1. Ceramics
Term ceramics comes from the greek word keramikos – “burnt stuff”
Ceramics are typically formed during high temperature heat treating – “Firing”
Traditionally ceramics included:
China
Porcelain
Bricks (both construction and refractory)
Tiles
Glasses
Over the last 60 years or so…there has been an explosion in new technologies similar to other areas of material science

2. Ceramic Bonding CaF2: large SiC: small • Bonding:
-- Mostly ionic, some covalent.
-- % ionic character increases with difference in
electronegativity.
• Amount of ionic bond character:
CaF2: large
SiC: small
Eq 2.10: % ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100
XA, XB are electronegativities of components A and B

3. Ionic Ceramics
Crystal structure are composed of electrically charged ions
Cations (Fe3+) – Positive Charge Typically metals
Anions (O2-) – Negatively Charged Typically non-metals
Two characteristics influence crystal structure:
The magnitude of charge on the component ions
Stoichiometry must balance
Overall charge neutrality is required
Relative sizes of the component ions
small
large
Note that size of ion is affected by charge:
For iron: r(Fe2+) = nm, r(Fe3+) = nm, r(Fe) = nm

4. Criteria of Site Selection
Which sites will cations occupy to form stable crystal structure?
Size of sites
does the cation fit in the site
Stoichiometry
if all of one type of site is full the remainder have to go into other types of sites.
Covalent Bond Hybridization

5. Ionic Bonding & Structure
1. Size - Stable structures:
--maximize the # of nearest oppositely charged neighbors. - +
unstable - + - +
stable
stable
• Charge Neutrality:
--Net charge in the
structure should
be zero.
--General form:
CaF 2 : Ca 2+
cation F -
anions + A m X p
m, p determined by charge neutrality

6. Coordination # and Ionic Radii
cation
anion
• Coordination # increases with
How many anions can you arrange around the cation? 2 r
cation
anion
Coord #
< 0.155 3 4 6 8
linear
triangular TD OH
cubic
ZnS
(zincblende)
NaCl
(sodium
chloride)
CsCl
(cesium
Purely geometrical argument

7. Geometrical Derivation of Site Size
Determine minimum rcation/ranion for OH site (C.N. = 6)
a = 2ranion

8. Site Selection II Stoichiometry
If all of one type of site is full the remainder have to go into other types of sites.
Ex: We know that an FCC unit cell has 4 OH and 8 TD sites.
If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites, then
4 in OH
2 in TD

9. Site Selection III Bond Hybridization – significant covalent bonding
the hybrid orbitals can have impact if significant covalent bond character present
For example in SiC
XSi = 1.8 and XC = 2.5
89% covalent bonding
both Si and C prefer sp3 hybridization
Therefore in SiC get TD sites

10. Example: Predicting Structure of FeO
• On the basis of ionic radii, what crystal structure
would you predict for FeO?
Cation
Anion Al 3+ Fe 2 + Ca 2+ O 2- Cl - F
Ionic radius (nm)
0.053
0.077
0.069
0.100
0.140
0.181
0.133
• Answer:
based on this ratio,
--coord # = 6
--structure = NaCl

11. Rock Salt Structure Example: NaCl (rock salt) structure
Same concepts can be applied to ionic solids in general
Example: NaCl (rock salt) structure
rNa = nm
rCl = nm
rNa/rCl = 0.564
cations prefer OH sites
AX Crystal Structure: equal number of Anion and Cation locations

12. MgO and FeO MgO and FeO also have the Rock Salt structure
O2- rO = nm
Mg2+ rMg = nm
rMg/rO = 0.514
cations prefer OH sites
So each oxygen has 6 neighboring Mg2+

13. 2nd Type of AX Crystal Structure
Cesium Chloride structure:
 cubic sites preferred
So each Cs+ has 8 neighboring Cl-

14. 3rd Type of AX Crystal Structures
Zinc Blende structure
Size arguments predict Zn2+ in OH sites,
In observed structure Zn2+ in TD sites
Why is Zn2+ in TD sites?
bonding hybridization of zinc favors TD sites
So each Zn2+ has 4 neighboring S2-
Ex: ZnO, ZnS, SiC

15. AX2 Crystal Structures Fluorite structure Calcium Fluorite (CaF2)
Cations in cubic sites
UO2, ThO2, ZrO2, CeO2
antifluorite structure –
cations and anions reversed
rC/rA for CaF2 is about 0.8 – coordination number of 8  cubic structure
But, stoichiometry calls for ½ as many Ca2+ as F- ions 8 cubes in unit cell

16. ABX3 Crystal Structures
Perovskite crystal structure
Ex: Barium Titanate – BaTiO3
Temperatures above 120oF –
cubic crystal structure

17. Summary of Common Structures

18. Close Packing of Anions
Coordination = 4
Coordination = 6
Since Anions are commonly packed in FCC structure – we can talk about close packed planes of anions
Can have both:
FCC Stacking – ABCABC
HCP Stacking – ABABAB
Cl- form FCC Lattice
Close packed planes are {111}

19. Mechanical Properties
Why are ceramics more brittle than metals?
Consider method of deformation
In metals we have dislocation motion along slip planes
Slip planes are the close packed planes
In ionic solids dislocation motion is very difficult
Why? Too much energy needed to move one anion past another anion