lecture3.pdf - Chapter 3 The structure of crystalline...

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1 Chapter 3: The structure of crystalline solids Outline Fundamental concepts Unit cells Metallic crystal structure Density and polymorphism Crystal systems Fundamental concepts Crystalline materials: atoms are situated in a repeating or periodic array over large atomic distances Crystalline structure: how atoms, ions, or molecules are arranged spatially Lattice: a three-dimensional array of points coinciding with atom position Fig 3.1 (a) a hard sphere unit cell (b) a reduced-sphere unit cell, (c) an aggregate of many atoms (c) 1 2
2 Unit cells Small repeating units in a crystal Basic structural unit or building block of the crystal structure Not unique A highest level of geometrical symmetry Close-packed directions are cube edges. • Atoms per unit cell = 8 x 1/8 =1 (Courtesy P.M. Anderson) Simple cubic structure (SC) Coordination # (nearest neighbors) = 6 Rare due to low packing density (only Po has this structure) 3 4