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

Summary 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].     

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

Summary Although melting is a most familiar physical phenomenon, the nature of the structural changes that occur when crystals melt are not known in detail. The present article considers the structural implications of the changes in physical properties that occur at the melting points, T_m, of the alkali halides. This group of solids was selected for comparative examination because the simple crystal lattices are similar and reliable data are available for this physical change. For most of these salts, the theoretical lattice energies for alternative, regular ionic packing in 4:4, 6:6 and 8:8 coordination arrangements are comparable. Density differences between each solid and liquid at T_mare small. To explain the pattern of quantitative results, it is suggested that the melt is composed of numerous small domains, within each of which the ions form regular (crystal-type) structures (regliq). The liquid is portrayed as an assemblage of such domains representing more than a single coordination structure and between which dynamic equilibria maintain continual and rapid transfers of ions. T_mis identified as the temperature at which more than a single (regular) structure can coexist. The interdomain (imperfect and constantly rearranging) material (irregliq) cannot withstand shear, giving the melt its fluid, flow properties. From the physical evidence, it is demonstrated that the structural changes on melting are small: these can accommodate only minor modifications of the dispositions of all, or most, ions or larger changes for only a small fraction. This proposed representation, the set/liq melt model, may have wider applicability.     

A mass-spectrometric determination of the work function of alkali metal halide crystals

Thermochemical cycles including experimentally measured enthalpies of sublimation of alkali metal halides in the form of M₂X⁺ and MX₂^∮ ionic clusters and MX molecules were used to calculate work functions for NaCl, NaBr, NaI, KCl, KBr, KI, RbCl, and RbI.     

Ammonium ions in alkali metal halide crystals: tunnelling and spin relaxation

We have used low energy inelastic neutron scattering spectroscopy to examine the tunnelling spectroscopy of the ammonium ion in the (NH4)0.02Rb(x)K(0.98-x)I system. The concentration of different species were varied as x increased, this was followed systematically and the first consistent assignment scheme for these features is given. Differences were also found for the relaxation rate of the spin temperature inversions that could be generated in these species. At a critical concentration--about x = 0.04 mole fraction--the relaxation rates of the species changed dramatically.     

Absorption spectra in alkali halide crystals

A phenomenological model is presented which predicts the absorption energy of the first fundamental band in alkali halide crystals as well as impurity absorptions such as the hydride ion. Its basis is the electron transfer model of Hilsch and Pohl, but is considerably more satisfying since it utilizes kinetic energy terms for the absorption energy, and there is good to excellent agreement with the virial theorem.     

Luminescence of alkali halide crystals by multiphoton excitation

Luminescence is observed for CsI and NaBr crystals due to two-photon excitation. Near UV fluorescence of a NaI crystal from three-photon excitation is reported. The mechanism of producing near UV luminescence is discussed.     

Thermotropic liquid crystals from alkali metal dihexadecylphosphates

Lithium, sodium, potassium, rubidium, and caesium dihexadecylphosphates were synthesized. Their thermal stability was checked by thermogravimetry. Their ability to show thermotropic mesomorphic behaviour was investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All these compounds were found to exhibit a lamellar structure in the crystalline state at room temperature and a columnar structure of hexagonal symmetry in the mesomorphic state at high temperature. The potassium, rubidium, and caesium derivatives were found to exhibit an additional Ia 3 d body-centred cubic structure in the temperature range between the crystal and the columnar phase. Structures and structural parameters are briefly discussed.     

An enthalpy landscape view of homogeneous melting in crystals

A detailed analysis of homogeneous melting in crystalline materials modeled by empirical interatomic potentials is presented using the theory of inherent structures. We show that the homogeneous melting of a perfect, infinite crystalline material can be inferred directly from the growth exponent of the inherent structure density-of-states distribution expressed as a function of formation enthalpy. Interestingly, this growth is already established by the presence of very few homogeneously nucleated point defects in the form of Frenkel pairs. This finding supports the notion that homogeneous melting is appropriately defined in terms of a one-phase theory and does not require detailed consideration of the liquid phase. We then apply this framework to the study of applied hydrostatic compression on homogeneous melting and show that the inherent structure analysis used here is able to capture the correct pressure-dependence for two crystalline materials, namely silicon and aluminum. The coupling between the melting temperature and applied pressure arises through the distribution of formation volumes for the various inherent structures.     

Semiempirical approximation for atomization energy of bivalent metal halide crystals

The ionicity and atomization energy of 27 bivalent metal halides have been estimated. The calculation was performed with experimental values of the interatomic distances. The atomization energy was a function of only the bond ionicity, which was determined by minimizing the energy. For layer structures, account was taken of the energy of anion polarization due to static fields induced on the anions by their asymmetric environment. The estimates of ionicity are close to those according tc Phillips. The error in calculating the atomization energy is no greater than I eV, averaging 3.5%.Published as a matter for discussion.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 708–713, November–December, 1985.     

Ab initio calculations for absorption and emission energies of alkali halide crystals doped with thallium

We calculate transition energies associated with optical properties of thallium doping in alkali halide crystals via an atomic cluster of minimal size where an sp‐valence‐shell impurity enters as a substitutional defect in the model crystal. Hartree–Fock (HF), density functional theory (DFT), and configuration interaction (CI) [CIS (CI with single excitation) and QCISD (single plus double and quadruple excitation)] calculations are performed to theoretically obtain the absorption and emission energies as vertical transitions evaluated at the ground and first excited‐state optimized geometries, respectively, where the optimization is carried out separately with the HF and DFT methods. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 785–790, 2000     

A new method of calculation of the melting temperatures of crystals of Group 1A metal halides and francium metal

A new method of calculation of melting temperatures of binary ionic crystals has been suggested. The method is based on finding a matrix relation between the ionic radii (the lattice energy U) and melting temperature of ionic crystals of the MX type, where M is a Group 1A metal, and X is a halogen. From the equation for the lattice energy U, a new equation has been derived for calculation of the melting temperature of ionic crystals with the use of only the ionic radii and the degree of bond ionicity ?: T _m = f(U, ?). The average error of determination of T _m for alkali-metal halides is 2.80%. The melting temperatures of francium halides and alkali-metal astatides (including FrAt) have been calculated. It has been shown that the accuracy of calculation of the melting temperature of ionic crystals depends on the degree of bond ionicity: the error increases with an increase in the covalent contribution. On the basis of the melting temperatures of metal halide crystals, a method has been developed for the calculation of the melting temperatures of corresponding metals. The melting temperature of francium has been calculated to be 24.861 ± 0.517°C.     

High quality large single crystals of metal halide perovskites for optoelectronic applications

正As the technological development of large single-crystalline wafers have revolutionized many industries including electronics and photovoltaics,one can predict that the availability of large single-crystalline perovskite crystals and wafers can revolutionize its broad applications in photodetectors,solar cells,LEDs,lasers,etc.In 2015,Liu et al.[1]at Shaanxi Normal University developed a reactive inverse-temperature crystallization(RITC)method and harvested high quality MAPbI_3 single     

Towards a comprehensive theory of lyoluminescence in alkali halide crystals: Some results and some perspectives for future research

Lyoluminescence is an interesting tool to study irradiation defects in solids and their behaviour after dissolution. A general kinetic scheme of the process valid for alkali halide crystals is presented, and some ideas about structure and properties of the interacting species, in the solid as well as in the liquid phase, are discussed.     

Effects of alkali metal halide salts on the hydrogen bond network of liquid water

Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.     

Sulfoxide-mediated Umpolung of alkali halide salts

A new protocol for the direct two-electron oxidative Umpolung of alkali halide salts is reported. This procedure, relying on the use of a commercially available sulfoxide as the oxidant, allows the electrophilic halogenation of carbonyl compounds as well as halolactonisation reactions to proceed from the corresponding sodium salts, at room temperature and under mild conditions.     

A method for controlled stoichiometry intercalation of alkali metals in layered metal dichalcogenide crystals

Crystals ofLi_yTiS₂ (0 < y < 1) and2HA_yTaX₂ (A =Li, Na, K; 0 < y < 1; X =S, Se) have been prepared by equilibration of the unintercalated crystals with a source powder of the desired alkali ion stoichiometry via the alkali vapor pressure of the compounds at 400–650°C. Crystals of 1 cm² area were intercalated. The final stoichiometry of the crystals is in good agreement with that expected from the source.     

Three empirical correlations connecting gaseous cluster energies and solvation energies of alkali metal and halide ions.

This paper gives two empirical correlations of formation Gibbs energies of gaseous clusters DeltaG(f)n as function of number of solvent molecules attached to the ion, n, and one correlation connecting the DeltaG(f)n for each individual cluster with the total DeltaG(o)hydr value. The experimental ratios of DeltaG(f)2/DeltaG(f)1 and DeltaG(f)3/DeltaG(f)1 for both alkali metal and halide ions are on average equal to 0.75 and 0.5, respectively. DeltaG(f)n values for n > or = 4 are correlated with n as DeltaG(f)n = [a/(n - 1)] DeltaG(f)1 + b DeltaG(f)1. For all available data on cluster energies and each individual cluster, the DeltaG(f)n's are straight-line functions of DeltaG(o)hydr. This well corresponds to another empirical rule stating that the Gibbs energies of transfer of ions between two solvents are often as well straight-line functions of DeltaG(o)(hydr) [J. Rais and T. Okada, J. Phys. Chem. A, 2000, 104, 7314]. Tentative models of the found behavior are proposed. A full data set of the gaseous cluster energies of formation based on inclusion of new, usually not used entries from the literature is provided.     

Vibronic and resonance Raman spectra of NO2impurity ions in the body-centered alkali halide crystals

The low-temperature resonance secondary radiation spectrum as well as the absorption and luminescence excitation spectra of NO₂⁻ impurity ions in cesium halides have been studied. The energy relaxation processes and NO₂⁻ equilibrium orientation and reorientation problems have also been discussed. It was shown that the systems under study were characterized by average Stokes' losses and strong lattice distortions, exemplified by the Generation of a number of low-frequency local and pseudolocal vibrations. The inhomogeneous broadening in CsCl-NO₂⁻ and CsI-NO₂⁻ spectra was extremely large for the simple molecular impurity systems, leading to the interesting peculiarities of energy relaxation processes. Unlike some alkali halide crystals with NaCl structure the impurity NO₂⁻ does not rotate in the lattices with CsCl structure. The NO₂⁻ equilibrium orientation in cesium halides was fixed: both the molecular axis and the axis perpendicular to the molecular plane were directed in (100) directions of the crystal.