V. P. Stepanov

Density of stratified ionic melts: Experiment and theory

Densities of phases co-existing in molten ionic systems with partial miscibility out of halides of alkali metals (KBr, KI, RbBr, RbI, CsCl, CsBr, CsI) with lithium fluoride are determined in a broad temperature interval by a method of hydrostatic weighing. A linear decrease in the difference between phase densities with increasing temperature is found for all systems. By studying a molten KBr-LiF mixture, this regularity is traced up to a critical miscibility point. The critical index of the order parameter, calculated from these data happens to equal 1/2 and coincide with results predicted by mid-field theory.

The Density of Fluoride—Iodide Melts with a Limited Mutual Solubility of Components

The densities of phases in stratifying ionic melts of lithium fluoride with potassium, rubidium, and cesium iodides were measured. The role played by the size and temperature factors in the stratification of systems with predominantly Coulomb interactions of particles is discussed.

Frequency dependence of potentials of minimum capacitance for electrodes of copper subgroup metals in alkali halide melts

The electrochemical impedance technique was use to obtain the potentials of the minimum capacitance of Cu, Ag, and Au electrodes in chlorides, bromides, and iodides of sodium, potassium, and cesium. Their dependence on the nature of the metal and electrolyte, temperature and ac signal frequency is studied. It is shown that the potential of the minimum capacitance tends at a decrease in the frequency to the zero charge potential measured under similar conditions. An assumption is ventured that the cause of this may be specific adsorption of halide ions on the electrode in the form of negatively charged dihalide complexes. It is found that in the systems where two minimums are observed, a decrease in the spacing between the cathodic and anodic minimums correlates with an increase in the strength of the metal-adsorbate bond.

Conductivity of fused mixtures of lithium fluoride with cesium halides in the stratification region

Conductivity of fused mixtures of lithium fluoride with cesium halides is measured in the stratification region. The difference of conductivities of coexisting equilibrium phases is shown to increase with the increase in the halide anion radius at equal temperatures and to decrease with the increase in the temperature.

The adiabatic compressibility of two-phase systems of salt mixture melts based on lithium fluoride

The adiabatic compressibility coefficients of stratifying liquid lithium fluoride mixtures with potassium, rubidium, and cesium bromides and iodides were determined by the acoustic method with invoking density data. The difference of the compressibilities of phases along the saturation line was found to depend on the ratio between component bond energies. A compressibility jump was observed experimentally in the passage through the critical point.

Sound Wave Propagation in Immiscible AgBr+LiCl Melts

Abstract The velocity of sound wave propagation was measured for the biphasic region of AgBr+LiCl melts using the pulse method at temperatures from melting point to mixing temperature. It was found that temperature dependences of sound velocities on the saturation lines have opposite signs because of the different effect of thermal motion of the particles and the phases’ composition at the sound velocity. The difference between the sound velocities in the coexisting phases, Δu, is described by the exponential equation Δu≈(Tc−T)θ where the exponent is 0.865. This index is close to the values found in salt families formed with silver iodide and lithium and sodium halides but occurs at a lower value than that found for alkali halide melts between each other. The sound velocities in the phases take the same value 1612 m s−1 at the upper critical consolute temperature Tc=843 K. For a family of stratified silver halide containing melts, the versatility of the temperature dependence of the sound velocity near the critical point of mixing is declared. The sound velocity is discussed from the viewpoint of the different character of chemical bonds of salts.

Capacitance of the double electrical layer on the copper-group metals in molten alkali metal halides

The electrochemical impedance is measured to study the capacitance of the double electrical layer of metallic Au, Ag, and Cu as a function of potential and temperature in nine molten salts, namely, the chlorides, bromides, and iodides of sodium, potassium, and cesium. The C–E curve of a gold electrode has an additional minimum in the anodic branch. This minimum for silver is less pronounced and is only observed at low ac signal frequencies in cesium halides. The additional minimum is not detected for copper in any salt under study. This phenomenon is explained on the assumption that the adsorption of halide anions on a positively charged electrode surface has a predominantly chemical rather than an electrostatic character. The specific adsorption in this case is accompanied by charge transfer through the interface and the formation of an adsorbent–adsorbate covalent bond.

Adiabatic compressibility of an immiscible molten NaCl–AgI salt mixture

Adiabatic compressibility β of an immiscible 0.5NaCl + 0.5AgI liquid mixture in the immiscibility range is studied experimentally and theoretically using the model of charged hard spheres. The compressibility is calculated by the relationship β = 1/u2ρ studied using sound velocity u measured by a pulse method and density ρ determined by hydrostatic weighing. It is shown that the compressibility of the upper phase decreases and that of the lower phase increases when the temperature increases because of the superposition of the effects of the thermal motion of ions and the phase compositions. The temperature dependence of the difference between the compressibilities of the equilibrium phases is described using the empirical equation Δβ = (Tc–T)0.442, which is close to the mean-field theory description. The results of the model calculations adequately reproduce the experimentally observed temperature dependence of the compressibility of the coexisting phases. However, the theoretically predicted critical exponent (1/2) differs from the experimentally determined exponent by 13%. These results are discussed in terms of the nature of chemical bond in silver iodide.

Capacitance of a polycrystalline silver electrode in Na, K, and Cs bromide melts

The differential capacitance of the polycrystalline silver-NaBr, KBr, or CsBr salt melt interface is measured. At high and medium ac signal frequencies, the potential dependence of the capacitance of all salts represents a parabolic curve with one minimum. At frequencies of several or several tens of hertzs, an additional minimum appears in the anode potential region; it decreases to zero with increasing temperature. The temperature and frequency dependences of the cathode and anode minima are analyzed.

Ultrasound velocity and adiabatic compressibility of LaCl3 + MCl (M = Li, Na, K, Rb, and Cs) melts

The ultrasound velocity in binary LaCl3 + MCl (M = Li, Na, K, Rb, and Cs) melts is measured, and their adiabatic compressibility is calculated as a function of temperature and composition. A relation is established between the relative deviations of these properties from their values for hypothetic ideal salt mixtures, on the one hand, and the ionic potentials of the alkali-metal cations, on the other.