Ionically Bonded Solids

a structure or packing of the ions has to be assumed and the various interactions between the ions have to be taken into account. Begin with NaCl, which has one of the simplest ionic structures known (Fig. 2.5a), 

This topic is discussed in greater detail in the next chapter and depends on the size of the ions involved, the nature of the bonding, etc. 

The second term in parentheses is an alternating series that converges to some value a, known as the Madelung constant. Evaluation of this constant. 

Strictly speaking, this is not exact, since in Eq. (2.17) the repulsive component of the ions that were not nearest neighbors was neglected. If that interaction is taken into account, an exact expression for Esumis given by 

The total electrostatic attraction for 1 mole of NaCl in which there are twice Avogadro s number of ions but only N/. bonds is 

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Diffusion in ionically bonded solids is more complicated than in metals because site defects are generally electrically charged. Electric neutrality requires that point defects form as neutral complexes of charged site defects. Therefore, diffusion always involves more than one charged species.9 The point-defect population depends sensitively on stoichiometry for example, the high-temperature oxide semiconductors have diffusivities and conductivities that are strongly regulated by the stoichiometry. The introduction of extrinsic aliovalent solute atoms can be used to fix the low-temperature population of point defects. 

Liquids formed from covalently bonded solids are different from ionic-bond solids in another way as well. Covalently bonded liquids do not contain free-floating charged particles. Instead, they contain tightly-bonded, self-contained, neutral molecules. As a result, a liquid produced from a covalently bonded solid does not conduct electricity well at all. These liquids are good insulators. 

Motivated by the application of ZnO in gas sensors and catalysis and by the more general desire to understand surface properties of ionically bonded solids, electronic properties of ZnO surfaces have been investigated for many years [20,76-80]. An overview of the early work on ZnO surface properties is included in the book of Henrich and Cox [81]. 

This is an important result for two reasons. First it confirms that our simple model for the interaction between ions in a solid is, for the most part, correct. Second, it supports the notion that NaCl can be considered an ionically bonded solid. 

Wafer bonding is a process by which two mirror-polished wafers of any materials adhere to each other at room temperature without the application of any macroscopic gluing layer or external force. The bonding is achieved through van der Waals force. Wafer bonding achieved at room temperature is usually relatively weak compared to metallically, covalently, or ionically bonded solids. Therefore, for many applications, the room-temperature bonded wafers have to undergo a heat treatment to strengthen the bonds across the interface. 

Ionic bonding gives rise to ionically bonded solids. 

The stability of ionically bonded solids depends on the lattice energy. The greater the lattice energy, the more stable the solid will be. The stability in turn determines the melting temperature, thermal expansion, and stiffness of the solid. More stable solids will possess greater melting points, less thermal expansion, and greater stiffness. 

Achvahon of ionically bonded solids may be by exposure to electron, photon, or ion radiahon, which creates point defects. Elechon and photon radiahon of insulator and semiconductor surfaces prior to film deposihon have been used to enhance the adhesion of the him, probably by generahng charge sites and changing the nucleahon behavior of the adatoms. 

FIGURE 1 4 An ionic bond IS the force of attrac tion between oppositely charged ions Each Na ion (yellow) in the crystal lattice of solid NaCI IS involved in ionic bonding to each of six surrounding Cl ions (green) and vice versa... 

Solid Dispersion If the process involves the dispersion of sohds in a liquid, then we may either be involved with breaking up agglomerates or possibly physically breaking or shattering particles that have a low cohesive force between their components. Normally, we do not think of breaking up ionic bonds with the shear rates available in mixing machineiy. 

Sq can be calculated from the theoretically derived U(r) curves of the sort described in Chapter 4. This is the realm of the solid-state physicist and quantum chemist, but we shall consider one example the ionic bond, for which U(r) is given in eqn. (4.3). Differentiating once with respect to r gives the force between the atoms, which must, of course, be zero at r = rg (because the material would not otherwise be in equilibrium, but would move). This gives the value of the constant B in equation (4.3) ... 

Typically, ionic compounds are solids at room temperature and have relatively high melting points (mp NaCl = 801°C, CaCl2 = 772°C). To melt an ionic compound requires that oppositely charged ions be separated from one another, thereby breaking ionic bonds. 

If you were given a sample of a white solid, describe some simple experiments that you would perform to help you decide whether or not the bonding involved primarily covalent bonds, ionic bonds, or van der Waals forces. 

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

In semi-conducting compounds, we know that some of the electrons form bonds between the cation and the anion, either as covalent or ionic bonds (or somewhere in between). What happens to the rest Do they remeun around the parent atom Why are some solids conductive while others are not The following discussion addresses these questions. Obviously, we cannot be exhaustive but we can examine the main features of each phenomenon to show what happens in the solid. We will not derive the equations associated with each subject. This aspect is left to more advanced studies. 

Chemical Partially-ionic- covalent-bond solids Dissolution is based on neutralization Aluminum hydroxide in acid or base Al(OH)3 + 3 H+ Al3+ + 3 H20 A1(0H)3 + 0H-->A10(0H)2 (aq) + H20... 

Ionic radii are quoted in Tables 2.3 and 2.5 for a large number of cations including those of the elements in groups 13, 14, 15, and 16, which do not form predominately ionic bonds. These values were obtained by subtracting the fluoride or oxide ion radius obtained from predominantly ionic solids from the length of a bond that is not predominantly ionic. The very small values for the radii of cations obtained in this way do not bear much relation to the real size of the atom in the crystal or molecule. 

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