Lattice Systems

Question 1

What are the 7 different lattice unit cell shapes?

Answer 1

Cubic 
Tetragonal
Hexagonal
Trigonal
Orthorhombic 
Monoclinic
Triclinic

Question 2

What is the draw back of the macroscopic structure?

Answer 2

The macroscopic structure of the crystal does not always tell you what the crystal system is

Question 3

What do you need to define for 3D lattices?

Answer 3

You need to define axes, angles and dimensions

Unit cells are defined in terms of length a, b and c and angles alpha, beta and y

Question 4

How many bravais lattices are there ?

Answer 4

There are 14 difference space lattices in 3D

The lattice points are not necessarily occupied by atoms

Question 5

Describe the 4 Bravais lattices you need to know

Answer 5

Cubic P- primitive, there is no point in the middle, just at 4 corners
Cubic I- body centred- points are 4 corners and in the middle
Cubic F- all face centred- points are in 4 corners and one in the middle of each face
C- one face centred- points are in 4 corners and only one face has point in the middle

Question 6

Why is there no cubic C?

Answer 6

A cubic C is actually a tetragonal P

Question 7

What is a close packed sphere?

Answer 7

Spheres are packed as close together so the spaces between the atoms are minimised and number of nearest neighbours is maximised
Hard inner core held together by a sea of electrons

Question 8

Why is the close packed sphere a good model for metals?

Answer 8

Non directional interactions so it is a good model for s systems
This is because s orbitals are spherical whereas p orbitals are highly directional

Question 9

In 2D what is the most efficient packing?

Answer 9

Hexagonal packing

Question 10

Why are there two choices of packing for 3D structures?

Answer 10

This is because there are 2 choices for the third layer because not all depressions between the second layer are identical

Question 11

What are the two 3D close packed spheres?

Answer 11

Hexagonal close packed (hcp)

Cubic close packed (ccp or fcc)

Question 12

What is hexagonal close packed?

Answer 12

This is ABABAB stacking
Coordination number 12
There are 6 nearest neighbours in same row, 3 above and 3 below
The second layer sits in the holes of the second layer
The third layer is identical to the 1st layer

Question 13

What type of metals do hcp tend to be?

Answer 13

These tend to be hard metals

Question 14

What is coordination number?

Answer 14

This is the number of nearest neighbouring atoms

Question 15

What is cubic close packed?

Answer 15

This is ABCABC stacking
This has coordination number 12
There are 6 nearest neighbours in same row, 3 above and 3 below
The second layer sits in the holes of the first layer
The 3rd layer is the opposite way round to the 1st

Question 16

What is the difference between hcp and ccp?

Answer 16

In hcp, the third layer is placed directly on top of the first layer (identical)
In ccp, the third layer is placed above the empty spaces in the first layer (points in opposite direction)

Question 17

What type of metals do ccp tend to be?

Answer 17

They tend to be soft metals

Question 18

How can you tell if a structure is close packed?

Answer 18

If the atoms in each layer lie in the depressions between the atoms in the layer below then the structure is close packed

Question 19

What is a slip plane?

Answer 19

A plane surface through a crystal along which slip can take place

Question 20

Why are ccp softer metals than hcp?

Answer 20

When a metal is under stress, layers of atoms can slip past each other along a slip plane
The more slip planes there are, the softer the metal
Ccp has 8 slip planes whereas hcp has 1 therefore ccp are softer

Question 21

Describe the cubic close packed unit cell

Answer 21

The atoms in the vertices do not touch but touch those on adjacent faces
The atoms in each face contribute 1/2 to the unit cell
The atoms on the vertices each contribute 1/8
There are 4 atoms in total in the unit cell

Question 22

Describe the coordination number of cubic cell units

Answer 22

FCC or CCP is the only close packed unit cell- coordination number is 12
All other unit cells have lower coordination numbers
BCC( body centred cubic) has coordination number 8
PC has coordination number 6

Question 23

Why are BCC harder than close packed metals?

Answer 23

When a metal deforms, the atomic planes must slip over each other which is more difficult for the BCC structure