Ionic alkali halide

The sodium chloride structure is adopted by a large number of compounds, from the ionic alkali halides NaCl and KC1, to covalent sulfides such as PbS, or metallic oxides such as titanium oxide, TiO. Slip and dislocation structures in these materials will vary according to the type chemical bonding that prevails. Thus, slip on 100 may be preferred when ionic character is suppressed, as it is in the more metallic materials. 

We see in the AB structure map at the end of the book that the two most common structure types, NaCl and CsCl, are well ordered into separate domains. The ionic alkali halides containing NaCl and CsCl are themselves located in the upper left-hand corner of the map. Running across to the right... 

A wide open series of solid solution systems, such as ionic alkali halides KCl j jBr binary and pseudobinary metallic or semiconducting alloys Ag—Cu AI—-Cu... 

SIMPLE IONIC MODEL FOE. THE ALKALI HALIDES The total potential energy for an ionic alkali halide molecule is... 

Insulators in which atoms have completely filled electronic shells, such as the noble elements or the purely ionic alkali halides (composed of atoms from columns I and VII of the Periodic Table) are actually the simplest cases the atoms or ions in these solids differ very little from isolated atoms or ions, because of the stability of the closed electronic shells. The presence of the crystal produces a very minor perturbation to the atomic configuration of electronic shells. The magnetic behavior of noble element solids and alkali halides predicted by the analysis at the individual atom or ion level is in excellent agreement with experimental measurements. 

Rasaiah J C 1970 Equilibrium properties of ionic solutions the primitive model and its modification for aqueous solutions of the alkali halides at 25°C J. Chem. Phys. 52 704... 

The ionic bond is the most obvious sort of electrostatic attraction between positive and negative charges. It is typified by cohesion in sodium chloride. Other alkali halides (such as lithium fluoride), oxides (magnesia, alumina) and components of cement (hydrated carbonates and oxides) are wholly or partly held together by ionic bonds. 

A problem with studies on inert gas is that the interactions are so weak. Alkali halides are important commercial compounds because of their role in extractive metallurgy. A deal of effort has gone into corresponding calculations on alkali halides such as LiCl, with a view to understanding the structure and properties of ionic melts. Experience suggests that calculations at the Hartree-Fock level of theory are adequate, provided that a reasonable basis set is chosen. Figure 17.7 shows the variation of the anisotropy and incremental mean pair polarizability as a function of distance. 

Turning next to an ionic crystal, where the ions may be regarded as spheres, the total volume of the crystal is equal to the volumes of these spheres, together with the appropriate amount of void space between the spheres. To take the simplest case, it is convenient to discuss a set of substances, all of which have the same crystalline structure—for example, the 17 alkali halide crystals that have the NaCl structure. 

We have performed ionic mobility studies on mica and in alkali halide surfaces. Here we shall describe some results obtained on mica with different surface ions. Alkali halides will be discussed in detail in the next section. 

Hanlon JE, Lawson AW (1959) Effective ionic charge in alkali halides. Phys Rev 113(2) 472-478... 

Based on the ionic radii, nine of the alkali halides should not have the sodium chloride structure. However, only three, CsCl, CsBr, and Csl, do not have the sodium chloride structure. This means that the hard sphere approach to ionic arrangement is inadequate. It should be mentioned that it does predict the correct arrangement of ions in the majority of cases. It is a guide, not an infallible rule. One of the factors that is not included is related to the fact that the electron clouds of ions have some ability to be deformed. This electronic polarizability leads to additional forces of the types that were discussed in the previous chapter. Distorting the electron cloud of an anion leads to part of its electron density being drawn toward the cations surrounding it. In essence, there is some sharing of electron density as a result. Thus the bond has become partially covalent. 

Some values for the enthalpy of formation of Schottky defects in alkali halides of formula MX that adopt the sodium chloride structure are given in Table 2.1. The experimental determination of these values (obtained mostly from diffusion or ionic conductivity data (Chapters 5 and 6) is not easy, and there is a large scatter of values in the literature. The most reliable data are for the easily purified alkali halides. Currently, values for defect formation energies are more often obtained from calculations (Section 2.10). 

Frenkel defects on the anion sublattice show only anion migration and hence have fa close to 1. The alkali halides NaF, NaCl, NaBr, and KC1 in which Schottky defects prevail and in which the cations and anions are of similar sizes have both cation and anion contributions to ionic conductivity and show intermediate values of both anion and cation transport number. 

The simple theory of electronegativity fails in this discussion because it is based merely on electron transfer energies and determines only the approximate number of electrons transferred, and it does not consider the interactions which take place after transfer. Several calculations in the alkali halides of the cohesive energy (24), the elastic constants (24), the equilibrium spacing (24), and the NMR chemical shift 17, 18, 22) and its pressure dependence (15) have assumed complete ionicity. Because these calculations based on complete ionicity agree remarkably well with the experimental data, we are not surprised that the electronegativity concept of covalency fails completely for the alkali iodide isomer shifts. 

Overlapping Ion Model. The ground-state wave function for an individual electron in an ionic crystal has been discussed by Lowdin (24). To explain the macroscopic properties of the alkali halides, Lowdin has introduced the symmetrical orthogonaliz tion technique. He has shown that an atomic orbital, x//, in an alkali halide can be given by... 

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

The concept that the ionic radius is relatively independent of the structure of the solid arose intuitively from experimental observations carried out on alkali halides, which are ionic solids par excellence. Figure 1.3 shows the evolution of interatomic distances in alkali halides as a function of the types of anion and cation, respectively. Significant parallelism within each of the two families of curves may be noted. This parallelism intuitively generates the concept of constancy of the ionic radius. 

Fumi F. G. and Tosi M. P. (1964). Ionic sizes and Born repulsive parameters in the NaCl-type alkali halides, I The Huggins-Mayer and Pauling forms. J. Phys. Chem. Solids, 25 31 3. 

With ionic crystals, there are some rather interesting possibilities. A large part of the perturbation which a free surface introduces is associated with the change in the electrostatic environment of an ion in going from the interior to the surface. If the normally filled valence band is associated with the anions (as is the case with the alkali halides and with certain n-type semiconducting oxides), the surface perturbation acts in the direction of producing a band of surface states with its center lying above the center of the normal anion band. This anion surface band will normally be completely filled. Conversely, for the normally empty cation band (the... 

The failure of Pauling s criterion for the fraction of ionic character of a bond (/ijer) in the case of alkali halides stems from the fact that the criterion fails to include the far from negligible polarization deformation of the ions in these completely ionic substances Rittner, Ref. 17, p. 1035). 

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