Lecture 7

Question 1

Crystalline and Amorphous

Answer 1

Crystalline- possesses long range order

Amorphous- possesses short range order (tetrahedral coordination of Si by O

Question 2

Crystalline to Amorphous

Answer 2

A crystalline solid is heated –> atoms begin to vibrate due to increase in energy –>bonds break and long range order is lost

Question 3

Amorphous to Crystalline

Answer 3

Amorphous liquid is cooled –>atoms lose energy –> they assemble into crystalline clusters –> clusters grow by addition of atoms from the liquid

Question 4

Glass formation

Answer 4

When a liquid is cooled it can be difficult for atoms to be arranged in a crystalline array (involves bond breaking/making and the molecules are large) –> as it is cooled the atoms lose mobility–> amorphous liquid structure is frozen solid –> amorphous or glassy solid

Question 5

Silica glass

Answer 5

Silicon and oxygen are covalently bonded and to transform the amorphous liquid into a crystalline

Question 6

Unit cells

Answer 6

Single crystals formed from crystals of high symmetry have properties which are largely isotropic
Single crystals formed from unit cells of low symmetry have properties which may be highly anisotropic

Question 7

Metallic crystal structures

Answer 7

Non- directional
Desirable to have electrons orbit the maximum number of nuclei
Maximum packing density of atoms

Question 8

Metallic crystal structures pt 2

Answer 8

Face centred cubic
-Al, Cu, Ni, Ag, Au, Pt, Pb
Body centred cubic 
-Cr, Mo, Ta, W
Hexagonal close packed
-Zn, Cd, Co

Question 9

Face centred cubic

Answer 9

Atoms touch lies along the face diagonal
Each atom is coordinated by 12 nearest neighbours
Filling space for spherical atoms of 74%

Question 10

Body centred cubic

Answer 10

Centre and corner atoms touch along the body diagonal
Coordination number is 8
Space filling at 68%

Question 11

Hexagonal close packed

Answer 11

Each atom is coordinated by 12 nearest neighbours

Space filling of 74%

Question 12

Polymorphism

Answer 12

The ability to adopt different crystal structures depending on the temperature (and pressure). Structural changes are accompanied by a volume change

Question 13

Polymorphism (Iron)

Answer 13

bcc fcc bcc

Question 14

Polymorphism (Titanium)

Answer 14

hcp bcc

Question 15

Metals have simple crystal structures because

Answer 15

• non-directional bonding
• all atoms have the same size
• close packing giving the lowest energy

Question 16

Ionic bonding leads to crystal structures which

Answer 16

-ensures that ions are coordinated by ions of opposite charge
Maintain an overall charge neutrality
Maintain ions in contact for a stable ionic bonded structure

Question 17

AX- Type crystal structures

Answer 17

An equal number of anions and cations. The magnitude of charge must be the same. Coordination of A by X and X by A must be the same.
NaCl- 6
CsCl- 8
ZnS-4

Question 18

AmXp

Answer 18

The different number of anions (X) and cations (A). The magnitude of charge must not be the same. Coordination of A by X and X by A must be different. CaF –> Ca by F 8, F by Ca 4

Question 19

AmBnXp

Answer 19

Two different types of cation (A and B)
Barium titantate (BaTiO3)

Question 20

Polymorphism

Answer 20

Ionically bonded materials exhibit polymorphism (Zirconia and Barium titanate)