Ball Model (Rock Salt)

X

M

First fill all the FCC sites with X ions

Then fill all the octahedral sites with M ions Lect 3, Page 1

Wurtzite (e.g. ZnO)

A O2- at z = 0

B O2- at z = ½

A O2- at z = 1

Zn2+ at z = 1/8

Zn2+ at z = 5/8

Lect 6, Page 2

Filling of Spheres: Corundum

A O2- (1st layer)

B O2- (2nd layer)

A O2- (3rd layer)

Al3+ (1st layer)

Al3+ (2nd layer)X

X

X X

X

X

X

X

X

x vacant octahedral site

Whole structure consists of 6 layers of oxygen

Al3+ (3rd layer)

Lect 6, Page 4

Filling of Spheres: Ilmenite

A O2- (1st layer)

B O2- (2nd layer)

A O2- (3rd layer)

Fe2+ (1st layer)

Ti4+ (2nd layer)X

X

X X

X

X

X

X

X

x vacant octahedral site

Whole structure consists of 6 layers of oxygen

Fe2+ (3rd layer)

Lect 6, Page 8

Projection on {1010} Plane: Ilmenite

[0001]

Lect 6, Page 9

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

Lect 6, Page 10(atoms need to be differently coloured)

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)

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

Rutile

½ of the Octahedral Sites Filled

Lect 6, Page 11(atoms need to be differently coloured)

Unit-cell of Rutile

Lect 6, Page 12(atoms need to be differently coloured)

Polyhedra Model of Rutile

Lect 6, Page 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

Structure of ReO3

Oxygen

Cation Lect 6, Page 13(atoms need to be differently coloured)

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