1. Then fill all the octahedral sites with M ions
Ball Model (Rock Salt) X M
First fill all the FCC sites with X ions
Lect 3, Page 1
Then fill all the octahedral sites with M ions

2. Wurtzite (e.g. ZnO) A O2- at z = 0 Zn2+ at z = 1/8 B O2- at z = ½
Lect 6, Page 2

3. Filling of Spheres: CorundumA
O2- (1st layer) X
Al3+ (1st layer) X X B
O2- (2nd layer)
Al3+ (2nd layer) A
O2- (3rd layer)
Al3+ (3rd layer) x
vacant octahedral site
Whole structure consists of 6 layers of oxygen
Lect 6, Page 4

4. Filling of Spheres: IlmeniteA
O2- (1st layer) X
Fe2+ (1st layer) X X B
O2- (2nd layer)
Ti4+ (2nd layer) A
O2- (3rd layer)
Fe2+ (3rd layer) x
vacant octahedral site
Whole structure consists of 6 layers of oxygen
Lect 6, Page 8

5. Projection on {1010} Plane: Ilmenite
[0001]
Lect 6, Page 9

6. Lithium Niobate (LiNbO3)
Structure is similar to Al2O3 except that Al sub-lattice is substituted in a ordered manner by Li and Nb in the same layer unlike in alternating layer in FeTiO3
Ferroelectric nature
Highly anisotropic refractive index
Birefringence
Changeable by electric field
Used in electro-optic devices
Mixed Li and Nb occupancy
(atoms need to be differently coloured)
Lect 6, Page 10

7. LiNbO3
Bond strength principle can be applied to check the stoichiometry.
Charge dipole along [001] is responsible for ferroelectricity.
Charge Dipole
Lect 6, Page 10
(atoms need to be differently coloured)

8. Rutile Structure Polymorph of titanium di-oxide or TiO2
Other forms are Anatase and Brookite
It is formed by quasi-HCP packing of anions
Half of the octahedral sites filled by cations
Resulting structure is tetragonal due to slight distortion
Anisotropic diffusion properties of cations in TiO2
Large and anisotropic refractive index
High Bi-refringence
Used as pigments and is non-toxic
Lect 6, Page 11

9. Rutile ½ of the Octahedral Sites Filled Lect 6, Page 11
(atoms need to be differently coloured)

10. Unit-cell of Rutile Lect 6, Page 12
(atoms need to be differently coloured)

11. Polyhedra Model of Rutile
Lect 6, Page 12

12. ReO3 Structure Stoichiometry : MX3
Atomic Positions         M      0, 0, 0         X      ½, 0, 0
Primitive cubic unit-cell
Coordination Numbers/Geometry         M        CN=6     Octahedral coordination         X         CN=2     Linear coordination
Can be visualized as perovskite ABO3 structure with empty B-sites
Representative Oxides
ReO3, UO3, WO3
Used for gas sensing and electrochromic applications
Lect 6, Page 13

13. Structure of ReO3 Oxygen Cation Lect 6, Page 13
(atoms need to be differently coloured)

14. Summary Anions form the base lattice
Interstices can be completely or partially filled
Pauling’s rules play important role in structure determination
Deviations lead to structural distortions
Most compounds follow three common structures
FCC packing of anions
HCP packing of anions
Primitive cubic structures
Lect 6, Page 14