Structure of crystalline solids

Question 1

What makes a crystal not dense?

Answer 1

the random packing

Question 2

What makes a crystal dense?

Answer 2

ordered packing

Question 3

Dense, ordered packed structures tend to have lower or higher energies?

Answer 3

Lower

Question 4

What are properties of crystalline materials?

Answer 4

atoms pack in periodic, 3D arrays

Question 5

What are properties of non-crystalline materials?

Answer 5

atoms have no periodic packing

• complex structures
• rapid cooling

Question 6

What are amorphous materials?

Answer 6

non-crystalline

Question 7

what are examples of crystalline materials?

Answer 7

metals
many ceramics
some polymers

Question 8

What properties of metallic crystal structures?

Answer 8

densely packed

have the simplest crystal structures

Question 9

Why are metallic crystal structures densely packed? (4)

Answer 9

• Typically, only one element is present, so all atomic radii are the same.
• Metallic bonding is not directional.
• Nearest neighbour distances tend to be small in
  order to lower bond energy.
• Electron cloud shields cores from each other

Question 10

What are the three simplest crystal structures?

Answer 10

Simple cubic structure
Body centered cubic structure
Face centered cubic structure

Question 11

What are simple cubic structures? (2)

Answer 11

Rare due to low packing density (only Po has this structure)
Close-packed directions are cube edges

Question 12

How many coordination (nearest neighbours) does it have? SC

Answer 12

6

Question 13

What is the APF of a simple cubic structure?

Answer 13

0.52

Question 14

How many atom/unit cell are there is a SC?

Answer 14

1 atom/unit cell

8 x 1/8 = 1

Question 15

What are body centered cubic structures?

Answer 15

Atoms touch each other along cube diagonals

Question 16

What are examples of BCC structures?

Answer 16

Cr, W, Fe(a), Tantalum, Molybdenum

Question 17

What is the coordination of a BCC?

Answer 17

8

Question 18

How many atoms are there per cell in a BCC?

Answer 18

1 center + 8 corneres x 1/8 = 2 atoms/unit cell

Question 19

What are the close-packed directions in a BCC?

Answer 19

length = 4R = √3 a

Question 20

What is the APF for a BCC?

Answer 20

0.68

Question 21

What is a face centred cubic structure?

Answer 21

Atoms touch each other along face diagonals

Question 22

What are examples of face centred cubic structure?

Answer 22

Al, Cu, Au, Pb, Ni, Pt, Ag, Si

Question 23

What is the coordination of a FCC?

Answer 23

12

Question 24

How many atoms are there per cell in a FCC?

Answer 24

6 face x 1/2 + 8 corners x 1/8 = 4 atoms/unit cell

Question 25

What is the APF for a face-centered cubic structure?

Answer 25

0.74 which is the maximum achievable APF

Question 26

What are the close-packed directions for a FCC?

Answer 26

length = 4R = √2a

Question 27

What is the Hexangonal close-packed structure?

Answer 27

6 then 3 then 6 stacking structure

Question 28

What is the coordination for the HCP?

Answer 28

12

Question 29

What is the APF for the HCP?

Answer 29

0.74

Question 30

How many atoms/unit cell are there in a HCP?

Answer 30

6

Question 31

What are examples of HCP?

Answer 31

Cd, Mg, Ti, Zn

Question 32

What is c/a in HCP

Answer 32

1.633

Question 33

Level these from highest to lowest density: metals, ceramics and polymers

Answer 33

ƿmetals > ƿceramics > ƿpolymers

Question 34

Why do metals have higher densities than ceramics and polymers? (2)

Answer 34

- close-packing (metallic bonding) - often large atomic masses

Question 35

Why do ceramics have lower density than metals but a higher density than polymers? (2)

Answer 35

* less dense packing | * often lighter elements

Question 36

Why do polymers have the lowest density?

Answer 36

* low packing density (often amorphous) | * lighter elements (C,H,O)

Question 37

What are polycrystals?

Answer 37

Crystals that have single crystal grains that are randomly oriented, the overall competent properties are not directional

Question 38

What is the difference between single and polycrystals? (properties)

Answer 38

Single crystal properties vary with direction: anisotropic Polycrystals properties may/may not vary with direction and are randomly oriented: isotropic, if grains are textured, anisotropic

Question 39

What is polymorphism?

Answer 39

Two or more distinct crystal structures for the same material (allotropy/polymorphism)