Ionic crystals electronic localization

In a metallic compound the valence electrons form a collective belonging to the whole crystal. In a non-metallic compound, on the other hand, it is a useful approximation to consider the bonding valence electrons as localized between cation and anion in covalent crystals or on the anion in purely ionic crystals. Moreover, the electron balance is not influenced by the degree of covalency of the bonds, so that formally we can treat all cation-anion bonds as if they were ionic. For the valence electrons of a normal ionic compound Mm X the following relation holds ... 

A single-particle effect that adds features in the X-ray absorption spectrum of molecules not present in that of atoms is the shape resonance (74, 75). (In the case of solids this effect, caused by a modification of the density of states due to the presence of the other atoms in the molecule, is automatically accounted for in band calculations.) Localization of the excited electron inside the molecule in states resulting from an effective potential barrier located near the electronegative atoms in the molecule causes strong absorption bands in free molecules and near the inner-shell ionization limits of positive ions in ionic crystals (74). Consequently, molecular inner-shell spectra depart markedly from the corresponding atomic spectra. The type of structure of an inner-shell photoabsorption spectrum depends on the geometry of the molecule, the nature of its ligands, etc., and can sometimes be used to determine the structure of the molecule. 

The valence electrons in metals have considerable freedom of movement (see Section 1.3.4). This contrasts with ionic crystals where the bonding electrons are normally locahzed within the ions, and with covalent and molecular crystals where electrons are localized in the constituent bonds. 

Like in molecular crystals, the electronic charge distribntions in ionic crystals are highly localized in the neighborhood of the ion cores. However, some electrons are now bonnd to the component of the opposite type. As snch, ionic crystals can be viewed as molecnlar crystals in which the constitnents are not atoms or molecnles, bnt ions. Ionic crystals are also insnlators. 

One of the characteristics of chemisorption is that it permits the formation of different types of bonds between a given adsorbed species and the same adsorbent. Thus, an atom can be attached to an ionic crystal by a weak covalent bond, a strong covalent bond, or an ionic bond. The first is characterized by a localized electron and an induced dipole moment that may be larger by several orders of magnitude than the moment due to physical adsorption. When bonding is augmented by a free electron from the crystal lattice, the adsorbed atom (in the case of monovalent electropositive atom) is held by a strong covalent bond. On the other hand, localization of a hole near a weakly adsorbed atom leads to the formation of an ionic bond. Thus the same atom can represent an acceptor or a donor at the same time. 

Exciton - A localized excited state consisting of a bound electron-hole pair in a molecular or ionic crystal. The exciton can propagate through the crystal. 

In solid state physics, polarons are electrostatically induced local lattice distortions caused by an electron in a ionic crystal. In conducting polymers radical cations (lone electrons associated with positively charged holes) have a similar effect. 

Polaron. This mechanism occurs only in ionic crystals and involves the interaction between the electron and the ions in the crystal. The electron can cause local distortion of the lattice known as a polaron as illustrated in Figure 30.3. When the interaction is suffi-... 

A carbon atom has four unpaired electrons and can share four electrons wifli other atoms and form four covalent bonds. Such covalent bonds can be extremely strong (diamond), and the electrons can be locally and strongly bound. Therefore, sohd covalent crystals generally exhibit no electrical conductivity—neither electronic nor ionic. In biomaterials, covalent bonds with carbon are very important, and biomaterials usually have no molecular ionic or electronic conductivity. However, the charges in such a molecule may be far apart thus, very large dipole moments and strong electric polarization may occur (Table 2.3). 

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