Structure of NaCl and related compounds

The data listed in Table 10.1.4 are useful in understanding the structure and properties of many compounds. Two examples are presented below. 

The effective ionic radii listed in Table 4.2.2 can be calculated from the experimentally determined values of the ionic distances or the parameters of the unit cells. For example, NaH has the structure of NaCl type with a = 488 pm, so the Na+ H- distance is 244 pm. Taking the radius of Na+ as 102 pm, the radius of H- in the NaH crystal is 142 pm. 

The alkaline-earth oxides MO are composed of M2+ and O2- stabilized by electrostatic forces. The electrostatic Coulombic energies are directly proportional to the product of the two ionic charges and inversely proportional to the interionic distance. The alkaline-earth oxides MO are 2 2 valence compounds and have the same electronic charge for M2+, but the radii of M2+ and the 

Various kinds of packing defects exist in the ionic crystals of NaCl type. A pair of cation and anion may be shifted from their stable positions toward the surface of the crystal, thus leaving behind a pair of vacancies. This is called the Schottky defect. The cation may leave its stable position and enter into an interstitial site. The formation of an interstitial cation and a vacancy is called the Frenkel defect. In addition to these two common kinds of defects, the presence of impurity atoms, atoms of varied valence, vacancies, and/or interstitial atoms is also possible. Some other important defects are discussed below. 

A color center or F-center is formed from diffusion of a small quantity of M+ ion into an ionic crystal MX. Since the crystal must keep its charge neutrality, additional electrons readily move to fill the vacancies normally occupied by anions. Thus the composition of the crystal becomes (M+)i+i(X e ). The origin of the color is due to electronic motion, and a simple picture of an electron in a vacancy is illustrated by the particle in a three-dimensional box problem, which is discussed in Section 1.5.2. 

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