L3 - Crystallographic Structures Flashcards
1
Q
What type of stacking is used by HCP?
A
- ABABABAB
- atoms in each layer touch each other
- B layer atoms are on top of gaps in the A layer
- A layers are always in same position
- Close packed within layers
- not necessarily between layers though
2
Q
What determines the stacking sequence of a structure?
A
- The order in which different layers are placed in a given structure
3
Q
What does HCP stand for?
A
- Hexagonal Close Packing
4
Q
How many atoms in a HCP unit cell?
A
6
- HCP unit cell is hexagonal shaped
- 6 (1/3) + 1 in middle in A(/B)
- therefore 3
- 3 in B(/A)
5
Q
What sort of stacking is used by FCC?
A
- ABC
- A and B layers same as HCP
- C layers are above gaps in B layer
- Planes run diagonally through unit cell
- Planes are all close packed within each other
- Planes are oriented in many different directions
6
Q
What sort of stacking is used in BCC?
A
- none given as has no close packed planes
- has close packed directions instead
7
Q
What are close packed directions?
A
- directions within a structure where atoms touch each other
8
Q
What are close packed planes?
A
- planes in which there are the most atoms per unit area
- or planes in which atoms are touching each other
9
Q
What is polymorphism?
A
- When materials change crystalline structure
- usually changes as temp changes
10
Q
How are directions defined?
A
- square brackets []
- from origin to coordinate of exit from unit cube
- general direction is for symmetrical directions
- denoted by <>
11
Q
How are planes defined?
A
- denoted by curly brackets ()
- take intercepts of plane and axes
- take reciprocals of intercepts
- take the integer values of this
