Crystallography

Question 1

What is crystallinity?

Answer 1

• Solids without an ordered structure are Non-cyrstalline or amorphous materials.
• Therefore Crystallinity are when there is an ordered structure and arrangment of atoms, in solids.

Question 2

What is meant by crystal structures?

Answer 2

Variabiliity in the spatial arrangment of atoms in different materials

Question 3

What is meant by a unit cell?

Answer 3

It is the minimum number of atoms to represent the repetitive pattern

Question 4

What is Hexagonal Close Packing (HCP)?

Answer 4

• This is known as ABABAB stacking.
• While each layer is stacked closely horizontally within, the layers vertically aren’t.
• The stacking structure is shown in the image below:

Question 5

What is Face Centred Cubic Packing (FCC)?

Answer 5

• This is known as ABCABC stacking.
• Each layer is closely packed together in all directions.
• Resulting shape is a cube, so it has a high degree of symmetry.
• Cube formed has the ABC layers going at an angle to form the cube as shown below:

Can remember that each face has a molecule, so thats the C

Question 6

What is Body Centred Cubic (BCC)?

Answer 6

• This is a non-close packed structure.
• It is highly symmetrical
• It does have close-packed directions, all the molecules touch along a diagonal, shown below:

Question 7

What is Polymorphism / Allotropy?

Answer 7

This is when materials can have more than one crystal structure.
* For example: Fe goes from BCC to FCC and back again when heating up to high temperatures

Question 8

How do you identify Crystal directions?

Answer 8

• Imagine a unit cell with lengths (a) on each side in the x,y,z axis.
• We can then work out how far in integers the line has gone for each direction, shown below.
• If a direaction doen’t go a whole integer in length, just extend the box out till it goes a whole integer in distance.
• A direction is always denoted by square brackets
• A family is a classification of directions that can be thought as the same, just translated into place. This gives a general direction. It is denoted by greater and less than sybmols <>.

You may have to shift the origin sometimes!!

Question 9

How do you identify Crystal planes?

Answer 9

• A plane is where you intecept any of the axis’.
• Planes are denoted by round brackets ().
• Any direction where the plane is parrallel to them, then it goes to infinity, and we take the reciprical, so it will be 0. Any other distance, such as a/1, will make 1 or 2, again has to be an integer.
• A family of planes is a classification of many planes that have similar indices. This is denoted by curly brackets {}.