Ionic solids formation

Measurements of photoconductivity and of the Hall potential [367] are accurate and unambiguous methods of detecting electronic conduction in ionic solids. Kabanov [351] emphasizes, however, that the absence of such effects is not conclusive proof to the contrary. From measurements of thermal potential [368], it is possible to detect solid-solution formation, to distinguish between electronic and positive hole conductivity in semi-conductors and between interstitial and vacancy conductivity in ionic conductors. 

Born-Habcr cycle A closed series of reactions used to express the enthalpy of formation of an ionic solid in terms of contributions that include the lattice enthalpy. 

Figure 2.3 The potential-energy curve for Li+ + F ionic bond formation according to quantum mechanics (solid line) or classical electrostatics (dotted line).

Ionic bonding involves the transfer of electrons from one atom to another. The more electronegative element gains electrons. The less electronegative element loses electrons. This results in the formation of cations and anions. Usually, an ionic bond forms between a metal and a nonmetal. The metal loses electrons to form a cation. The nonmetal gains electrons to become an anion. The attraction of the opposite charges forms an ionic solid. 

Lattice enthalpies of ionic solids can be predicted from several equations, which account for the coulombic interactions [45-47,49]. The estimates can then be used to derive the standard enthalpies of formation, by equation 2.47. However,... 

The MEG model has been extensively used to determine lattice energies and interionic equilibrium distances in ionic solids (oxides, hydroxides, and fluorides Mackrodt and Stewart, 1979 Tossell, 1981) and defect formation energies (Mack-rodt and Stewart, 1979). Table 1.21 compares the lattice energies and cell edges of various oxides obtained by MEG treatment with experimental values. 

A Born-Haber cycle is the application of Hess s Law to the enthalpy of formation of an ionic solid at 298 K. Hess s law states that the enthalpy of a reaction is the same whether the reaction takes place in one step or in several. A Born-Haber cycle for a metal chloride (MCI) is depicted in Figure 1.56 the metal chloride is formed from the constituent elements in their standard state in the equation at the bottom, and by the clockwise series of steps above. From Hess s law, the sum of the enthalpy changes for each step around the cycle can be equated with the standard enthalpy of formation, and we get that ... 

A variety of techniques has been employed to investigate aliovalent impurity-cation vacancy pairs and other point defects in ionic solids. Dielectric relaxation, optical absorption and emission spectroscopy, and ionic thermocurrent measurements have been most valuable ESR studies of Mn " in NaCl have shown the presence of impurity-vacancy pairs of at least five different symmetries. The techniques that have provided a wealth of information on the energies of migration, formation and other defect energies in ionic solids are diffusion and electrical conductivity measurements. Electrical conductivity in ionic solids occurs by the motion of ions through vacancies or of interstitial ions. In the case of motion through vacancies, the conductivity, a, is given by... 

This chapter consists of two sections, one being a general discussion of the stable forms of the elements, whether they are metals or non-metals, and the reasons for the differences. The theory of the metallic bond is introduced, and related to the electrical conduction properties of the elements. The second section is devoted to a detailed description of the energetics of ionic bond formation. A discussion of the transition from ionic to covalent bonding in solids is also included. 

Lattice energies can be related to the heats of formation AH of ionic solids through the Bom-Haber cycle, which is the counterpart of the thermochemical cycle for covalent compounds given in Section 2.7. 

In principle, we can use the Born-Haber cycle to predict whether a particular ionic compound should be thermodynamically stable, on the basis of calculated values of U, and so proceed to explain all of the chemistry of ionic solids. The relevant quantity is actually the free energy of formation, AGf, and this is calculable if an entropy cycle is set up to complement the Born-Haber enthalpy cycle. However, in practice AHf dominates the energetics of formation of ionic compounds. 

Extrinsic Crystal Self-Diffusion. Charged point defects can be induced to form in an ionic solid by the addition of substitutional cations or anions with charges that differ from those in the host crystal. Electrical neutrality demands that each addition results in the formation of defects of opposite charge that can contribute to the diffusivity or electronic conductivity. The addition of aliovalent solute (impurity) atoms to an initially pure ionic solid therefore creates extrinsic defects.10... 

Ionization Potential. In the formation of ionic solids from atoms we assumed several steps, as follows first, the detachment of an electron from one atom, producing a positive ion (Eq. 3.3) second, the attachment of an electron to another atom giving a negative ion (Eq. 3.4) finally, the condensation of a large number of positive and negative ions to make an... 

Given the enthalpy of formation of an ionic solid, an experimental lattice energy can be obtained by thermochemical analysis. For example, the formation of crystalline sodium chloride is broken down as follows ... 

We first look at the fluorides of barium. Only BaF2 is known, a typically ionic solid having the fluorite (8 4) structure. From Table 5.2, we see that the calculated lattice energy is very close to the experimental value in other words, we can calculate the enthalpy of formation of BaF2(s) almost within the limits of experimental uncertainty. Why have BaF3 and BaF not been prepared Presumably they are thermodynamically unstable with respect to other species. In order to verify this supposition, let us estimate the enthalpies of formation AHf of BaF(s) and BaF3(s), assuming these to be ionic. 

The donor/acceptor reactions considered so far all involve the formation of new bonds, using erstwhile nonbonding electron pairs. A closely-related type of reaction is one in which an ion is transferred from one molecule to another, to form an ionic solid, e.g. the proton-transfer reaction ... 

The principal exothermic term in a stepwise thermochemical analysis (such as we found useful in Chapter 5) will be the lattice energy of the product. This is not readily obtainable experimentally (unlike the lattice energies of simple ionic solids) and its magnitude is not amenable to any simple analysis. As we shall see a little later, a purely ionic description of such products is often inappropriate anyway. Let us focus attention on the ease of formation of AX x and BX +1. The removal of X- from AXm will be favoured by ... 

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