1.7: Crystallography
- Page ID
- 88066
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Learning Objectives
On completion of this TLP you should:
- Have an understanding of the basic concepts of crystallography, i.e. lattices, motifs, symmetry elements etc.
- Be able to identify lattices and symmetry elements within those lattices.
- Know about the different types of unit cell.
- Understand the idea of close-packing and packing efficiency.
- Be familiar with the different crystal systems and Bravais lattices.
- 1.7.1: Introduction to Crystals
- Crystallography is of importance not only to chemists and physicists, but also to geologists, amateur minerologists and "rock-hounds". In this lesson we will see how the external shape of a crystal can reveal much about the underlying arrangement of its constituent atoms, ions, or molecules.In this lesson we will see how the external shape of a crystal can reveal much about the underlying arrangement of its constituent atoms, ions, or molecules.
- 1.7.2: Cubic Lattices and Close Packing
- When substances form solids, they tend to pack together to form ordered arrays of atoms, ions, or molecules that we call crystals. Why does this order arise, and what kinds of arrangements are possible? We will limit our discussion to cubic crystals, which form the simplest and most symmetric of all the lattice types. Cubic lattices are also very common — they are formed by many metallic crystals, and also by most of the alkali halides, several of which we will study as examples.
- 1.7.3: Ionic and Ion-Derived Solids
- In this section we deal mainly with a very small but imporant class of solids that are commonly regarded as composed of ions. We will see how the relative sizes of the ions determine the energetics of such compounds. And finally, we will point out that not all solids that are formally derived from ions can really be considered "ionic" at all.