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Crystal Structures

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Title: Slide 1 Author: Patrick Woodward Last modified by: Patrick M. Woodward Created Date: 5/4/2006 3:48:27 PM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: Crystal Structures


1
Crystal Structures X-ray Diffraction
  • Chemistry 123
  • Spring 2008
  • Dr. Woodward

2
Crystals
3
Crystalline 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.

4
3D Crystal Systems (7)CubicTetragonalHexagonal
RhombohedralOrthorhombicMonoclinicTriclinic14
Bravais Lattices
The crystal systems each have distinctive
symmetry and unit cell dimensions
5
X-ray Diffraction
6
Diffraction 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

7
Braggs 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.
8
Symmetric Structures Simple X-ray Pattern Ex.
Gold Face centered cubic structure
Low symmetry Structures Complex X-ray Pattern Ex.
CuSO4 5H2O (triclinic structure)
9
Simple Cubic Crystal Structures
(a) CsCl
(b) NaCl
(c) Zinc blende
Decreasing coordination number
10
X-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.
11
MgS with the NaCl structure
ZnS with the Zinc Blende structure
12
Two 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
13
Region 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
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