Читать книгу Solid State Physics - Philip Hofmann - Страница 42

The term van der Waals bonding refers to a weak and purely quantum‐mechanical effect. The electron cloud around an atom or a molecule has no static charge distribution but one governed by quantum‐mechanical fluctuations. A simple atom with a closed shell can thus be viewed as a fluctuating dipole. The field of this dipole can polarize other atoms nearby, and the interaction of the two neighboring dipoles reduces the total energy of the pair of dipoles, that is, it can lead to bonding. This type of interaction is present in every solid but is much weaker than ionic, covalent, or metallic bonding. Typical binding energies per atom are in the meV range and, therefore, pure van der Waals bonding is only observable for solids that do not show any other bonding type – for example, noble gases. Van der Waals crystals of noble gases can only exist at very low temperatures. Since they are formed from inert atoms without any “real” bonds, they represent an almost ideal case of close‐packed spheres and they all crystallize in the face‐centered cubic structure. Van der Waals bonding can also be important in layered crystals such as graphite. Here, charge fluctuations may arise in the stacked graphene layers (see Figure 1.16). These fluctuations can happen over a much larger scale than in a single atom, and the van der Waals bonding between such layers can therefore be much stronger than between atoms.