The Purely Ionic Model

Marsh and Casabella induced elastic strain upon single crystals of NaCl and NaBr by plying static pressure (up to 6.9 MPa) and noted, as expected, that the C1 and Br NMR lineshapes broadened and became less intense. The authors determined that the purely ionic model of vK was inadequate to describe the changing field gradients at the chlorine and bromine nuclei with respect to changing pressure. It was concluded that ion orbital overlap between nearest and NNN atoms was a satisfactory model to rationalize their observations and that pure covalent effects did not need to be included. The effects of static elastic strain on chlorine SSNMR spectra were observed to determine the gradient-elastic tensors for LiCl and RbCl by Flackeloer and Kanert. ... 

The electron transfer from a partially filled band is obtained by multiplying Nj.j by the fractional occupation of the T band. The purely ionic model is transformed by the latter two equations into a model containing a bonding charge that reproduces the essential features of the electronic structure of NaCl-type compounds when the hybridization is reasonably weak. Whereas the atom and angular-momentum projected densities of states in the unhybridized model arose simply from the T unhybridized bands, they now contain additional parts due to hybridization with the T bands. Dashed lines have been added to complete the hybridized density of states in fig. 58. The number of electrons transferred between the atoms by this interaction is estimated from eq. (68) to be, with UN as example,... 

In the purely ionic model, the stabilization of these polar surfaces is achieved via a charge redistribution that increases the formal charge of Zn ions on the Zn-terminated surface from +2 to +3/2 and reduces the formal charge of O ions on the O-terminated surface from —2 to —3/2. The charge compensation can be achieved by electron transfer from the O- to the Zn-terminated surface, by removing surface ions, or by adsorption of charged species. 

For example, a purely ionic model of CrFe predicts a total binding energy (relative to Cr6+ plus six F isolated ions cf. Example 2.1) of 7.9832 a.u., more than 1000 kcal mol-1 less than the actual value. 

As is obvious from the table, Tc is almost doubled upon deuteration. These isotope effects are one of the largest observed in any solid state system. The question arises about isotope effects in non-hydrogen-bonded ferro- and antiferroelectrics. As already mentioned in the Introduction, within a mean-field scheme and in a purely ionic model it was predicted that these systems should not exhibit any isotope effect in the classical limit, which has been verified experimentally. Correspondingly, there was not much effort to look for these effects here. However, using a nonlinear shell-model representation it was predicted that in the quantum limit an isotope effect should... 

These agree rather well with the experimental values listed above, suggesting that the ionic model is a good one. On the other hand, the negative Fermi contact constant can only arise through polarisation of the electron spins in a covalent bond between the two atoms. The electric dipole moment also seems to be inconsistent with a purely ionic model, yet the quadrupole coupling constant eq0Q is very close to that of the ionic molecule LiF. 

Models of several types have been used for extended solids. They range from pure ionic models to full valence force fields, with a few hybrids falling in between. Brief mention will be made of the ionic models and the hybrids, but the bulk of this section discusses the full valence force fields. 

Although there is no sharp boundary between ionic bonding and covalent bonding, it is convenient to consider each of these as a separate entity before attempting to discuss molecules and lattices, in which both are important. Furthermore, because the purely ionic bond may be described with a simple electrostatic model, it is advantageous to discuss it first. The simplicity of the electrostatic, model has caused chemists to think of many solids as systems of ions. We shall see that this view needs some modification, and there are, of course, many solids, ranging from diamond to metals, which require alternative theories of bonding. 

Crystal field theory A theory of bonding in transition metal complexes in which ligands and metal ions are treated as point charges a purely ionic model. Ligand point charges represent the crystal (electric) field perturbing the metal s d orbitals that contain nonbonding electrons. 

To test our bonding model, as before, separate calculations must be made for the purely ionic case and the purely covalent case. However, from the outset we will assume that CN4 means covalent, and CN6 means ionic. The main test will be to see if we can match the values of in Table 5.5 with theoretical results, but... 

What we are doing in taking this approach to developing a model is mixing a purely ionic model with an atomic orbital model. The valence orbitals of the central atom are assumed to be influenced by the close approach of the ligands acting simply as points of negative... 

Herzog and Richtering determined the temperature dependence of the copper, bromine, and iodine linewidths. ° In all cases, as the temperature was increased, the linewidth associated with the anionic species was found to increase, while that of the copper decreased. This was interpreted by the authors as being due to the onset of Cu lattice mobility and enabled them to determine the activation energies for this process. Gunther and Hultsch were the first to report the highly shielded shift and short Ti( Br) of this compound when they studied Ti( Cu) temperature dependence in CuX (X = Cl, Br, I) systems.Spin-lattice relaxation values for lower temperatures (T = 78-300 K) in polycrystalline samples of CuBr were studied using Br NMR and found to obey the expected temperature dependence while near or above 9-a, in accord with the pure ionic vK model. 

It is universal practice to describe sihcates in terms of a purely ionic model. However, although we might write Si , the 4+ charge is unlikely on ionization energy grounds and is incompatible with the commonly observed... 

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