Topic 1 - Crystal Basics Flashcards
What is the definition of a crystal?
A regular, periodic arrangement of atoms, ions or molecules
What property imparts the defining characteristics of a crystal?
The underlying lattice - its periodicity
What is a crystal structure defined by?
It is defined by the lattice and a motif that decorates the lattice points
What’s the definition of a unit cell?
A repeating unit that can generate the entire crystal by translation in three dimensions
Draw out the 2D visualisation of a Face-centred cubic lattice
How many objects (atoms) are present in a FCC unit-cell?
Method 1:
8 x 1/8 = 1
6 x 1/2 = 3
Or
Method 2:
1 from corners of unit cell
1 from x-face
1 from y-face
1 from z-face
4 total
What are Miller indices used for?
Miller indices describe the planes and directions wrt the crystal lattice
They refer to a family of parallel planes separated by the same d-spacing
What structures do metals typically adopt?
Cubic close-packed (FCC)
Hexagonal close-packed
Body centred-cubic
How do FCC and HCP differ in structure?
FCC has 3 distinct layers; A, B & C which repeat periodically
HCP has 2 distinct layers; A & B which also repeat periodically
How do you determine the packing efficiency of a unit cell?
What expression gives Vatom in packing efficiency?
What expression gives Vunit cell in packing efficiency?
What properties arise from FCC?
Low corrugation of layers = layers can slip over each other easily
- results in FCC metals being ductile
What properties arise from HCP?
Low corrugation of layers
- fewer close-packed layers than in FCC
- results in HCP metals being more brittle
What properties arise from BCC?
Higher corrugation of layers - more difficult for layers to slip as atoms are not as close packed.
- Hence, BCC metals tends to be harder than FCC but more ductile than HCP
How are alloys made?
Alloys are metallic solids which contain atoms of more than one element
Additional elements disrupt periodicity of the crystal and introduce defects
- essentially solid solutions of elements
What are the different types of alloys?
Substitutional alloys
- exchange of atoms for one another
Interstitial alloys
- additional atoms sit in between atoms of host lattice
