Lattice types, ionic compounds

The relative sizes of the cation and anion determine the type of lattice an ionic compound adopts. For example, although caesium and sodium are both in the same group of the periodic table, the chlorides crystallize with different types of lattice. Sodium chloride adopts the simple cubic structure (Chapter 4), whereas caesium chloride adopts the lattice shown in Figure 15.10. In caesium chloride, the caesium ions cannot get as close to the chloride ions as the smaller sodium ions. Eight caesium ions can pack around a chloride ion if they are positioned at the corners of a cube. The structure of ionic lattices is determined by X-ray crystallography (see Chapter 21, and Chapter 22 on the accompanying website). 

From the standpoint of energy, the processes of separating the crystal lattice and solvating the ions can be related by means of a thermochemical cycle of the Born-Haber type. For an ionic compound MX, the cycle can be shown as follows ... 

The remaining compounds listed in Table II all adopt structures with infinite metal-metal bonded chains consisting of octahedral cluster units fused on opposite edges. However, because of the large difference in effective ionic radius of the cations concerned, very different lattice types are dictated. The compounds NaMoi 06 (19,22) and Bas(Moit06)8 (17) adopt tunnel structures with the Na+ or Ba2+ ions located in sites along the tunnels with 8-fold coordination by oxygen atoms. 

Before introducing experimental results for crystals grown by these methods, we shall consider the possible crystal defects of GaAs for a better understanding of experimental results. It is expected that at higher P, vacancies of Ga lattice sites or interstitial As may occur and at lower Pas vacancies of As or interstitial Ga may occur. Because GaAs is considered to be an ionic compound Ga-As, these defects at higher Pas act as donors (n-type) and those at lower P 1S act as acceptors (p-type). As shown below, the experimental results are not so simple. 

In order for an ionic compound to dissolve, the Madelung energy or electrostatic attraction between the ions in the lattice must be overcome. In a solution in which the ions are separated by molecules of a solvent with a high dielectric constant ( H 0 81.7 ) the attractive force will be considerably less. The process of solution of an ionic compound in water may be considered by a Bom-Haber type of cycle. The overall enthalpy of the process is the sum of two terms, the enthalpy of dissociating the ions from the lattice (the lattice energy) and the enthalpy of introducing the dissociated ions into the solvent (the solvation energy) ... 

The relationship between cubic close-packed (ccp) structures and ionic compounds of type B1 is obvious. Interstitial sites with respect to metal positions are at fractional coordinates of the type 00 and equivalent to the ionic sites in Bl. The Madelung constant of Al type metals with interstitially localized free electrons is therefore the same as that of rocksalt structures. It is noted that the interstitial sites define the same face-centred lattice as the metal ions. 

Metal oxides, the products of oxidation of metals are ionic compounds with the metal ions and oxide ions arranged in arrays in the crystal lattices. When the metal oxide contains excess metal ions in interstitial positions, they are known as n-type (or negative carrier type) oxides. When the metal oxides contain vacant sites (deficient in metal ions) in the lattice the oxides are known as p-type (positive carrier type). 

Metallic and nonmetalbc elements can react with each other to form com-ponnds by transferring electrons from the metal atoms to the nonmetal atoms. The ions formed attract each other becanse of their opposite charges, and these attractions are called ionic bonds. However, in a sobd ionic compound, a single pair of ions does not bond together instead, an almost inconceivably huge number of both types of ions forms a lattice that extends in three dimensions. The three-dimensional nature of the sodium chloride structure (Figure 5.9) is typical of ionic solids. 

The ionic nature of these compounds (the fact that charged particles are present) can be shown by experiments in which the ions are made to carry an electric current. Pure water does not condnct electricity well. However, if a compound that consists of ions is dissolved in water and the solution is placed between electrodes in an apparatns like that shown in Figure 5.10, the solution will conduct electricity when the electrodes are connected to the terminals of a battery. Each type of ion moves toward the electrode that has the opposite charge of that of the ion. That is, cations migrate to the negative electrode, called the cathode, and anions migrate to the positive electrode, called the anode. (The names cation and anion were derived from the words cathode and anode.) For electricity to be conducted, the ions must be free to move. In the solid state, an ionic compound will not conduct because the ious are trapped in the lattice. However, if the compound is heated until it melts or if it is dissolved in water, the liquid compound or solution will conduct electricity because the ions are free to move. 

A strong electrolyte in aqueous solution may be represented as separate ions because the ions of each type are free to move about independently of the ions of the other type. However, an ionic solid that is not dissolved in water is not written as separate ions the oppositely charged ions in the solid lattice of an ionic compound are not independent of each other (Figure 9.2). 

The atomic and ionic properties of an element, particularly IE, ionic radius and electronegativity, underly its chemical behaviour and determine the types of compound it can form. The simplest type of compound an element can form is a binary compound, one in which it is combined with only one other element. The transition elements form binary compounds with a wide variety of non-metals, and the stoichiometries of these compounds will depend upon the thermodynamics of the compound-forming process. Binary oxides, fluorides and chlorides of the transition elements reveal the oxidation states available to them and, to some extent, reflect trends in IE values. However, the lEs of the transition elements are by no means the only contributors to the thermodynamics of compound formation. Other factors such as lattice enthalpy and the extent of covalency in bonding are important. In this chapter some examples of binary transition element compounds will be used to reveal the factors which determine the stoichiometry of compounds. 

The actual arrangement will depend on the relative sizes and charges on the ions involved, but there will always be a regular arrangement, called a crystal lattice. This is generally true for any ionic compound, except that the type of arrangement and overall shape will be different in different materials. 

The experimental (Born-Haber) and theoretical (Born-Mayer) estimates of lattice energy agree well for typically ionic compounds such as the Gp.IA halides, and show that the theoretical picture of complete electron transfer is satisfactory. Values for salts of 18-electron type cations, however, often show considerable discrepancies. Representative differences between C/ xpi and are Rbl, 4 kcal Cdig, 86 kcal PbOg, 212 kcal. These indicate a gradual departure from the purely ionic condition the small cations tend to retain some hold on their electrons and the binding acquires considerable covalent character. 

In simple ionic compounds, each ion only occupies one type of environment, with all the ions of the same type having exactly the same geometric relationship to all the other ions in the crystal lattice. In more complicated ionic compounds, it is possible for ions of one species to occupy one of a limited number of environments, but this is the exception rather than the rule at this level. 

The intracrystalline channel cavity-pore-cage system in zeolites is surrounded by the lattice and therefore is submitted to the zeolite crystal field. This results in solvent-like and even electrolyte-type properties. One has seen above how cations could be easily exchangeable. It may also exist an interaction between any occluded ionic compound and the zeolitic framework. Salts, especially salts of univalent anions, have been shown to penetrate the zeolite structure and fill the available space even if the openings of the cavities (as the 0 -ring of 0.24 nm in size in sodal te cage of Y zeolite) is smaller than the size of the anion (CIO, NO for instance). The interesting feature is then the enhanced thermal stability of the occluded salt. 

The binary halides of the elements span a wide range of stoichiometries, stmcture types and properties which defy any but the most grossly oversimplified attempt at a unified classification. Indeed, interest in the halides as a class of compound derives in no small measure from this very diversity and from the fact that, being so numerous, there are many examples of well-developed and well-graded trends between the limiting cases. Thus the fluorides alone include OF2, one of the most volatile molecular compounds known (bp —145°), and Cap2, which is one of the least-volatile ionic compounds (bp 2513°C). Between these extremes of discrete molecules on the one hand, and 3D lattices on the other, is a continuous sequence of oligomers, polymers and extended layer lattices which may be either predominantly covalent [e.g. CIP, (MoPs)4,... 

Ionic bonds are described as well. The transition from covalent over polar covalent to ionic bonds is veiy fluent and depends on the difference in electronegativity between the atoms. The covalent bonds consist of sharing an electron pair and ionic bonds are electrostatic interactions between a cation and an anion. Solid ionic compounds are often arranged in lattice structures with many similarities with the lattice structures that we saw for the metallic compounds. The type of lattice structure for solid ionic compound depends on the ration between the radius of the cation and anion. 

There are 7 erystal systems and 14 types of unit cells that occur in nature, but we will be eoneerned primarily with the cubic system, which gives rise to the eubic lattiee. The solid states of a majority of metallic elements, some covalent eompounds, and many ionic compounds occur as cubic lattices. (We also describe the hexagonal unit eell a bit later.) There are three types of cubic unit cells within the cubie system ... 

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