Title: Crystal Structures
1Crystal Structures X-ray Diffraction
- Chemistry 123
- Spring 2008
- Dr. Woodward
2Crystals
3Crystalline Solids
CaF2 Unit Cell
-
- Crystal Lattice A 3D array of points where each
point has an identical environment. - Unit Cell The repeating unit (a unit cell is to
a crystal, like a brick is in a house). In a
given crystal all unit cells are identical.
43D Crystal Systems (7)CubicTetragonalHexagonal
RhombohedralOrthorhombicMonoclinicTriclinic14
Bravais Lattices
The crystal systems each have distinctive
symmetry and unit cell dimensions
5X-ray Diffraction
6Diffraction Demo
- Take home message
- The diffraction pattern is related but not equal
to the grid pattern - Diffraction is most effective for monochromatic
light whose wavelength is similar to the spacing
of slits - For crystals X-rays have a wavelength comparable
to spacings of atoms
7Braggs Law
A
B
d
The extra path length traveled by wave B (shown
in red) is equal to 2d sin q, where d is the
spacing between planes. Constructive
interference will only occur if this distance is
an integer multiple of the wavelength (nl). When
we use Braggs Law we generally set n1.
8Symmetric Structures Simple X-ray Pattern Ex.
Gold Face centered cubic structure
Low symmetry Structures Complex X-ray Pattern Ex.
CuSO4 5H2O (triclinic structure)
9Simple Cubic Crystal Structures
(a) CsCl
(b) NaCl
(c) Zinc blende
Decreasing coordination number
10X-Ray Powder Diffraction Pattern (Ex. Magnesium
Sulfide, MgS)
020 planes
220 planes
Each peak corresponds to scattering from a
different set of lattice planes. Two planes are
shown above for MgS, which has the same structure
as NaCl.
11MgS with the NaCl structure
ZnS with the Zinc Blende structure
12Two Component Phase Diagram
D
Liquid
Liquid
C)
C)
Eutectic Point
Eutectic Point
MgS
(
ss
)
(
ss
)
A
ZnS
(
ss
)
Liquid
Liquid
Temperature (
Temperature (
ZnS
(
ss
)
(
ss
)
Liquid
Liquid
MgS
(
ss
)
(
ss
)
2
-
phase region
2
-
phase region
C
MgS
(
ss
)
ZnS
(
ss
)
(
ss
)
(
ss
)
B
0
25
50
75
100
0
25
50
75
100
Mole
ZnS
Mole
Pure
ZnS
Pure
MgS
Pure
Pure
13Region A MgS-type solid solution
Region B ZnS-type solid solution
Region C Two-phase region
X-ray Powder Diffraction Pattern
X-ray Powder Diffraction Pattern
X-ray Powder Diffraction Pattern
Microscopic View
Microscopic View
Microscopic View
Zinc Blende type Zn1-xMgxS
Rock salt type Mg1-xZnxS