Silver halides lattice energies

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

The extra field of the subgroup ions certainly plays a part in the low solubility of these compounds, but it cannot be said that this alone reduces solubility since AgF, in which the fluorine ion is particularly small, is freely soluble in contrast to AgCl and the other silver halides. It must be kept in mind that, in general, lattice energy... 

Table CLI. Lattice Energies of Silver and Copper Halides...

Because of the difficulty of assigning basic radii in some crystals, notably those of the halides of silver and thallium, Ladd and Lee (7 ) have extended the above expression, which eliminates B and hence the basic radii, to the case where other forces, that is, dispersion energy terms, are included. They obtain for the lattice energy Uo... 

If the lattice defect responsible for deep trapping is an impurity ion with partially filled orbitals which introduces vacant energy levels in the forbidden energy gap of the silver halide host, the trapped carrier may relax into these levels causing a change in the formal valence state of the impurity. Defects that undergo such valence state changes may be electrostatically attractive, neutral, or repulsive to the carrier that is bound. 

Holes. Compared to the situation for photoelectrons in the silver halides, the properties of holes are less well understood. In the effective mass approximation, the binding energy is moderated by the background dielectric constant. When a mobile carrier is close to a trapping center, it does not experience the full dielectric constant of the perfect lattice. If the effective mass is sufficiently large or the dielectric constant sufficiently small, the... 

The dominant defect in silver halides is a Frenkel defect, in which a silver ion moves to an interstitial site. To calculate the energy required to form this defect we simply remove a silver ion from one position, put it in its new position and compare the energy of the crystal lattice with that of the perfect lattice. 

Similarly, the lattice energy—which is determined experimentally from the heat of sublimation—is also susceptible to theoretical estimation. To obtain good agreement, one must, of course, take into consideration the exchange forces of the electron shells in close proximity and the zero energy in addition to the two purely electrostatic terms of equation (34). J. Mayer has done this for the halides of thallium and silver and has found very satisfactory agreement with experimentally determined data. Table 59 is a proof of this. 

Lattice Energies op Silver and Thaluum Halides in Cals. 

A Comparison of Born-Haber and Born-Lande Lattice Energies for Sodium, Silver, and Thallium Halides... 

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