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

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

Filling of Spheres: Corundum A 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

Filling of Spheres: Ilmenite A 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

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

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