Unit 8 - the crystal structure of solids

What is a crystalline material?

the atoms are situated in repeating or periodic array over large atomic distances

usually metals and soilds

What are non crystalline amorphous solids?

do not have a regular structure, no long-range atomic order

glasses and polymers

rapid cooling favours this as there is little time for the ordering process

what is a lattice?

three dimensional array of points coinciding with sphere centres

What is a unit cell?

the basic structural unit of the crystal structure

not in amorphous structures

How many groups of bravais lattices are there?

7

what are the angles and side lengths in a cubic?

a=b=c

90

what are the angles and side lengths in a tetragonal

a=b =/c

90

what are the angles and side lengths in a orthorhombic?

a=/b=/c

90

what are the angles and side lengths in a monoclinic?

a=/b=/c

a and gamma 90

what are the angles and side lengths in a hexagonal

a=b=/c

alpha and beta 90, gamma 120

what are the angles and side lengths in a rhombohedral

a=b=c

alpha and beta 90

what are the angles and side lengths in a triclinic?

nothing equal

How many possible unit cells are there?

14 possible repetitive unit cells which causes the complete filing of a 3D space

Why do half of elemental solids have FCC or HCP packing?

efficient close packing and hence low energy

What is a simple cubic unit cell

Atomic packing factor - 0.52

coordination number - 6

what is a body centered cubic unit cell?

atomic packing factor - 0.68

coordination number - 8

harder and less malleable metals, slip is harder

What is a face centred cubic unit cell?

atomic packing factor - 0.74

coordination number - 12

what is the atomic packing factor?

fraction of solid sphere volume in a unit cell assuming the hard sphere model.

fcc has 74% which is the maximum packing possible for spheres to have the same diameter

What is the hexagonal closed packed crystal structure

atomic packing factor - 0.74

coordination number 12

same as FCC same volume of atoms per unit cell the only difference is the stacking sequence

The HCP and FCC stacking sequence

FCC - ABCABC

HCP - ABAB

Why is graphite and diamond so different to each other?

graphite - soft, dark-coloured material

diamond - hard, transparent

the reason they are so different is the 2 different crystal structures

the higher the apf, easier deformation