Structure of crystalline solids Flashcards
What makes a crystal not dense?
the random packing
What makes a crystal dense?
ordered packing
Dense, ordered packed structures tend to have lower or higher energies?
Lower
What are properties of crystalline materials?
atoms pack in periodic, 3D arrays
What are properties of non-crystalline materials?
atoms have no periodic packing
- complex structures
- rapid cooling
What are amorphous materials?
non-crystalline
what are examples of crystalline materials?
metals
many ceramics
some polymers
What properties of metallic crystal structures?
densely packed
have the simplest crystal structures
Why are metallic crystal structures densely packed? (4)
- Typically, only one element is present, so all atomic radii are the same.
- Metallic bonding is not directional.
- Nearest neighbour distances tend to be small in
order to lower bond energy. - Electron cloud shields cores from each other
What are the three simplest crystal structures?
Simple cubic structure
Body centered cubic structure
Face centered cubic structure
What are simple cubic structures? (2)
Rare due to low packing density (only Po has this structure)
Close-packed directions are cube edges
How many coordination (nearest neighbours) does it have? SC
6
What is the APF of a simple cubic structure?
0.52
How many atom/unit cell are there is a SC?
1 atom/unit cell
8 x 1/8 = 1
What are body centered cubic structures?
Atoms touch each other along cube diagonals
What are examples of BCC structures?
Cr, W, Fe(a), Tantalum, Molybdenum
What is the coordination of a BCC?
8
How many atoms are there per cell in a BCC?
1 center + 8 corneres x 1/8 = 2 atoms/unit cell
What are the close-packed directions in a BCC?
length = 4R = √3 a
What is the APF for a BCC?
0.68
What is a face centred cubic structure?
Atoms touch each other along face diagonals
What are examples of face centred cubic structure?
Al, Cu, Au, Pb, Ni, Pt, Ag, Si
What is the coordination of a FCC?
12
How many atoms are there per cell in a FCC?
6 face x 1/2 + 8 corners x 1/8 = 4 atoms/unit cell
What is the APF for a face-centered cubic structure?
0.74 which is the maximum achievable APF
What are the close-packed directions for a FCC?
length = 4R = √2a
What is the Hexangonal close-packed structure?
6 then 3 then 6 stacking structure
What is the coordination for the HCP?
12
What is the APF for the HCP?
0.74
How many atoms/unit cell are there in a HCP?
6
What are examples of HCP?
Cd, Mg, Ti, Zn
What is c/a in HCP
1.633
Level these from highest to lowest density: metals, ceramics and polymers
ƿmetals > ƿceramics > ƿpolymers
Why do metals have higher densities than ceramics and polymers? (2)
- close-packing
(metallic bonding) - often large atomic masses
Why do ceramics have lower density than metals but a higher density than polymers? (2)
- less dense packing
* often lighter elements
Why do polymers have the lowest density?
- low packing density (often amorphous)
* lighter elements (C,H,O)
What are polycrystals?
Crystals that have single crystal grains that are randomly oriented, the overall competent properties are not directional
What is the difference between single and polycrystals? (properties)
Single crystal properties vary with direction: anisotropic
Polycrystals properties may/may not vary with direction and are randomly oriented: isotropic, if grains are textured, anisotropic
What is polymorphism?
Two or more distinct crystal structures for the same material (allotropy/polymorphism)
