Shell Model halides

Shell models have been used successfully in a wide variety of systems. The greatest number of applications have been in the simulation of ionic materials,especially systems including alkali halides, oxides,and zeolites. " The shell model is also commonly used for the simulation of molten and shell-type models... 

Scattering by Alkali Halides Melts A Comparison of Shell-Model and Rigid-Ion Computer Simulation Results. 

CsF is the only alkali halide with rocksalt structure in which the cation makes the greater contribution to the electronic polarizability. Although there have not been any Shell Model calculations on this material, its mirror compound Nal should provide at least some guide as to the energetics of... 

The effects of relaxation on the calculated surface phonon dispersion in Rbl have apparently been verified, particularly by the observation of a surface optical mode which lies above the bulk phonon optical bands. Except for the mysterious acoustic band mode in Rbl, the Shell model calculations have generally been quite accurate in predicting surface vibrational mode energies in both high-symmetry directions of the alkali halide (001) surfaces. 

The frequency dependence of the damping and shift of the TO-mode at q = 0 has also been calculated for NaCl [5.50]. A breathing shell model was used to provide frequencies and eigenvectors necessary for these calculations. The influence of anharmonicity on the TO and LO optical phonons at q = 0 has been studied experimentally by means of the far-infrared dielectric response for 18 alkali and thallium halides [5.51], for the silver and thallium halides... 

The closed-shell model is even less satisfactory for compounds containing polyvalent ions (e.g., MgO). The qualitative explanation for these cases is that the electronic distribution of each ion is not sufficiently close to that of the isoelectronic rare gas atom, e.g., the charge distribution of is greatly different from that of Ne. This point has been confirmed by empirical studies of ionic polarizabilities based on the Clausius-Mossotti model. These show that the polarizability of the halide ions is nearly constant for each ion (independent of the compound in which it finds itself), but that the polarizability of varies by as much as a factor of three from one compound to another. It is possible that introduction of polarization data beyond the simple multipole model, in a manner similar to that used in successful treatments of covalently bonded compounds (see Section 4), would improve matters in these cases. 

The geometric structure of the covalent binary halides, whether neutral or complexed ions, can be explained on the basis of the Nyhotm-Gillespie rules known as the Valence Shell Electron Pair Repulsion Model (VSEPR) theory the geometrical arrangements of the bonds around an atom in a species depends on the total number of electron pairs in the valence shell of the central atom, including both bonding... 

Spohr [190] studied the adsorption of on the Pt(lOO) surface. The free energy barrier towards iodide adsorption that is produced by the layers of adsorbed water is associated with a significant intermediate increase in coordination number, before the hydration number decreases at short ion-metal distances for geometrical reasons. Philpott and Glosli [109] observed in a series of MD studies of ion adsorption on charged electrodes that the Li+ hydration shell structure in the vicinity of a model metal surface does not depend on the halide counterion (F, Cl , Br , I ). In this study, no specific interactions between the metal surface and water molecules or ions were employed. 

The trivalent elements form complexes formally derived from the trihalides by addition of further halide ions or other donors. The stereochemistries can be understood by considering the presence of the lone pair in the valence shell and employing the models outlined in Chapter 4. 

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