Structure Of Alkali Halides Research Articles

Transferable Deformation-Dipole Model for Ionic Materials

A model for the ionic interactions in polyvalent metal halides was originally built for chloroaluminate clusters using an analysis of data on static and dynamic structure of their molecular monomers [for a review see M. P. Tosi, Phys. Chem. Liquids 43, 409 (2005)]. Recently, by continuing the deformation-dipole model calculations, the transferability of the halogen parameters was tested through the calculation of the structure of alkali halides and alkaline-earth halides. In this work we test the usefulness of the deformation-dipole model in the study of ionic materials by examining the transferability of the overlap parameters for the halogen ions across families of halide compounds. Following a comparative discussion of alkali and alkaline-earth halide monomers near equilibrium, results on alkaline-earth halides are given. By using the transferable ionic potential model we also calculate the equilibrium structure of the molecular clusters, as well as the vibrational frequencies of ACl4 compounds (where A = U, Np, Pu, Am and Th).

Hybrid Inorganic−Organic Host−Guest Compounds: Open-Framework Cadmium Oxalates Incorporating Novel Extended Structures of Alkali Halides

Four unusual inorganic−organic nanocomposites containing extended alkali halide structures, present as layers or as three-dimensional units, have been synthesized by a metathetic reaction carried out under hydrothermal conditions. These compounds, with cadmium oxalate host lattices and the alkali halide structures as guests, have the compositions [RbCl][Cd6(C2O4)6]·2H2O, I; 2[CsBr][Cd(C2O4)]·H2O, II; 2[CsBr][Cd2(C2O4)(Br)2]·2H2O, III; and 3[RbCl][Cd2(C2O4)(Cl2)]·H2O, IV. Crystal data for I, rhombohedral, space group = R-3 (no. 148), a = 9.3859(3), c = 23.9086(8) A; V = 1824.05(1) A3, Z = 18, M = 1359.51, ρcalcd = 3.702 g cm-3, μ(Mo Kα) = 7.375 mm-1, R1 = 0.04, wR2 = 0.0904 [537 observed reflections with I > 2σ(I)]; for II, orthorhombic, space group = Pbcm (no. 57), a = 6.0854(6), b = 11.0793(11), c = 16.889(2) A; V = 1138.7(2) A3, Z = 8, M = 644.06, ρcalcd = 3.745 g cm-3, μ(Mo Kα) = 15.219 mm-1, R1 = 0.04, wR2 = 0.08 [408 observed reflections with I > 2σ(I)]; for III, orthorhombic, space group = Cmcm (no....

Fascinating Alkali Halide Structures of Different Dimensionalities Incorporated in Host Lattices

Ein Cadmiumoxalatgitter dient als Wirt für ausgedehnte Alkalimetallhalogenid-Strukturen mit unterschiedlichen Dimensionen. Das Bild zeigt beispielsweise, wie die Cl−-Ionen eines dreidimensionalen KCl-Gitter in einem Cd6O24-Cluster eingebaut sind.

Fascinating Alkali Halide Structures of Different Dimensionalities Incorporated in Host Lattices

Fascinating Alkali Halide Structures of Different Dimensionalities Incorporated in Host Lattices

Fascinating Alkali Halide Structures of Different Dimensionalities Incorporated in Host Lattices

Fascinating Alkali Halide Structures of Different Dimensionalities Incorporated in Host Lattices

A density functional study of small alkali halide systems

The electronic structures of lithium-, sodium-, and potassium-containing alkali halide monomers and dimers are calculated using the local density method. In particular, for lithium and sodium halides, the calculated equilibrium bond lengths and vibrational frequencies are in excellent agreement with experimental gas-phase results. For potassium halides, we find small but systematic deviations between calculated properties and experiment. Inclusion of nonlocal exchange corrections improves the agreement between theory and experiment for these systems. The investigation shows that the density functional approach provides an efficient and accurate method for the calculation of the electronic structure of alkali halides.

X-ray-absorption near-edge structure of alkali halides: The interatomic-distance correlation.

Cl L-edge x-ray-absorption near-edge-structure (XANES) total-electron-yield spectra have been recorded for LiCl, NaCl, KCl, as well as NaCl-KCl solid solutions, in the region of 190--280 eV, using synchrotron radiation. The spectra are interpreted using a model previously developed and based on a 1/${\mathit{R}}^{2}$ correlation. All post-edge features may be correlated with interatomic distances and lattice spacings. The model also assigns the XANES features to interatomic distances and lattice spacings in Na and F K-edge spectra of alkali halides.

Empirical energy parameters for structural modeling of alkali halide crystals in standard formalisms

Potential energy parameters, describing Coulombic, van der Waals, and repulsion energies, have been established for each of the nine common alkali halide ions in two forms of the exp-6 potential, with use of the standard, ambient-stable alkali halide crystal structures for reference. The derived potential parameters fir the lattice energies of the stable NaCl-type structures to better than 1 kcal/mol and predict the lattice energies of the stable CsCl-type structures to better than 3 kcal/mol. For the unstable alkali halide structures, lattice constants are predicted to better than 0.2 {angstrom}, in general, while the lattice energies obtained are more positive than for the corresponding stable strucures and agree well with published values where available.

Mechanism of B1-B2 transition of ionic crystals under pressure

Under pressure the B1 structure of alkali halides transforms to the B2 structure. There is a preferred orientation relation between these two structures. Lattice-dynamically, the TA[100] mode of B1 structure becomes unstable under pressure. It is shown that this instability is caused by the repulsive term which increases under pressure and is weakened by deformation of the TA[100] mode. Discussions on thermodynamics are given; there is no difference between the Gibbs and the internal energies for lattice-dynamical studies, but for finite deformation what must be dealt with is the Gibbs free energy.

Structure and thermodynamic properties of molten strontium chloride

Self-consistent calculations of pair distribution functions and thermodynamic properties are presented for a pair potentials model of molten strontium chloride. The calculations extend to a strongly asymmetric ionic liquid, an earlier assessment of bridge diagrams in a modified hypernetted chain approach to the liquid structure of alkali halides. Good agreement is found with computer simulation data obtained by de Leeuw (1978) with the same set of pair potentials, showing that the present approach incorporates genuine general features of liquid structure theory for multicomponent liquids with strong relative ordering of the component species. It is further shown that the strong correlations between the divalent cations, both in the model and in real molten strontium chloride, can be approximately reproduced on the basis of a simple one-component-plasma model, provided that dielectric screening is allowed for in the real liquid. This allows the authors tentatively to attribute the significant level of disagreement between a pair potentials model of this liquid and the neutron diffraction data of McGreevy and Mitchell (1982) to many-body distortions of the electronic shells of the ions.

Mechanism of Instability of B1 Structure of Alkali Halides under Pressure

Upon applying pressure on alkali halide model-crystals of B1 structure, the oscillation of TA[100] mode becomes unstable. Due to deformation of TA[100] mode, the repulsive force between the nearest neighbor ions is weakened, energy is lowered, and the instability sets in. For the discussion of onset of instability, the internal energy and Gibbs free energy lead to the same conclusion. For a finite deformation, the Gibbs free energy must be used.

Optical properties and electronic structure of alkali halides by thermoreflectivity

We report detailed measurements of thermoreflectivity spectra of NaCl, KCl, RbCl, NaBr, KBr, RbBr, NaI, KI, and RbI in the 5-9-eV region at several temperatures. Through an analysis of the thermoreflectivity line shapes and temperature coefficients, we try to find a general interpretation of the optical-spectra structures for all crystal families in terms of $\ensuremath{\Gamma}$, $L$, and $X$ transitions, on the basis of present understanding of band calculations. The spin-orbit splitting of the $\ensuremath{\Gamma}$ exciton is determined to be 0.13 \ifmmode\pm\else\textpm\fi{} 0.02 eV in chlorides, 0.49 \ifmmode\pm\else\textpm\fi{} 0.02 eV in bromides, and 1.10 \ifmmode\pm\else\textpm\fi{} 0.02 eV in iodides.

Exciton Polarons of Molecular Crystal Model. II. Optical Spectra

Absorption and emission spectra of excitons interacting with optical or intramolecular vibrations are calculated with the dynamical coherent-potential approximation proposed previously. Investigations are made for various situations to which limiting concepts of nearly-free, self-trapped and vibronic excitons are not applicable straightforwardly. An interpretation is proposed that the absorption structure of alkali halides around the exciton peak originates from a situation where the self-trapped state is energetically close to the lowest edge of the nearly-free state. In this case, where the exciton-phonon and the excitation-transfer interactions are comparable to each other and strong, configuration mixing between the two types of states is small. When the two types of interaction are weak and comparable to each other, an intermediate state between the nearly-free and the vibronic-exciton states is realized. This situation explains the progressional structure in the exciton absorption band of crystallin...

Short‐Range Three‐Body Potential in the First‐Order Exchange Energy and the Relative Stability of Alkali Halide Crystals

AbstractUsing the Heitler‐London approach a simple expression for the first‐order exchange energy between a system of interacting atoms or ions is obtained in the S2 approximation. It is shown that this expression leads to two‐body, three‐body, and four‐body interactions only. A specific expression for the three‐body interaction is evaluated using several different approximations. Assumptions which lead to the expression previously obtained by Lundqvist are discussed. It is found that these expressions vanish for neutral particles. Some modification in the assumption is made and an expression for three‐body interaction for neutral particles is obtained. Numerical calculations are made for argon, xenon, and helium and the results compared with those obtained by others. The three‐body interaction is also applied to study the relative stability of different alkali halide structures. It is found that this three‐body potential has a strong preference for CsCl structure.

Pseudopotential approach to the band structure of alkali halides

A pseudopotential approach to the calculation of the conduction band structure of ionic crystals is proposed along the lines suggested by Heine and Abarenkov. An improved model potential originally suggested by Griuliano and Ruggeri is adopted and discussed in connection with other choices of the potential. Detailed calculations are performed in LiCl, NaCl and KC1, and the results are similar to those obtained with the KKR and the OPW methods, but are more free electronlike, as suggested by Phillips. The minimum of the conduction band is F1 in NaCl and KC1 andL1 in LiCl. The lowest conduction band is nondegenerate and overlaps very little with the higher conduction bands. In the case of KC1 a well-defined secondary minimum exists atX3, while in LiCl and NaClX3 is more likely a saddle point. The strong absorbtion peak above ionization in KC1 is attributed to a resonant exciton associated to the minimum atX3, rather than to a peak in the density of states. The electron-hole interaction should be taken into account to interpret the entire optical excitation spectrum.

Excitonic Structure of Alkali Halides

A new technique was used for the study of the reflectivity on KI, NaI, and KBr in vacuum ultraviolet; the cleavage of the samples was done in liquid helium at 1.8\ifmmode^\circ\else\textdegree\fi{}K, avoiding any pollution of the surface. With this method we observed new structures and, in particular, for the first time the pure triplet exciton state predicted a few years ago by Onodera and Toyozawa.

Application of New Madelung Summation Method to Close-Packed Alkali Halide Structures

Application of New Madelung Summation Method to Close-Packed Alkali Halide Structures

Effect of ionic forces shown by the liquid structure of alkali halides and their aqueous solutions

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