NaCl CsCl Research Articles

Stability of thermally-induced martensitic transformations in bi-atomic crystals

Some of the most interesting, and technologically important solid–solid transformations are the first order diffusionless transformations that occur in certain equiatomic, ordered, bi-atomic crystals. These displacive transformations include thermally-induced, reversible, proper martensitic transformations as seen in shape memory alloys such as NiTi (where group–subgroup relationships exist between the symmetry groups of the crystal phases) and the reconstructive martensitic transformations seen in certain ionic compounds such as CsCl (where no group–subgroup relationship exists between the phases). In contrast to existing continuum mechanics approaches, the present work constructs a continuum energy density function W( F ,θ) (as a function of a uniform deformation gradient and temperature) of a perfect periodic bi-atomic crystal from temperature-dependent atomic pair-potentials. Equilibrium solutions and their stability are examined as a function of temperature for crystals under no external stress. The full problem is solved numerically, and an asymptotic theory is employed to guide the numerical solution near multiple bifurcation points. Using pair-potentials and enforcing constrained kinematics (uniform deformation of a cubic CsCl-type crystal), lower symmetry crystals, such as orthorhombic, monoclinic, and rhombohedral structures are predicted. The first two of these are unstable within the chosen temperature window for our particular case, while the third is stable for higher temperatures. In addition, a hysteretic transformation was discovered in which the CsCl structure is stable at high temperatures and the NaCl structure is stable at low temperatures. These two cubic phases are connected by an unstable rhombohedral path corresponding to the transformation mechanism proposed by Buerger (1951). The CsCl–NaCl transformation suggested by the numerical results is a reconstructive transformation with a group–nonsubgroup relationship between the symmetry groups of the two phases.

Electrochemical impedance investigations of redox mechanisms of refractory metal compounds in molten salts. I. Niobium chloride and oxychloride in CsCl–NaCl eutectic melt

Electrochemical studies of the redox mechanisms of niobium chloride and oxychloride in CsCl–NaCl eutectic melts at 550°C are reported. Cyclic voltammograms and electrochemical impedance spectra have been measured in situ over a wide potential range between the limits of chlorine evolution and metal deposition at various oxide concentrations 0≤Ox/Nb≤3. In analyzing the impedance spectra we have focused on the determination of the Warburg impedance and coefficients, respectively, which reflects the variation of the concentration of redox species as a function of electrode potentials. Taking into account the relevant redox equilibria these Warburg coefficient versus potential curves can be quantitatively described by simple model calculations. This is presented for the first time in detail for a high temperature impedance investigation. In comparison with the corresponding voltammograms this analysis yields the following main results for the redox mechanisms. For pure NbCl 5 in CsCl–NaCl eutectic melts evidence is found for a three step reduction mechanism according to Nb(V)–Nb(IV)–Nb(III)–Nb. With oxide addition to a Nb(IV) containing melt oxocomplexes form which are reduced in two steps from Nb(V) to Nb(IV) to Nb-metal or metal oxide deposits. Clear indications for dioxocomplexes are derived from the impedance analysis which form in melts with Ox/Nb=2. Comparison with the corresponding melts containing tantalum chloride or oxychloride is discussed where dioxocomplexes are not found.

Phase transitions of lanthanide monophosphides with NaCl-type structure at high pressures

Lanthanide monophosphides LnP (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Tm and Yb) with a NaCl-type structure have systematically been prepared at high temperatures. Using synchrotron radiation, X-ray diffractions of LnP have been studied up to 61 GPa at room temperature. The NaClCsCl transition for CeP is found at around 25 GPa. First-order phase transitions of LnP (Ln = La, Pr and Nd) with the crystallographic change occur at around 24, 26 and 30 GPa, respectively. The structure of the high pressure phases of these phosphides is a body center tetragonal structure (Ln: 0, 0, 0; P: 1 2 , 1 2 , 1 2 ; space group P4/mmm), which can be seen as the distorted CsCl-type structure. The PrP distance in the high pressure form of PrP is 2.789 Å. This almost agrees with the sum of covalent radii of Pr and P. The PrP bond has the covalent character at very high pressures. Similar results are also obtained for LaP and NdP. The pressure-induced phase transitions of SmP, GdP, TbP, TmP and YbP occur at around 35, 40, 38, 53 and 51 GPa, respectively. The structure of the high pressure phase is unknown. The phase transitions of LnP with many f-electrons are not due to the mechanism of the ordinary NaClCsCl transition. The transition pressures of LnP increase with decreasing the lattice constants in the NaCl-type structure, which decrease with increasing atomic number of the lanthanide atoms.

Electrochemical impedance and cyclic voltammetry study of the influence of oxide on the redox chemistry of niobium and tantalum in chloride melts

The redox process of penta- and tetravalent niobium and tantalum species dissolved in CsClNaCl eutectic melts (65-35 mol%) have been investigated by electrochemical impedance spectroscopy and cyclic voltammetry. In particular, the influence of oxide has been studied. A strong correlation between the structures of the cyclic voltammograms and the potential dependence of the Warburg diffusion impedance is observed, which is determined by the concentrations of the redox species under consideration. Without any oxide added to melts containing TaCl5 or NbF72− the TavTaIV and NbVNbIV redox process, respectively, is observed in the cyclic voltammograms. This is also the case after equilibrating the melts with the respective metals. Plots of the Warburg coefficients as a function of the electrode potential exhibit minima near the equilibrium potentials. This confirms the presence of both redox species. Addition of oxide, added as Na2O, strongly influences the redox structures of TaVTaIV and NbVNbIV in the volammograms and the minima of the Warburg coefficient curves. When the structure is completely suppressed, for example, when TaV is titrated with an equivalent amount of oxide, the corresponding minima disappear. From the impedance spectra additional informations have been obtained: Indications of electronic conduction, most pronounced for tantalum, and an adsorption process at the electrode in NbIV systems.

Density Trends in Equiatomic Binary Intermetallic Alloys

Though correlations of 59 solid atomic elements, we find that cohesion and melting temperatures are dependent on the electron concentration of net-bonding valence electrons while mass density is not or subtly dependent. Examination of 390 mass densities of AB-type binary intermetallic compounds revealed a relation like that of Vegard's Law of melting points. There exists a density trend, the correlation between mass density of the compound and average mass densities of constituent elements, regardless of how the compounds are formed: either between metal - metal, metal - nonmetal, or between simple - simple, simple - transition and transition - transition element, regardless of the structural type of the compounds between B1(CF8)NaCl and B2(CP2)CsCl

Potential measurements of thorium tetrachloride in alkali halide solutions

The thermodynamic behaviour of ThCl4 – alkali chloride solutions was investigated using galvanic and formation cells of the type[Formula: see text]and[Formula: see text]where ACl represents NaCl, KCl, CsCl, or equimolar mixtures of NaCl–KCl, NaCl–CsCl, and KCl–CsCl. In addition, the thermodynamic behaviour of thorium in mixed alkali chloride – alkali fluoride melts was studied using an electrometric titration technique. It was found that these melts are highly nonideal due to the formation of octahedrally coordinated complexes. Key words: ThCl4 solutions, potentials, thermodynamic properties, titration.

Multi-Ionic Potential and Membrane Permeability Matrix. III. Diffusion and Concentration of Ions within Membrane Phase as a Controlling Factor to Ion Permselectivity

Abstract Membrane concentrations of cations in the NaCl–CsCl and NaCl–CaCl2 systems with a highly selective cation-exchange membrane were measured under the extensive external electrolyte conditions. The modes of membrane/solution distribution and diffusion of cations, as a controlling factor to the membrane permselectivity, were correlated consistently with the electrochemical and cation flux data, as well as with the estimated membrane permselectivity parameters. The NaCl–CsCl uni-uni valent cation system was characterized well by the Na+/Cs+ concentration ratio of external solution, since the Na+/Cs+ concentration ratio in the membrane phase was related directly to the external ratio. Na+/Cs+ diffusion coefficient ratio varied from 0.5 to 3 with increasing external Na+/Cs+ concentration ratio from 1/100 to 100. While in the NaCl–CaCl2 uni-bi valent cation system, the high and constant membrane concentration of Ca2+ regulated the transport process. The Na+/Ca2+ diffusion coefficient ratio ranged from 0.5 to 500 with external Na+/Ca2+ ratio from 1/100 to 100. Analyses on the phenomenological coefficients, describing the inter-cationic correlations within the membrane phase, as a function of the external cation concentration ratio were also carried out.

Pressure-volume behaviour of UP, USb and UTe

The pressure-volume dependences of UP, USb and UTe have been determined by X-ray powder diffraction in a diamond anvil cell. To obtain reliable variations it is essential to account for the effects of anisotropic stress components. UP maintains its cubic NaCl-type structure at least up to 25 GPa while USb undergoes a cubic NaClCsCl type transformation at 8 GPa. The experimental data have been fitted to a Birch equation of state and the bulk moduli so obtained are compared to theoretical values. The presence of a Kondo effect in UTe is assumed to be responsible for its low bulk modulus.

Thermodynamic properties of the dilute solutions of silver chloride and aluminum trichloride in alkali chlorides by electromotive force measurements

Thermodynamic data for dilute solutions of AgCl and AlCl3 in molten alkali chlorides were derived from the emf's of formation and of galvanic cells, respectively, given as, Ag/AgCl–ACl/C,Cl2 and Ag/AgCl–ACl//AlCl3–ACl/Al. ACl represents NaCl, KCl, CsCl, or the equimolar mixtures of NaCl–CsCl and KCl–CsCl. It was found that the thermodynamic properties of the ternary melts could be predicted from the results of the binary melts using the relationship (17, 20, 21), [Formula: see text] where Z represents any partial molar property. The ternary solution is denoted by the subscript 1, 2, 3 while the two binaries are denoted by 1, 2 and 1, 3; t is a concentration variable given as t = X3/(X2 + X3). These expressions are valid when the binary and the ternary solutions have the same content of either AgCl or AlCl3, respectively.

Evaluation of sound velocity in binary molten electrolytes

A comparison of sound velocity evaluated from Nomoto's relation, ideal mixing relation Jacobson's free length theory and Schaaffs' collision factor theory has been made in six binary molten electrolytes (NaClKCl, NaClRbCl, NaClCsCl, KClLiCl, KClRbCl, KClKBr) at 800°C. The agreement between theoretical and experimental values is quite satisfactory. Discrepancies in the sound velocity values are maximum in the case of the NaClCsCl system where maximum interactions are observed. Non-ideality in the mixtures has been discussed in terms of sound velocity.

Multi-ionic Potential and Membrane Permeability Matrix. I. NaCl–CsCl Bi-ionic System with a Cation Exchange Membrane

Abstract This paper is concerned with analysis of membrane transport processes on the basis of nonequilibrium thermodynamics. Inter-ionic correlations between permeating ions are quantitatively estimated by applying the membrane permeability matrix theory to the system in which the NaCl and CsCl solution phases are divided by a highly selective cation exchange membrane. The membrane permeability matrix of the system may be expressed by four ionic correlation terms concerning with Na+ and Cs+. The electroconductive and diffusional membrane permeabilities are expressed as a function of the matrix elements having the dimension of permeating speed of ions. The conductive and diffusional permeabilities as well as the matrix elements are estimated from membrane potential, conductance, and ionic flux data. The external electrolyte concentration dependence of these permeability parameters is discussed. The conductive and diffusional permeabilities considerably differ from each other and their ratio changes with the external electrolyte concentration, whereas the permeability ratio of Na+ to Cs+ is constant. Electroconductive and diffusional characteristics of membrane transport processes are consistently analyzed in terms of the memberane permeability matrix theory.

Calculation of phase diagrams of binary salt mixtures with a common anion

In order to calculate excess chemical potentials of the components in binary molten salt mixtures, a simple model is proposed which requires three physical parameters for each salt: the crystal lattice energy, the latent heat of melting and the sum of the ionic radii. The model has been used to calculate the liquidus temperature for various compositions of the binary system. Calculated and experimentally determined phase equilibria for the following systems are compared: LiF–KF, LiF–NaF, NaF–KF, LiCl–KCl, NaCl–CsCl, LiNO3–NaNO3, CaF2–MgF2, LiF–CaF2, NaF–CaF2 and NaCl–SrCl2.

Neutron Scattering Study of Phonon Dispersion Relations along [110] in TlCl

The phonon dispersion relations of TlCl along the [110] direction have been studied by inelastic neutron scattering at 80 K, partly at 293 K. The energies of four zone-boundary phonons at the \(M(\frac{1}{2}\), \(\frac{1}{2}\), 0) point associated with the indirect band-gap transition between the X (0, 0, \(\frac{1}{2})\) and \(R(\frac{1}{2}\), \(\frac{1}{2}\), \(\frac{1}{2})\) points were determined. The softening of the TA 2 branch ( e //[001]) was found around the zone-boundary. The zone-boundary phonon softens from 1.75 meV (293 K) to 1.28 meV (4.5 K). The mechanism of this phenomenon is discussed in connection with the CsCl–TlI or CsCl–NaCl type transitions which can be accomplished by a shear of the (110) planes corresponding to the TA 2 mode in the CsCl structure.

Theory of Enthalpy of Mixing in Reactive Charge Asymmetrical Molten Salt Systems. Part II. Ternary Solutions

The enthalpies of mixing for a reacting charge asymmetrical ternary molten salt system have been calculated on the basis of an extension of a previously developed model for binary systems. The enthalpy of mixing is considered to consist of a reaction term and a mixing term; the former results from the formation of the tetrahedrally coordinated complex species [Formula: see text] and the latter from the mixing of the products of reaction according to quasichemical theory modified to account for charge asymmetry.The ternary model enables the prediction of the integral enthalpies from a knowledge of the interaction parameters determined for the two charge asymmetrical binary systems. The calculations are compared with experimentally measured integral enthalpies of mixing in the ternary system MnCl2–NaCl–CsCl, and shown to give accurate predictions of the behaviour of the system.

Calculation and Interpretation of the NMR Chemical Shift and Its Pressure Dependence in the Alkali Halide Lattices

The symmetrical orthogonalization technique of Löwdin has been applied to the alkali halide outer-shell s and p wavefunctions. The resulting distorted ionic wavefunctions have been used to calculate the pressure dependence of the NMR chemical shift within the framework of the Kondo—Yamashita overlapping-ion model. The computed values of the rate of change of the chemical shift with respect to pressure, the change of the chemical shift at the NaCl–CsCl phase transition, and the chemical shift of the ionic reference solutions are compared to Baron's experimental data. Reasonable agreement is found although discrepancies exist and are discussed. No adjustable parameters are used in these calculations.