Crystal Structures

Question 1

Whats the difference between a crystalline and amorphous solid?

Answer 1

In a crystalline solid, the atoms are arranged in a repeating or periodic structure
An amorphous solid has no atomic order

Question 2

What is the basic structural unit or building block of a crystal structure? What does it determine?

Answer 2

Unit Cell. The atomic arrangement and geometry of the unit cell define the crystal structure of the material

Question 3

What is the atomic packing factor

Answer 3

The ration of the volume of atoms in a unit cell and the volume of the unit cell

Question 4

What is the coordination number of a unit cell?

Answer 4

The number of atoms touching the central atom

Question 5

How many atoms are in an FCC unit cell?

Answer 5

4

Question 6

Whats the coordination number of an FCC unit cell?

Answer 6

12

Question 7

Side length ‘a’ of an FCC cell?

Answer 7

2Rsqrt(2)

Question 8

How many atoms are in a BCC cell?

Answer 8

2

Question 9

Whats the oordination number of a BCC cell?

Answer 9

8

Question 10

Whats the ‘a’ side length of a BCC cell?

Answer 10

4R/sqrt(3)

Question 11

Whats the APF of BCC?

Answer 11

0.68

Question 12

APF for FCC?

Answer 12

0.74

Question 13

How many atoms in a Hexagonal closed pack cell (HCP)?

Answer 13

6

Question 14

Coordination number of HCP?

Answer 14

12

Question 15

What is the theoretical density of a unit cell?

Answer 15

The mass of atoms in a unit cell per volume of cell unit

Question 16

What is the term for when some metals and non metals have more than one crystal structure?

Answer 16

Polymorphism

Question 17

Whats the terminology for polymorphism in elemental solids?

Answer 17

Allotropy (where each phase or crystal structure has its own mechanical properties and characteristics

Question 18

What are the 6 lattice parameters and what doe they tell us about a crystal structure

Answer 18

Lattice parameters,
side lengths: a b c
Axial angles, alpha, beta, gamma
Provides information about unit cell configurations and atomic arrangements

Question 19

What do you call a line between two points in a crystalline material?

Answer 19

Crystallographic Direction

Question 20

For crystallographic directions what brackets do you use?

Answer 20

[abc]

Question 21

What does changing all the signs in a crystallographic direction give?

Answer 21

The antiparallel direction

Question 22

What brackets do you used to describe crystallographic planes

Answer 22

Regular - (abc)

Question 23

In crystallographic planes what does each index imply?

Answer 23

The point of intersection between the plane and the abc axis.

Question 24

If a plane lies directly on an axis, what should the index reduce to?

Answer 24

0

Question 25

What does it mean for crystallographic directions to be crystallographically equivalent?

Answer 25

It means the atomic spacing along each direction is identical

Question 26

What does it mean when two directions have the same linear density.

Answer 26

They have the same number of atoms centred per length of the direction vector

Question 27

What do you call the number of atoms centred on a plane per square area of the plane?

Answer 27

Planar density

Question 28

Where is slip most likely to occur?

Answer 28

On the most dense packed crystallographic planes and along the most densely packed directions

Question 29

What do you call a group of directions that are all crystallographically equivalent?

Answer 29

Family, and use

Question 30

What are the key features of a family of planes? (In a cubic system)

Answer 30

The planes all have the same indices, just in different order and or sign. {123} family includes (1-23) and (3-12) etc.

Question 31

What must occur for a crystalline solid to form as a single crystal?

Answer 31

The arrangement of atoms must be perfect and extend throughout the entirety of the specimen without interruptions

Question 32

What do you call a crystalline solid composed of many small crystals and grains?

Answer 32

Polycrystalline

Question 33

What do you call the region where 2 different grains meet?

Answer 33

Grain Boundary (atomic disorder and high energy)

Question 34

Whats the difference between isotropic and anisotropic material properties?

Answer 34

Anisotropic have different properties in different directions Isotropic means to be the same everywhere/every direction