Insulators ionic solids

The electronic levels of crystalline solids separate into bands of allowed and forbidden energies [53]. A solid whose highest occupied band (valence band) is completely filled and separated from the lowest unoccupied (conduction) band is an insulator. Ionic solids are typically insulators. In this one-electron band description, the lowest electronic excitation corresponds to a transition from the top of the valence to the bottom of the conduction band (a band-gap excitation). Direct band-gap transitions do not involve simultaneous emission or absorption of a phonon, whereas indirect ones do. 

It is to be expected that tire conduction data for ceramic oxides would follow the same trends as those found in semiconductors, i.e. the more ionic the metal-oxygen bond, the more the oxides behave like insulators or solid elee-trolytes having a large band gap between the valence electrons and holes, and... 

In metals, valence electrons are conduction electrons, so they are free to move along the solid. On the contrary, valence electrons in insulators are located around fixed sites for instance, in an ionic solid they are bound to specific ions. Semiconductors can be regarded as an intermediate case between metals and insulators valence electrons can be of both types, free or bound. 

The fully ionic solids (region I) afforded band insulators, 1 1 Mott insulators with ground states of antiferromagnets (E b(21) and F b(22) in Fig. 1) or spin-Peierls systems, ferroelectrics, ferromagnets, spin-ladders, and nonlinear transport materials (switching and memory). 

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

The same model can be applied to an ionic solid. In this case, for the example of MgO, Fig. 2.9 represents the transfer of electrons from anions to cations resulting in an electron in the conduction band derived from the Mg2+ 3s states and a hole in the valence band derived from the 2p states of the O2- ion. Because the width of the energy gap is estimated to be approximately 8eV, the concentration of thermally excited electrons in the conduction band of MgO is low at temperatures up to its melting point at 2800 °C. It is therefore an excellent high-temperature insulator. 

Electrical conductivity of metals is very high and is of the order of 106 108 ohm-1 cm-1 while that of insulators is of the order of 10-12 ohm-1 cm-1. Semi-conductors have intermediate conductivity which lies in the range 102 10-9 ohm-1 cm1. Electrical conductivity of solids may arise through the motion of electrons and positive holes (electronic conductivity) or through the motion of ions (ionic conductivity). The conduction through electrons is called n-type conduction and through positive holes is called p-type conduction. Pure ionic solids where conduction can take place only through motion of ions are insulators. However, the presence of defects in the crystal structure increases their conductivity. 

Besides being a probe of the presence of sites in different coordination states and of their different reactivity, near-UV excitonic bands of insulating oxides can be further analyzed to obtain insights into the electronic features of surface sites responsible for such transitions, and the reasons for the peculiar reactivity related to a type of surface site/structure. To achieve this, it must be recalled that the main model for the quantitative prediction of the energies of surface states of highly ionic solids has been developed by Levine and Mark [48], where the exciton gap for surface ions, E is expressed as ... 

Metals conduct electricity very well. In contrast, insulators do not. Insulators may consist of discrete small molecules, such as phosphorus triiodide, in which the energy necessary to ionize an electron from one molecule and transfer it to a second is too great to be effected under ordinary potentials.20 We have seen that most ionic solids are nonconductors. Finally, solids that contain infinite covalent bonding such as diamond and quartz are usually good insulators (but see Problem 7.5). 

Ionic compounds, like sodium chloride, are normally considered insulating in the solid state. This is due to the marked difference in electronegativity of the cations and anions, which creates a large band gap of typically 6-12 eV between the valence and conduction bands. Therefore, ionic solids have a band structure similar to an insulator (Figure 5.15). The ions and their associated electrons can be thought of as fixed on their lattice sites. 

Finally, ionic bonds are formed between elements from opposite sides of the periodic table (typically between a metal and a nonmetal), where there is a large difference in electronegativity between the atoms. Because metals have very low lEs and nonmetals have large EAs, an ionic bond is characterized by the transfer of one or more electrons from one atom to another to form a cation-anion pair. As a general rule, the nondirectional nature of the electrostatic attraction between the ions leads to fairly high melting and boiling points. Most ionic solids are insulators because the ions are fixed in place in the crystalline lattice however, they become electrical conductors when molten or dissolved in aqueous solution. 

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