Lesson_2 Behaviour of metals (9) - 76 CHAPTER 3 Atomic and...

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3-7 Crystal Structures of Ionic Materials Many ceramic materials (Chapter 15) contain considerable fraction of ionic bonds between the anions and cations. These ionic materials must have crystal structures that assure electrical neutrality and stoichiometry, yet permit ions of di¤erent sizes to be packed e‰ciently. As mentioned before, ionic crystal structures can be viewed as close-packed structures of anions. Anions form tetrahedra or octahedra, allowing the cations to fit into their appropriate interstitial sites. In some cases, it may be easier to visualize coordination polyhedra of cations with anions going to the interstial sites. Some typical structures of ionic materials are discussed here. Visualization of Crystal Structures Using Computers Before we begin to describe di¤erent crystal structures, it is important to note that there are many new software programs and tools that have become available recently. These are quite e¤ective in better understanding the crystal structure concepts as well as other concepts discussed in Chapter 4. One example of a useful program is the CaRIne TM software. Cesium Chloride Structure Cesium chloride (CsCl) is simple cubic, with the ‘‘cubic’’ interstitial site filled by the Cl anion [Figure 3-23(a)]. The radius ratio, r Cs þ r Cl ± 1 ¼ 0 : 167 nm 0 : 181 nm ¼ 0 : 92 TABLE 3-5 9
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  • Spring '14
  • Colombo
  • Physics, Material Science, Plastics, Manufacturing, Metals, Ion, Sodium chloride, radius ratio, sodium chloride structure

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