ABSTRACT

Figure 1 Dependence of potential energy on interatomic distance. The calculated values of the cohesive energy are compared with experimental results, which can be obtained by measuring the latent heat of sublimation at various low temperatures, and extrapolating to zero Kelvin. The empirical parameters relative to the cohesive forces can be evaluated starting from the inter-reticular distance, r0, and the compressibility, b, of the solid: 1/b = –Vdp/dV. The analysis of different types of solids can reveal the nature of different attractive forces. Outside of these general considerations, detailed explorations of crystalline cohesion are generally complex and do not rely on a single rule for all solids. Usually the type of bonds may be classified into one of four general categories and each of them is treated by specific simplifications. We follow these general rules but it must be pointed out that the approach by Harrison provides a coherent theory taking into account covalent solids, crystals of rare gases, ionic crystals, and simple and transitions metals [12]. 2. Rare Gas Crystals In addition to chemical bonding between atoms, there is another type of attractive force that exists between atoms, ions, or molecules known as van der Waals forces. These forces represent the main contribution to the cohesive energy for rare gas crystals, from Ne to Rn with eight outershell electrons/atom. The attractive energy between two atoms results in an induced dipole-dipole interaction (called the van der Waals-London interaction) which varies as r-6: W E P rp B A e e A2P E= - ◊ = - µ  e a a0 2 6/where a is the polarizability of the atom.