Lattice spacings, ionic crystals metals

In the case of organic molecules chemists consider a logical choice to express the potential in terms of valence bonds and valence electrons [20] while physicists often proceed with an unbiassed cartesian space which is sometimes transformed in that of a suitable model aimed at a specific physical property. Often the potentials used in treating the vibrations of ionic crystals or metals are transferred and constrained to the case of organic molecules. However the chemical reality of molecular covalent bonds and of their interactions is different from any type of "ionic" system. For example the popular "shell model" has not been as successful as the classical "valence force field" in the study of the lattice dinamics of "covalent crys-... 

Salts of the bases MOH are crystalline, ionic solids, colorless except where the anion is colored. For the alkali metal ions the energies required to excite electrons to the lowest available empty orbitals could be supplied only by quanta far out in the vacuum ultraviolet (the transition 5p6 —5p56s in Cs+ occurs at 1000 A). However, colored crystals of compounds such as NaCl are sometimes encountered. Color arises from the presence in the lattice of holes and free electrons, called color centers, and such chromophoric disturbances can be produced by irradiation of the crystals with X rays and nuclear radiation. The color results from transitions of the electrons between energy levels in the holes in which they are trapped. These electrons behave in principle similarly to those in solvent cages in the liquid ammonia solutions, but the energy levels are differently spaced and consequently the colors are different and variable. Small excesses of metal atoms produce similar effects, since these atoms form M+ ions and electrons that occupy holes where anions would be in a perfect crystal. 

Conditions are simplest if the same force is effective in all three spatial directions in the given crystal in this case true ionic lattices, true atomic, metallic and molecular lattices are obtained. It may happen, however,— and these cases will later be shown to be of special interest— that in one and the same lattice different types of forces prevail in the different directions in space, and result in mixed lattices and transition phenomena which will be discussed after the simple cases. 

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