Common Structures

Report
Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
e
CHEM1002 [Part 2]
Dr Michela Simone
Weeks 8 – 13
Office Hours: Monday 3-5, Friday 4-5
Room:
412A (or 416)
Phone:
93512830
e-mail:
[email protected]
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Summary of Last Lecture
Crystal structures I
•
•
•
•
•
Hexagonal close packing arises when close packed layers
repeat every third layer (ABABABAB)
Cubic close packing arises when close packed layers repeat
every fourth layer (ABCABCABC)
The coordination number in both types of close packing is 12
Unit cells are the simplest building blocks from which the
whole solid can be built
Atoms on corners of unit cells are shared between 8 cells,
atoms on faces of unit cells are shared between 2 cells,
atoms on edges of unit cells are shared between 4 cells and
atoms at the centres of unit cells are unshared.
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Crystal Structures II
Lecture 9:
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Packing efficiency
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Octahedral and tetrahedral interstitial holes
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Ionic crystal structures
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Blackman Chapter 7, Section 7.4 (pages 265-268)
Next week’s lectures
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Lectures 10 and 11: solubility
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Blackman Chapter 10, Sections 10.1 – 10.4
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Revise equilibrium calculations from CHEM1001!
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Lecture 12: introduction to coordination chemistry
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Blackman Chapter 13, Sections 13.1 – 13.4
Slide 4/20
x
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Packing Efficiency I: Atoms in Cell
The unit cell for CCP is face centred cubic (FCC)
 atoms on each corner and atoms on each face of the cube
•
Atoms on corners:
• Atoms on faces:
• Total:
Slide 5/20
x
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Packing Efficiency II: Volume Occupied by Atoms
The unit cell for CCP is face centred cubic (FCC)
 atoms on each corner and atoms on each face of the cube
• If each atom has radius r,
volume of atom =
• Number of atoms =
• Volume occupied by atoms =
Slide 6/20
x
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Packing Efficiency III: Volume of Unit Cell
The unit cell for CCP is face centred cubic (FCC)
 atoms on each corner and atoms on each face of the cube
• Close packed direction is
diagonal of face
a
• Length of diagonal:
r
a
r
• Length of side:
r
r
• Volume of cubic unit cell:
Slide 7/20
x
•
Packing Efficiency IV: FCC
The unit cell for CCP is face centred cubic (FCC)
 atoms on each corner and atoms on each face of the cube
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Volume occupied by atoms
• Volume of unit cell
• Packing efficiency
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Cubic Close Packing: CCP and HCP
• Cubic close packing:
 Layers repeat ABCABCABC
 Number of atoms in unit cell = 4
 Packing efficiency = 74%
 Coordination number = 12
•
Hexagonal close packing
 Layers repeat ABABAB
 Packing efficiency = 74%
 Coordination number = 12
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Other Metal Structures
• Simple cubic
 Number of atoms in unit cell = 1
 Packing efficiency = 52%
 Coordination number = 6
• Body centred cubic
 Number of atoms in unit cell = 2
 Packing efficiency = 68%
 Coordination number = 8
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Interstitial Holes - Oh
• Even in the close packed structures, there are spaces for extra atoms
• Octahedral interstitial holes
 at the centre of the cube and on each edge
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Interstitial Holes - Td
• Even in the close packed structures, there are spaces for extra atoms
• Tetrahedral interstitial holes
 in the centre of the cube corners
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Common Structures
• For every n close packed atoms, there are
 n octahedral holes and
 2n tetrahedral holes
• Almost all of the common structures can be thought as of being
derived from
 Close packed (CCP or HCP) anions
 Cations in octahedral and/or tetrahedral interstitials
Slide 13/20
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Common Structures - Rocksalt
• NaCl (rocksalt)
 CCP Cl- anions with Na+ in all of the octahedral interstitials
Slide 14/20
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Common Structures - Zinc Blende
• Zinc Blende (ZnS)
 CCP S2- anions with Zn2+ in half of the tetrahedral interstitials
Slide 15/20
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Common Structures - Anti-Fluorite
• Anti-Fluorite (Na2O)
 CCP O2- anions with Na+ in all of the tetrahedral interstitials
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Common Structures - Fluorite
• Fluorite (CaF2)
 CCP Ca2+ cations with F- in all of the tetrahedral
interstitials
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Common Structures
Anion
Packing
ccp
hcp
Filling of Interstitial
Sites
Structure
Examples
Td
Oh
-
all
rock-salt
NaCl, MgO, LaN
-
1/2
cadmium chloride
CdCl2, NiCl2
half
-
zinc blende
(sphalerite)
ZnS, CuCl, BN
all
-
anti-fluorite
Na2O, Li2S
1/4
1/2
spinel
MgAl2O4
-
all
nickel arsenide
NiAs, FeS
-
1/2
cadmium iodide
CdI2, Mg(OH)2
all
-
wurtzite
ZnS, BeO
-
1/2
rutile
TiO2, CrO2
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Summary: Crystal Structures II
Learning Outcomes - you should now be able to:
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Complete the worksheet
Work out the number of atoms in a unit cell
Understand the calculation of packing efficiency
Recall the packing efficiency and coordination
numbers of HCP, CCP, BCP and simple cubic
structures
Be able to rationalize ionic structures in terms of
filling of interstitial holes
Next lecture:
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Solubility
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Practice Examples
1. How many atoms are there in the body-centred cubic unit cell of tungsten?
A. # atoms = 1
B. # atoms = 1 + 1/8 × (8) = 2
C. # atoms = 1/2 × (6) = 3
D. # atoms = 1/2 × (6) + 1/8 × (8) = 4
E. # atoms = 1 + 1/2 × (6) + 1/8 × (8) = 5
2. Verify that the efficiency of simple cubic packing is 52% and BCP is 68%.
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