cation

1
Ionic Compounds
anion
cation
Ceramics
2
More Complex Structures

• Multiple cations
• Perovskite ABO3
• Capacitors
• Spinel AB2O4
• Magnetic properties

• Covalency
• Zinc blende
• Semiconductors
• Diamond
• Semiconductors
• Silicates
• Minerals

?
RB lt RA RO
3
Spinel

• Spinel AB2O4 B ? boron
• A2(B3)2O4 MgAl2O4
• Occurs when RA RB lt RO
• CCP array of oxygen anions
• To attain stoichiometry, fill 3 cation sites per
  4 anions (¾ 1)
• All octahedral 1 1
• 1/2 tetrahedral 1 1
• arrange occupied sites in a regular manner like
  zinc blende to keep cations far apart
• unit cell is 8x regular CCP unit cell
• A ? B site occupancy ? magnetic properties

1/2 octahedral 1/8 tetrahedral
1/2 cation 1 anion
1/4 cation 1 anion
normal B in oct, A in tet
inverse A B in oct, B in tet
4
Magnetism in Spinel

• unpaired electron spin
• atoms in B-site add
• atoms in A-site subtract
• tune A and B-site occupancy to tune magnetic
  behavior
• (details given in lecture)

5
Covalent Compounds
sp3
s2p2
s2p4
s2p1
s2p3
s2
semi-conductors
6
Covalent Structures
? both species tetrahedral
Recall zinc blende
ZnS 2 -2 GaAs 3 -3
single element C or Si or Sn
or sp3
4
4
diamond
How many lattice points per unit cell?
7
Brief Review

• Metals
• Close-packed structures (CN 12)
• Cubic close-packed, hexagonal close-packed
• Subtle reasons for selecting one over the other
• Slightly less close-packed
• Body centered cubic (CN 8)
• Influence of covalency
• Ionic structures
• Close-packed with constraints
• Covalent structures
• Not close-packed, bonding is directional
• Any can be strongly or weakly bonded (Tm)

8
Diamond vs. CCP
8 atoms/cell, CN 4
4 atoms/cell, CN 12
½ tetrahedral sites filled
9
Computing density

• Establish unit cell contents
• Compute unit cell mass
• Compute unit cell volume
• Unit cell constant, a, given (or a and c, etc.)
• Or estimate based on atomic/ionic radii
• Compute mass/volume, g/cc
• Example NaCl
• Contents 4 Na 4 Cl
• Mass 4(atom mass Na atomic mass Cl)/No
• Vol a3
• Units

10
Single Crystal vs. Polycrystalline
Rb3H(SO4)2
Ba(Zr,Y)O3-d
Regions of uninterrupted periodicity amalgamated
into a larger compact
Periodicity extends uninterrupted throughout
entirety of the sample External habit often
reflects internal symmetry
grains delineated by grain boundaries
11
Polycrystallinity