Metallic crystal structures

Question 1

In terms of A,B,C… what does a hexagonal close-packed (hcp) structure look like?

Answer 1

ABABABABA

Question 2

Describe the HCP structure

Answer 2

1st and 3rd layer identical, 2nd layer fits into dimples of first either tetrahedral or octahedral

Question 3

What is a tetrahedral hole?

Answer 3

4 surrounding atoms

Question 4

What is an octahedral hole?

Answer 4

6 surrounding atoms

Question 5

In terms of A,B,C… what does a cubic close-packed (ccp) structure look like?

Answer 5

ABCABCABC

Question 6

Describe the CCP structure

Answer 6

2nd layer sits in the dimples of the first layer and the third layer sits in the dimples of the second not in line with the first

Question 7

What is a unit cell?

Answer 7

The unit cell for simple crystals is the simplest arrangement of spheres which, when repeated, will reproduce the whole structure.

Question 8

What is the unit cell of a CCP structure called?

Answer 8

Face-centred cube (FCC)

Question 9

What is the difference between body-centred cube (BCC) structure and face-centred cube (FCC) structure?

Answer 9

More space between the atoms in BCC than FCC
bcc ― 68% of the space is occupied by the spheres.
fcc ― 74% of the space is occupied by the spheres.

Question 10

What is an open structure?

Answer 10

BCC is an open structure:
Each atom is surrounded by 8 other. The atoms are NOT close packed.
e.g. Li, Na, K, Rb, Cs, Cr, V, W