STRUCTURE TYPES

Rock Salt (NaCl)•Fcc with respect to anions.•Cations occupy octahedral voids.•Octahedral sites are present at edge

centres and body centre.•There are 4 octahedral sites per unit cell.•General formula AX.•CN of anion=8•CN of cation=8

• Each ion and its neighbours can be represented by apt. polyhedra.

• Octahedron has 12 edges.• Each edge shared between two

octahedron.• Octahedron centred on Na ions.• Most halides & hydrides of

alkali metals and Ag+ have this structure.

• Mostly ionic, but TiO metallic.

Zinc Blende or Sphalerite (ZnS)•Fcc w.r.t anions.•Two types of tetrahedral sites are there : T+ , T_

•Cations occupy tetrahedral sites, either T+ or T_ rest are empty.

•Tetrahedral sites are present at (1/4,1/4,3/4) position and its equivalent points.

•There are total 8 tetrahedral sites, 4 T+ and 4 T_ •General formula AX.•CN of anion=4•CN of cation=4

•Contains ZnS4 or SZn4 tetrahedra.

• Each corner shared by four tetrahedra.

• Bonding is less ionic than rock salt structure.

• Oxides do not have this structure

•Exception ZnO- dimorphic with zinc blende and wurtzite.

•Covalent compounds of Be, Zn, Cd and Hg have this structure.

Antifluorite (Na2O)•Fcc w.r.t anions.•Tetrahedral sites T+ and T_ are occupied

by cations. Octahedral sites are empty.•General formula A2X•CN of anion=8•CN of cation=4•Structure shown by oxides and

chalcogenides of alkali metals.

•For CN of anion displace unit cell along body diagonal by ¼, so that cation becomes new origin of unit cell.

•It contains cations at corners, edge centres, face centres and body centres.

•Centre of smaller cube is primitive, hence CN 8.

Structure can be described in two ways:1. 3-D network of tetrahedra :

▫ eight NaO4 tetrahedra▫ Each edge shared between two

tetrahedra2. 3-D network of cubes :

▫ four ONa8 cubes ▫ Each corner common to 4 cubes▫ Each edge common to 2 cubes

Fluorite (CaF2)•Fcc w.r.t cation.•Tetrahedral sites T+ and T_ are occupied

by anions. Octahedral sites are empty.•General formula AX2.•CN of anion=4•CN of cation=8•It includes fluorides of large, divalent

cations and oxides of large tetravalent cations.

•Arrangement of cubes shows MF8 coordination in fluorites.

•Anions at the centre of cube i.e. , in voids.•Cations at corners and face centres.

CsCl Type•Simple cubic w.r.t anion.•Anions present at corners.•Cation present in cubic void i.e., body

centre.•General formula AB.•CN of anion=8•CN of cation=8

Wurtzite (ZnS)• HCP w.r.t anion.• Cations occupy

tetrahedral sites, either T+ or T_ rest are empty.

• c/a ratio=1.633 (assuming anions are in contact)

• Tetrahedral site at a distance 3/8 above anion.

• 12 tetrahedral voids are present, only 6 are occupied.

Nickel Arsenide (NiAs)• HCP w.r.t anion.• Cations occupy octahedral voids.• 6 octahedral voids are present.• Ni and As have the same CN but

not the same coordination environment unlike rock salt structure.

• AsNi6 trigonal prisms which link up by sharing edges.

• c/a ratio varies for different compounds whereas same for wurtzite.

Rutile (TiO2)• Distorted HCP oxide array or tetragonal packed oxide

array.• ½ octahedral sites occupied by Ti.• Alternate rows of octahedral sites are full and empty.• Oxides of tetravalent metal ions & fluorides of small

divalent ions exhibit this structure.• M4+ and M2+ are too small to form fluorite structure

with O2- and F- .

In CdI2 layers of octahedral sites are occupied and these alternate with empty layers.In CdI2 anions are HCP whereas in CdCl2 they are CCP.

Silicate Structures• Composed of cations and silicate anions.• Mostly built of SiO4 tetrahedra.

Exception: In SiP2O7 , Si is octahedrally coordinated to oxygen.

• SiO4 tetrahedra links up by sharing corners. Never share edges or faces.

• A corner is shared by maximum of 2 tetrahedra.• To relate formula to structure Si:O ratio is important.• Si:O ratio is variable.• Two types of oxygen atoms: bridged and non-bridged.• Exact structure cannot be determined.

Lattice Energy•Net potential energy of arrangement of

charges.•Denoted by U.•Equivalent to sublimation energy.

•U depends on :▫Crystal structure▫Charge on ions▫Inter nuclear separation

•Attaractive force given byF=(Z+ Z- e2 / r2 )

Potential energy V=(- Z+ Z- e2 / r )•Repulsive energy V=(B/ rn )

B=Born exponent5<n<12 (for large n V--» 0)

•U is a combination of electrostatic attaraction and Bohr repulsion.

•U

A is Madelung constant. Depends on geometrical arrangement of point charges.

Born Haber Cycle• Lattice energy is equivalent to heat of formation

from one mole of its ionic constituents in gaseous phase. Cannot be measured experimentally.

• ΔHf can be measured. Can be related to U via a thermodynamic cycle known as Born Haber cycle.

• Stability of compounds can be determined by finding ΔHf .

• Difference between theoretical & calculated lattice energies provides evidence for non-ionic bonding.