Ultrasound velocity in dissolving alkali halide melts

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Abstract

Ultrasound velocities in the molten exsolving mixtures (LiF + CsCl, LiF + KBr, LiF + RbBr, LiF + CsBr, LiF + KI, LiF + RbBr, and LiF + CsBr) were obtained along the saturation line over a wide temperature range using the acoustic method. The temperature dependences of properties far away from the critical temperature are close to linear and the temperature and composition factors are highly correlated. The difference between the magnitudes of sound velocity for the coexisting phases increases with the radii of ions in the mixtures. The linearity of the temperature dependence of the velocity, which is typical for all systems at relatively low temperatures, is violated when approaching the critical point of mixing, mainly due to sharp changes of the light phase state. Our results suggested a classical (mean-field) critical behavior of ionic melts.

Introduction

Alkali halides form the basis of molten electrolytes for a number of electrochemical processes because of a unique combination of their physicochemical properties (high decomposition voltage, conductivity and thermal stability, low vapor pressure and viscosity). From a theoretical point of view, they represent an interesting class of liquids with a predominantly Coulombic particle interactions. It is convenient to investigate the effects of size factors on the physical and chemical properties of ionic melts using them as models. The available data suggest that mixing of these halide melts does not obey the ideal laws because of the creation of complex ionic groups in the salt mixtures. Our investigation revealed that the more significant the differences are between the sizes of the mixed ions, the greater the stability of the complexes and consequently the greater deviation from ideal mixtures. In the liquid state, these salts are usually capable of being mixed unlimitedly with each other.

However, there is one insufficiently explained feature of halide melts namely, the ability of some ionic salt systems to stratify in the molten state [1]. This phenomenon is surprising in itself, considering the identity of the chemical bonds within the components of the systems. To all appearances, the ratio of the cation and anion radii is limited to when their composition becomes unstable. As a result, the mixture of such components splits into two liquid phases below the critical temperature. The increasing differences in the sizes of the ions are accompanied by a rise in the critical temperature and the displacement of the dome immiscibility to the component with the smallest ionic radius. At present, there is information on the density of the coexisting liquid phases from molten mixtures of lithium fluoride with potassium, rubidium and cesium halides [2]. These results show that, with a higher temperature and smaller differences in the radii of the ions, there are smaller differences in the densities of the phases along the saturation line.

Other physical and chemical properties of stratifying ionic melts have not been reported in the literature. However, the potential for molten media as coolants in molten salt nuclear reactors, as media for extraction processes and for actinides and fission products necessitates more intensive and extensive investigation. The results of these investigations will contribute to the development of the theory underlying the condensed ionic liquids in critical conditions, including the liquid–liquid phase transition.

In recent years, theoreticians [3], [4] have shown a growing interest in critical phenomena in ionic systems; however, there is still no common opinion in regards to the type of criticality and the role of charge fluctuations in the liquid-liquid phase transitions. Disagreements exist about whether to take into account or ignore short-range forces, and experimental data for ionic fluids are mostly based on salt solutions or mixtures of the room temperature melted ionic liquid. To clarify this debate, it is imperative to include the incompletely miscible molten alkali halides where intermolecular boundaries are obliterated and Coulomb long-range interactions dominate. In earlier publications [5], [6], [7], the acoustic method was shown to be very informative for the development of model representations about the structure of liquid media and the redistribution of the interparticle binding energy caused by mixing of components. However, no information exists about the use of this method for the investigation of exsolving ionic melts.

The purposes of this work were to measure the sound velocity in coexisting phases of molten ionic systems, to determine the temperature dependences of the velocity in each of the phases, and to study the influence of the ion size on the miscibility of liquid salts. The selected the melts for investigation were (in mole fractions) 0.7LiF–0.3KBr, 0.8LiF–0.2RbBr, 0.7LiF–0.3CsBr, 0.7LiF–0.3KI, 0.8LiF–0.2RbI, 0.7LiF–0.3CsI, and 0.7LiF–0.3CsCl. These compositions corresponded to the top of the immiscibility gap.

Section snippets

Experimental

The ultrasound velocity measurements were made via the pulse method, by determining the time Δt of sound travel through the test medium between the plane-parallel faces of cylindrical acoustic waveguides when the coaxial waveguide were displaced by a fixed distance Δh. A schematic of the working cell is shown in figure 1. The melts under study were placed in a platinum crucible (1) in which an end of the monocrystal tungsten waveguide (2) with a piezoelectric transducer (3) was fixed at the

Results and discussion

First, we determined the extent and state of the saturated phases by probing the entire thickness of the melts under study with a small displacement of the upper sound guide. Figure 2 illustrates how the sound velocity changed with the immersion depth of the sound guide in the LiF–KBr melt at 1183 K. The plot shows the constant sound velocity in each phase at some distance from their interface, indicating the thermodynamic equilibrium of the system. The pattern is reproduced in full with the

Summary

Seven liquid mixtures formed with lithium fluoride and halides potassium, rubidium, and cesium halides, which are stable diagonals of the reciprocal ternary systems, were investigated to elucidate peculiarities of the liquid-liquid phase separations in ionic systems. By measuring the sound velocities in co-existing phases for (LiF + KBr) and (LiF + CsCl) mixtures, we showed that these salts exhibit classical (mean-field) criticality. This type of unmixing was also obtained for some ionic fluids [11]

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