Chapter-3 - Chapter 3 Structure of Crystalline Solids...

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1 3-0 Chapter 3 Structure of Crystalline Solids Learning Objectives • Classify materials according to the arrangement of atoms/ ions • Describe structure of crystalline materials in terms of crystal structure, unit cell, and lattice • Quantitatively compare materials in terms of density and packing factors 3-1 3.2 Crystal Structure- Fundamental Concepts •Matter can be structurally classified in terms of the arrangement of atoms and ions •Solids that exhibit repetitive atomic positions are referred to as crystalline •Solids that lack repetitive atomic positions are referred to as noncrystalline or amorphous noncrystalline SiO 2 crystalline SiO 2
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2 3-2 •A crystalline solid that exhibits only one orientation of arrangement is referred to as a single crystal •Solids that exhibit more than one orientation are referred to as polycrystalline. • Regions of similar orientation are known as grains . • These regions are typically separated by amorphous structures known as grain boundaries . 3-3 •For crystalline materials, we will describe structure in terms of a crystal structure . •Crystal structures can be systematically differentiated in terms of its: Lattice - a periodic and geometric arrangement of points in space. These points are occupied by atoms or ions. •The specific atom or ion type (identity and number) is known as the motif or basis of the crystal structure. Crystal= lattice+basis.
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3 3-4 The smallest division of the crystal structure that represents both the lattice and basis is known as the unit cell . 3-5 3.4 Metallic Crystal Structures Face Centered Structures Hexagonal Close- Packed Structures Body Centered Structures
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4 3-6 Many common metals exist in one of the three crystal structures. Atomic radii are deduced from X-ray data and geometric consideration of crystal structures. 3.4 Metallic Crystal Structures 3-7 We will characterize crystal structures in four quantitative fashions. Cube edge length ( a ) Atomic packing factor Coordination number Density 2 2 (FCC) 4 (BCC) 3 a R R a = = 3.4 - 3.5
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5 3-8 We will characterize crystal structures in four quantitative fashions. Cube edge length ( a ) Atomic packing factor Coordination number Density volume of atoms in unit cell volume of unit cell APF = 3-9 The number of atoms per unit cell refers to the contribution/ presence of an atom or ion to/within the unit cell. Correctly establishing the number of atoms per unit cell is essential to calculations of packing factors and density.
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6 3-10 The coordination number refers to the number of nearest neighbors ( touching atoms) for a specific lattice point.
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