Structures of Ionic Solids

Like metallic solids, ionic solids tend to adopt structures with symmetric, close-packed arrangements of atoms. However, important differences arise because we now have to pack together spheres that have different radii and opposite charges. Because cations are often considerably smaller than anions —= (Section 7.3) the coordination numbers in ionic compounds are smaller than those in close-packed metals. Even if the anions and cations were the same size, the close-packed arrangements seen in metals cannot be replicated without letting ions of like charge come in contact with each other. The repulsions between ions of the same type make such arrangements unfavorable. The most favorable structures are those where the cation—anion distances are as close as permitted by ionic radii but the anion—anion and cation—cation distances are maximized. 

Is it possible for all atoms in an ionic compound to lie on the lattice points as 

Three common ionic structure types are shown in FIGURE 12.26. The cesium chloride (CsCl) structure is based on a primitive cubic lattice. Anions sit on the lattice points at the comers of the unit cell, and a cation sits at the center of each cell. (Remember, there is no lattice point inside a primitive unit cell.) With this arrangement, both cations and anions are surrounded by a cube of eight ions of the opposite type. 

The sodium chloride (NaCl) and zinc blende (ZnS) structures are based on a face-centered cubic lattice. In both structures the anions sit on the lattice points that lie on the corners and faces of the unit cell, but the two-atom motif is slightly different for the two structures. In NaCl the Na ions are displaced from the Cl ions along the edge of the unit cell, whereas in ZnS the Zn ions are displaced from the ions along the 

Do the anions touch each other in any of these three structures if not, which ions do touch each other  

Like metallic soHds, ionic solids tend to adopt structures with symmetric, close-packed arrangements of atoms. However, important differences arise because we now have to pack together spheres that have different radii and opposite charges. Because cations are often considerably smaller than anions ccD(Section 7.3), the coordination numbers in ionic compounds are smaller than those in close-packed metals. Even if the 

Compound Cation-Anion Distance (A) Lattice Energy (kj/mol) Melting Point (°C) 

How many cations are there per unit ceii for each of these structures How many anions per unit cell  

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In this part of the chapter, we begin with molecular solids and distinguish them from network solids. Then we examine metallic solids, which, if consisting of a single element, are built from identical atoms stacked together in orderly arrays. The structures of ionic solids are based on the same kinds of arrays but are complicated by the need to take into account the presence of ions of opposite charges and different sizes. 

The structures of ionic solids may be accounted for quite accurately by the use of a coulombic interaction potential between neighbouring ion pairs together with a suitable ion-core repulsion. 

We have studied the transformations of the CsCl + KCl and CsCl + CsBr solid solutions in order to find the limitations and applicability of the Born treatment in explaining the two entirely different behaviours of the solid solutions of these two systems. Such a study is of value since theoretical approaches to explain the relative stabilities of structures of ionic solids have not been quite successful, and it is important to explain the relative stabilities of at least the two simplest structure types in ionic solids, viz., the NaCl and CsCl structures. We also wished to find out whether the first order characteristics of Pm3m-Fm3m transitions are retained in the solid solutions. We have therefore examined the crystallography of the Pm3m and Fm3m phases of the solid solutions as functions of temperature from these data, coefficients of expansion of the two structures have been calculated. 

The ionic charge carriers in ionic crystals are the point defects.1 2 23,24 They represent the ionic excitations in the same way as H30+ and OH-ions are the ionic excitations in water (see Fig. 1). They represent the chemical excitation upon the perfect crystallographic structure in the same way as conduction electrons and holes represent electronic excitations upon the perfect valence situation. The fact that the perfect structure, i.e., ground structure, of ionic solids is composed of charged ions, does not mean that it is ionically conductive. In AgCl regular silver and chloride ions sit in deep Coulomb wells and are hence immobile. The occurrence of ionic conductivity requires ions in interstitial sites, which are mobile, or vacant sites in which neighbors can hop. Hence a superionic dissociation is necessary, as, e.g. established by the Frenkel reaction ... 

The structures of ionic solids will.be discussed in detail in Chapter 16. 

Ionic solids are stable, high-melting-point substances held together by the strong electrostatic forces that exist between oppositely charged ions. The principles governing the structures of ionic solids were introduced in Section 13.5. In this section we will review and extend these principles. 

The chapter will assume an understanding of Hess law and the thermodynamic terms enthalpy of formation and free energy, together with some prior knowledge of the structures of ionic solids in terms of the close packing of spheres. 

The general structure of ionic solids results in the following properties ... 

Before we embark upon a discussion of the structures of ionic solids, we must say something about the sizes of ions, and define the term ionic radius. The process of ionization (e.g. equation 5.4) results in a contraction of the species owing to an increase in the effective nuclear charge. Similarly, when an atom gains an electron (e.g. equation 5.5), the imbalance between the number of protons and electrons causes the anion to be larger than the original atom. 

We shall for the most part illustrate our general arguments by examples of typically ionic solids oxides, halides, hydrates, etc. However, it must be appreciated that factors important in determining the structures of ionic solids also decide the stereochemistry of discrete molecules and complex ions. We shall not, therefore, hesitate to discuss the stereochemistry of isolated molecules or complex ions in solution insofar as they are relevant to the general problem of the stereochemistry of metal ions. 

Describe in general terms the structure of ionic solids such as NaCl. How are the ions packed in the crystal ... 

The structures of ionic solids can be visualized best by thinking of the ions as spheres packed together as efficiently as possible. For example, in NaCl the larger Cl ions are packed together much like one would pack balls in a box. The smaller Na ions occupy the small spaces ("holes") left among the spherical Cl ions. 

Describe, in general, the structures of ionic solids. Compare and contrast the structure of sodium chloride and zinc sulfide. How many tetrahedral holes and octahedral holes are there per closest packed anion In zinc sulfide, why are only one-half of the tetrahedral holes filled with cations ... 

When spheres of a given size are close-packed, the spaces between the layers of spheres (the voids or interstices) can be filled with smaller spheres. If the spheres represent cations and anions, the structures of ionic solids can be visualized. There are two types of interstices between layers of close-packed atoms - tetrahedral holes or interstices and octahedral holes or interstices. Tetrahedral holes are formed when one sphere in a layer fits over or under three spheres in a second layer. Octahedral holes are formed when three spheres in one layer fit over or under three spheres in a second layer. The two types of holes have different numbers per close-packed sphere, different sizes, and different coordination numbers and coordination geometries. The coordination number of the anion would be the number of cations in contact with the anion. The coordination geometry of the anion would be the geometrical arrangement of the cations which surround the anion. Related statements can be made regarding the coordination number and coordination geometry of the cation. 

We examine the structures and properties of solids held together by the mutual attractions between cations and anions. We learn how the structures of ionic solids depend on the relative sizes of the ions and their stoichiometry. 

Use the molecular-orbital model to qualitatively predict the trends in melting point, boiling point, and hardness of metals. [Section 12.4] Predict the structures of ionic solids from their ionic radii and empirical formula. [Section 12.5]... 

We should mention that in the few cases in which the variation in electron density in a crystal has been accurately determined (e.g. NaCl), the minimum electron density does not in fact occur at distances from the nuclei indicated by the ionic radii in general use e.g. in LiF and NaCl, the minima are found at 92 and 118 pm from the nucleus of the cation, whereas tabulated values of rLj+ and rNa+ are 76 and 102 pm, respectively. Such data make it clear that discussing the structures of ionic solids in terms of the ratio of the ionic radii is, at best, only a rough guide. For this reason, we restrict our discussion of radius ratio rules to that in Box 6.4. 

Predict the structures of ionic solids from their ionic radii and empirical formula. (Section 12.5)... 

Boyce JB, Mikkelsen JC (1985) Local structure of ionic solid-solutions—extended x-ray absorption fine-structure study. Phys Rev B31 6903-6905... 

Before we discuss the structures of ionic solids, we must say something about the sizes of ions, and define the term ionic radius. The process of ionization (e.g. eq. 6.4) results in a... 

Simple interatomic potentials for describing the structure of ionic solids consist of two-body terms only and have the form (1) with Ysr = Yb-... 

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