viscosity is computed as a function of temperature from the experimental data on α/ f 2 and U, density and shear (dynamic) viscosity, thermal conductivity and heat capacities at constant volume and pressure. Linear equations are obtained for its decreasing as temperature is raised. The volume viscosity of molten alkali halides is considered in terms of the model of their autocomplex structure.

X-ray Structural Analysis of Molten LiF — LiCl System

The short range structures of molten LiF - LiCl ( 1 : 2 ) and ( 1 :1 ) mixtures with the common cation were investigated by an X -ray diffraction method and compared with those found in the pure melts and the molten alkali halide mixtures with the common anions. In both the mixtures, the nearest neighbor arrangements of the unlike ion pairs were similar to those in the pure melts. On the other hand, the nearest distances between like ions changed by mixing.

Thermal conductivity of molten alkali halides and their mixtures

Thermal conductivity of molten alkali halides and their mixtures with common anions is measured as a function of temperature. Use is made of the most reliable method of coaxial cylinders made of platinum, with radiation heat transfer being taken into account. It is found to vary directly with temperature. Expressions for its temperature dependence are derived from the experimental observations. Linear relations of the thermal conductivity to the molar volume are established for molten alkali fluorides, chlorides, bromides, iodides and their mixtures with common anions. Deviations from the additive quantities towards smaller values are observed in the case of the mixtures. They are in a good accordance with inverse deviations of their adiabatic compressibility. The heat conduction of ionic melts is semiquantitatively examined. Elementary quantities of heat transferred from cations to anions and vice versa are estimated.

Temperature invariance ofS 0-parameter of polymers

The parameterS 0 which is characteristic of molten alkali halides and liquids has also been shown to be characteristic of a wide variety of polymers. Calculated data using the volume expansivity of the polymer establish the temperature invariance and constancy of theS 0-parameter which retains, on an average, a constant value of 1.11 for polymers. Further understanding of the significance of fractional free volume andS 0-parameter in describing various thermoacoustic properties and the anharmonic behaviour in polymers, has been developed.