Polarizable-ion model

This conclusion is further strengthened considerably by the theoretical calculation of CBE originally performed by Pearson and Gray (102) and later on somewhat modified by Pearson and Mawby (8). Values of CBE are calculated according to three models, viz. the hard sphere model, the polarizable ion model and the localized molecular orbital model. Only the last one, treating the bonds as covalent, is able to account in a satisfactory way for the values found experimentally for such halides as HgCl2 and CdCl2. For LiCl and NaCl, on the other hand, an acceptable fit with the experimental values is obtained already by the hard sphere model, which certainly indicates a predominantly electrostatic interaction. 

The polarizable ion model for the monomeric alkali metal halides... 

Fig. 5.5. The polarizable ion model. Above spherical, unpolarized, ions. Below left polarization of the anion. Below right polarization of the cation. The direction of the z-axis has been chosen in snch a way that the dipole moment of the molecule is negative. This choice of axis, in turn, implies that the induced dipole moments are positive.

Dipole moments calculated from the polarizable ion model, equation (5.32), using the polarizabilities listed in Table 3.4 and experimentally determined bond distances, reproduce the experimental dipole moments with an average deviation of only 5% as compared to 32% for the spherical ion model [5]. 

As we have seen in the preceding chapter, the properties of the gaseous monomeric alkali metal halides are in very good agreement with those predicted by the polarizable ion model. Their experimental dipole moments are all very large, ranging from 6.28 D in T.iF to 11.69 D in Csl. The electric dipole moment of LiH, 5.88 D, is nearly as large as... 

The polarizable ion model and the shape of the heavier Group 2 metal haUdes... 

When an atom or ion is placed in an electrostatic field, its electrons are polarized, as shown schematically in Figure 1. The polarized species then exerts an altered effect on its surroundings in comparison with its unpolarized form. Attempts to account for the substantial polarizability of atoms and ions that exist in many solids have elicited the development of additional potential models. Most notable are the point polarizable ion model and the shell model. 

Fig. 4.7 Left The IR absorption spectrum (arbitrary units) calculated for models of amorphous S102 in which a simple pair-wise potential is used (RIM—dashed line) and when this potential is supplemented with an account of polarization effects (PIM—solid line) [1]. Right Imaginary part of the dielectric function calculated from the DIPole-Polarizable Ion Model (DIPPIM) potential [67], compared to first principles molecular dynamics (FPMD) [78] and experimental results [65, 79]. Figures taken from [1, 67]...

A realistic model must account not only for the classical electrostatic interaction Ugi, but also for three interactions accounting for the quantum nature of electrons. The exchange-repulsion, or van der Waals repulsion U gp is a consequence of the Pauli principle, while the dispersion (van der Waals attraction) arises from correlated fluctuations of the electrons. Last, the induction term reflects the distortion of the electron density in response to electric fields, including incipient charge transfer associated with bond formation. In molten salts, all these interactions can be taken into account in molecular dynamics (MD) simulations in the framework of the polarizable ion model [1],... 

Interaction potentials of the Polarizable Ion Model type were parameterized for a series of molten fluorides and chlorides including cations with a wide range of valencies (Li ", Na" ", K" ", Rb+, Cs" ", Be ", Ca " ", Sr " ", AP" ", Y +, La, Zr, Th" ) [2-5]. The procedure was successfully validated in the case of LiF-BeF2 mixtures, which allows us to propose the prediction of properties for many other molten salts. It is worth noting that the... 

The interaction potential used in these simulations derives from the polarizable ion model [15]. It can be described as the sum of four different contributions charge-charge, dispersion, overlapped repulsion, and polarization, as previously described by Salanne et al. [16].This code is dedicated to calculations in ionic liquids. Via classical MD calculations, it allows generating the trajectories of ions inside a periodically replicated simulation cell, and then extracting the relevant physico-chemical properties of the melt. 

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