Alkali Halide Melts Research Articles

Estimation of ion-induced dipole interactions to the thermodynamics of alkali halide melts

The statistical-thermodynamic model to account for the ion-induced dipole interactions in molten alkali halides through thermodynamic perturbation theory is proposed. The main contribution to the free energy of these melts was described on the basis of charged hard spheres model as the reference system. The expression for the charge-induced dipole term to the free energy in Fourier space was also presented. Estimations and analysis of the extra contribution to the thermodynamic characteristics of alkali halide melts were carried out. The perturbation due to the induced dipoles is confirmed to be negative for all alkali halides, while the reduced value of the extra term lies in the range from a few to 10% for free energy. The absolute values of the ion-induced dipole term are greater in those melts where the basic Coulomb interaction is also greater. Generally, the proposed model is quantitatively successful when describing the enthalpy and its temperature dependence without fitting parameters.

Structure and stability of alkali gallates structurally reminiscent of hollandite

AbstractSingle crystals of CsGa7O11, RbGa7O11, and RbGa4In5O14 were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa7O11 and RbGa7O11 adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO6 octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa4In5O14 is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa7O11 and RbGa7O11. CsGa7O11 and RbGa4In5O14 were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa7O11 decomposes to GaO(OH) xH2O when heated in warm aqueous solutions. CsGa7O11 undergoes ion exchange in both an aqueous solution of RbCl and a RbNO3 melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO3 melt.

Quantum-Chemical Study of the Titanium Complexes Stability in the Model System M2+ [Ti(3)F6]3-+12MCl2

In our previous works, a basic approach is to calculate the parameters of model systems containing an anionic complex with an outer sphere (OS) cationic shell. For similar systems nM+⋅[NbF7], nM+⋅[CrCl6] (n = 1 – nmax) and others (system I), it has been demonstrated that the most thermodynamically stable compositions have some intermediate number of OS cations n < n max, where n max is the maximal number of OS cations held by a given complex. In another our papers, an approach was proposed for estimating the stability of particles with different composition of the second coordination sphere of complexes in more complex model systems –M+ 3Cr(3)Cl6+18MCl and M2+Ti(4)F6+12MCl2, where M2+ - Mg, Ca, Sr, Ba (system II). The aim of this study is the quantum-chemical substantiation of the existence stable particles in alkali halide melts of formed by a titanium halide complex and an OS cationic shell of a certain composition in a model system M2+∙[Ti(3)F6]3-+12MCl2. The geometry optimization of structures was performed with the Firefly program package, partially based on the source code of the GAMESS(US) program, by the density functional theory DFT/UHF method with the use of the B3LYP hybrid functional. For the F and Cl atom quasi-relativistic basis set Stuttgart RLC ECP was used; for Ti, Ca, Sr and Ba – Stuttgart RSC 1997 ECP basis set and for Mg – CRENBL ECP. In all cases, the search for the optimized geometry was accompanied by control calculation of vibrational frequencies, and, thus, all the data reported correspond to true minimum of the potential energy surface (there are no imaginary frequencies). Different from system type I, in the system II it was possible direct calculation of the interaction energy between particle and rest part of the model system. If the interaction energy between the OS shell and titanium complexes exceeds the energy of its interaction with the rest environment, there are presence in the system of such dynamic equilibrium, which ensures the existence of sufficiently stable complex particles of a certain OS composition. The analysis of interactions in systems I and II on the example of Ti(3) complexes showed that there are discrepancies between the composition of the most stable complex particles in these systems. Allowance for the interaction of the particle "complex plus OS shell" with the surrounding ions gives a much more accurate estimate of the composition of the electroactive particles. Thus, using the model system of nM2+ +Ti(3)F6 for example, it was shown that the calculation parameters of this system may be used just as the initial assessment.

The thermo-elastic instability model of melting of alkali halides in the Debye approximation

ABSTRACTThe Debye model of lattice vibrations of alkali halides is used to show that there is a temperature below the melting temperature where the vibrational pressure exceeds the electrostatic pressure. The onset temperature of this thermo-elastic instability scales as the melting temperature of NaCl, KCl, and KBr, suggesting its role in the melting of the alkali halides in agreement with a previous more rigorous model.

Crystal structure of LnTe3, where Ln = La, Ho

LnTe3 (Ln = La, Ho) single crystals are produced by the interaction of elementary compounds in an alkali halide melt. The structures of the obtained compounds are determined by single crystal X-ray diffraction. It is found that LaTe3 crystallizes in the space group Ama2 with the following parameters: a = 4.4195(2) A, b = 26.2644(11) A, c = 4.4028(2) A. As for HoTe3, the space group Cmcm with the following parameters a = 4.2994(18) A, b = 25.393(10) A, c = 4.3021(17) A is determined.

Electrical conductivity of biphasic mixtures of molten silver iodide and lithium fluoride, chloride, and bromide

The electrical conductivity (EC) method was used for the biphasic systems of AgI with LiF, LiCl, or LiBr. The difference between the magnitudes of the conductivities for the equilibrium phases of the LiCl+AgI and LiBr+AgI melts decreases with an increase in temperature, becoming zero at 1250 and 983 K. For these temperatures, the values of critical conductivity are κ c = 4.70 S cm−1 and κ c = 3.90 S cm−1, respectively. The melt containing lithium fluoride exists in two phases up to a temperature of 1245 K. The temperature dependence of the differences between the conductivities for the coexisting phases is described as an exponential equation, with the critical exponent 0.89. This value is 11% less than that found for alkali halide melts. The covalent bonding between the silver and halide ions can be understood as causing the difference between the critical exponents of the alkali halide melts and those of silver iodide-containing mixtures.

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.

The Effect of the Second Coordination Sphere on Electrochemistry of Niobium Complexes in Alkali Halide Melts

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer (ks) for the Nb(V)/Nb(IV) redox couple in chloride, chloride-fluoride and fluoride melts has been presented. By using cyclic voltammetry, the standard rate constants of charge transfer were determined for chloride melts: ks (CsCl) < ks (KCl). The redox process in the NaCl–KCl (equimolar mixture) – NbCl5 melt was reversible. The standard rate constants of charge transfer were experimentally determined for chloride-fluoride melts: ks (KCl) < ks (CsCl) < ks (NaCl–KCl). The dependence of the standard rate constants was obtained for pure fluoride melts: ks (CsF) < ks (KF) < ks (NaF–KF). The influence of the first coordination sphere composition on the charge transfer standard rate constants was found experimentally: ks (fluoride melts) < ks (chloride-fluoride melts) < ks (chloride melts), both at glassy carbon and platinum electrodes. It was found that the ks increases with the temperature increase and substitution of a glassy-carbon electrode for a platinum one.

Analysis of the Interrelationship Between Melting and Fracturing of Alkali Halides

On the basis of the lattice potential energy with two forms proposed by Born–Mie and Born–Mayer, respectively, the critical interionic separations $${{ r}}_{\mathrm{i}}$$ where the lattice is fractured due to tensile force have been evaluated for alkali metal halides. The theoretical results are analyzed together with the interionic separations $${{ r}}_{\mathrm{m}}$$ determined by the melting temperature and the help of the isobaric equation of state. A new and simple interrelationship between $${{ r}}_{\mathrm{m}}$$ and $${{ r}}_{\mathrm{i}}$$ is obtained, and the crystal melting behavior can be accordingly predicted for NaCl-structure ionic crystals.

An anion effect on the separation of AgI-containing melts using sound waves

Sound velocities in molten ((LiF+AgI)) and ((LiBr+AgI)) mixtures have been measured to investigate the relationship between the sound velocity and the temperature and the role of the anion in the (liquid+liquid) phase transition. Our results show that the ((LiBr+AgI)) system is biphasic between the melting point and T=984K and becomes monophasic above this temperature. We show that the upper consolute critical temperature for the AgI-containing melts increases with decreasing anion size in the series F−>Cl−>Br−. The ((LiF+AgI)) melt remains biphasic at all temperatures investigated up to T=1218K. The temperature coefficients for the sound velocities in the upper and lower phases of the ((LiBr+AgI)) system have opposite signs because of the superposition of the temperature and composition factors. The difference between the magnitudes of the velocities for the coexisting phases decreases exponentially with increasing temperature and is described by a critical exponent of 0.85 for the ((LiBr+AgI)) melt near the critical temperature. This value is 15% less than that found for alkali halide melts, in which long-range Coulomb forces between ions prevail. This difference may result from the fact that silver halides are intermediate between the typical ionic salts and the fully covalently bonded ones.

Sound Velocities for Dissolving AgI + LiCl Melts

The ultrasonic velocities of a molten stratified mixture at a molar ratio of 0.3AgI and 0.7LiCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line over a wide temperature range using the pulse method to establish the characteristics of mixing salts comprising different chemical bonds. It is shown that the coefficients of the temperature dependences for the sound velocities in the coexisting phases have opposite signs as a result of the superposition of the temperature and concentration factors. It is found that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1250 K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the sound velocity difference, Δu, for the system studied is described by the equation Δu ≈ (Tc – T)β, where β = 0.900, which is close to the value 0.896 obtained for the previously investigated AgI+NaCl melt but is less than the value found for alkali halide melts (β = 1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide.

Sodium-potassium interdiffusion in potassium-rich alkali feldspar II: Composition- and temperature-dependence obtained from cation exchange experiments

Na-K interdiffusion in disordered potassium-rich alkali feldspar was studied experimentally using cation exchange between gem quality sanidine from the Eifel and alkali-halide melt at temperatures of 800 °C to 1000 °C and at close to ambient pressure. Sodium-potassium interdiffusion coefficient DNaK was determined for potassium mole fractions in the range 0.65 ≤ XOr ≤ 0.99. At 0.65 ≤ XOr ≤ 0.95 the sodium-potassium interdiffusion coefficient is largely independent of composition. At XOr ≥ 0.95, it rises sharply with increasing potassium mole fraction. Diffusion perpendicular to (001) is about one order of magnitude faster and less strongly temperature dependent than perpendicular to (010). The parameters of the Arrhenius equation describing the temperature dependence of the sodium-potassium interdiffusion coefficient DNaK = D0Exp (−EA/RT) were estimated as The results of our direct determinations are compared with theoretical calculations using the corresponding sodium- and potassium tracer-diffusion coefficients, and the processes underlying the observed composition- and temperature dependence of sodium-potassium interdiffusion are discussed.

The Composition of the Second Coordination Sphere and Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Halide Melts

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer (k s) for the Nb(V)/Nb(IV) redox couple in chloride-fluoride and fluoride melts is presented. By using cyclic voltammetry, the following series of the standard rate constants of charge transfer has been experimentally determined: k s (KCl) < k s (CsCl) < k s (NaCl–KCl). Quantum-chemical calculations showed that charge transfer activation energies could actually change in a nonmonotonic manner in the sodium-potassium-cesium series, resulting in a nonmonotonic change of charge transfer of rate constants in chloride-fluoride melts. In pure fluoride melts, the following dependence of the standard rate constants has been obtained: ks (CsF) < ks (KF) < ks (NaF–KF). It has been established that ks tend to increase in response to both temperature increase and substitution of a glassy-carbon electrode for a platinum one.

The Composition of the Second Coordination Sphere and Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Halide Melts

The effect of the second coordination sphere and the working electrode nature on the standard rate constants of charge transfer (k s) for the Nb(V)/Nb(IV) redox couple in chloride-fluoride and fluoride melts was studied.The standard rate constants for the Nb(V)/Nb(IV) redox couple were calculated based on cyclic voltammetry data and the Nicholson’s equation.The following unusual series of the standard rate constants of charge transfer is experimentally determined in chloride-fluoride melts: k s (KCl) < k s (CsCl) < k s (NaCl-KCl).Ab initio calculations carried out by using PC Gamess/Firefly quantum chemical program showed that charge transfer activation energies can actually change in nonmonotonic manner in the series sodium-potassium-cesium, which leads to nonmonotonic change of the charge transfer rate constants in chloride-fluoride melts.In pure fluoride melts, the following dependence of the standard rate constants was obtained using cyclic voltammetry: ks (CsF) < ks (KF) < ks (NaF-KF).It has been established that the standard rate constants of charge transfer increase with increasing temperature, as well as with replacement of the glassy carbon electrode with a platinum one.AcknowledgmentsThe work was financially supported by the Russian Foundation for Basic Research (project 12-08-01178-а).

Densities of a dissolving mixture of molten (AgI + NaCl)

The densities of a molten mixture of (0.5AgI+0.5NaCl) were measured along the saturation line over a wide temperature range by the hydrostatic weight method to establish the peculiarities of the mixing of salts with different chemical bonds. We showed that the difference between the magnitudes of the densities for the coexisting phases decreases with increasing temperature and becomes equal to zero at 1063.5K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the difference in densities, Δρ, is described by equation Δρ/ρc≈Tc-TTcβ, where ρc is the density at Tc. The index β=0.476 occurs at a lower value than that found for alkali halide melts (β=0.52) where long-range Coulombic forces between ions prevail.

Electroreduction of cerium(III) ions on a silver electrode in a chloride melt at 823 K

The electroreduction of cerium ions in a KCl-NaCl-CsCl eutectic melt at 823 K was studied. The electroisolation of metallic cerium from halide complexes was conducted at potentials higher than the potentials of the decomposition of alkali halide melts and shown to be the primary electrochemical process. The mechanism of the electrode process was determined.

Ultrasonic velocity for an equimolar mixture of molten AgI and NaCl in the biphasic region

The ultrasonic velocities of a molten stratifying mixture composed of 0.5 AgI and 0.5 NaCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line for a wide temperature range via the pulse method to establish the characteristics of mixing salts with different chemical bonds. We show that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1064K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the sound velocity difference, Δu, is described by the equation Δu≈(Tc−T)Θ, where Θ=0.896, which is less than that found for alkali halide melts (Θ=1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide.

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.

A view and a review of the melting of alkali metal halide crystals

A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].

Vibrational modes and structure of the LaCl 3[sbnd]CsCl melts

Raman spectra of the LaCl 3 CsCl melt mixtures at ten different compositions, including solid and molten LaCl 3 have been measured at temperatures up to 900 °C. The systematic spectral changes with composition and the invariance of the spectra on temperature are discussed in terms of the melt structure and in relation to other lanthanide halide–alkali halide melt mixtures. The data suggest that in CsCl-rich mixtures the predominant species present are the LnC l 6 3 − “octahedral” while in LaCl 3-rich melts a network-like structure of chloride bridged coordination polyhedra is formed. Most likely the predominant coordination of La 3+ remains six-fold at all compositions. The reduced isotropic and anisotropic Raman spectra are calculated and compared with the recently published simulated Raman spectra of the same melt mixtures.