Lattice Systems Flashcards
What are the 7 different lattice unit cell shapes?
Cubic Tetragonal Hexagonal Trigonal Orthorhombic Monoclinic Triclinic
What is the draw back of the macroscopic structure?
The macroscopic structure of the crystal does not always tell you what the crystal system is
What do you need to define for 3D lattices?
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
How many bravais lattices are there ?
There are 14 difference space lattices in 3D
The lattice points are not necessarily occupied by atoms
Describe the 4 Bravais lattices you need to know
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
Why is there no cubic C?
A cubic C is actually a tetragonal P
What is a close packed sphere?
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
Why is the close packed sphere a good model for metals?
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
In 2D what is the most efficient packing?
Hexagonal packing
Why are there two choices of packing for 3D structures?
This is because there are 2 choices for the third layer because not all depressions between the second layer are identical
What are the two 3D close packed spheres?
Hexagonal close packed (hcp)
Cubic close packed (ccp or fcc)
What is hexagonal close packed?
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
What type of metals do hcp tend to be?
These tend to be hard metals
What is coordination number?
This is the number of nearest neighbouring atoms
What is cubic close packed?
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
What is the difference between hcp and ccp?
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)
What type of metals do ccp tend to be?
They tend to be soft metals
How can you tell if a structure is close packed?
If the atoms in each layer lie in the depressions between the atoms in the layer below then the structure is close packed
What is a slip plane?
A plane surface through a crystal along which slip can take place
Why are ccp softer metals than hcp?
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
Describe the cubic close packed unit cell
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
Describe the coordination number of cubic cell units
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
Why are BCC harder than close packed metals?
When a metal deforms, the atomic planes must slip over each other which is more difficult for the BCC structure
