Close packing in crystals

How do atoms arrange themselves to form solids?
• Fundamental concepts and language
• Unit cells
• Crystal structures
?? Face-centered cubic
?? Body-centered cubic
?? Hexagonal close-packed
• Close packed crystal structures
• Density computations
• Types of solids
Single crystal
Polycrystalline
Amorphous
Types of Solids
Crystalline material: atoms self-organize in a periodic
array
Single crystal: atoms are in a repeating or periodic array
over the entire extent of the material
Polycrystalline material: comprised of many small
crystals or grains
Amorphous: disordered – lack of a systematic atomic
arrangement
Metallic Crystal Structures
?? Metals are usually (poly)crystalline; although formation
of amorphous metals is possible by rapid cooling
?? As we learned in Chapter 2, the atomic bonding in metals
is non-directional ? no restriction on numbers or
positions of nearest-neighbor atoms ? large number of
nearest neighbors and dense atomic packing
?? Atomic (hard sphere) radius, R, defined by ion core
radius - typically 0.1 - 0.2 nm
?? The most common types of unit cells are
• faced-centered cubic (FCC)
• body-centered cubic (BCC)
• hexagonal close-packed (HCP).