Contains information about various crystal types in solid state chemistry like Rock Salt, Wurtzite, Nickel Arsenide, Zinc Blende etc. It also gives a brief description of lattice energy and Born Haber cycle.

1. STRUCTURE TYPES

2. 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

3. • 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.

4. 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

5. • 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 ZnOdimorphic with zinc
blende and wurtzite.
• Covalent compounds of
Be, Zn, Cd and Hg have
this structure.

6. Antifluorite (Na O)
2
• 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.

7. • 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.

8. Structure can be described in two ways:
1. 3-D network of tetrahedra :
▫
▫
eight NaO4 tetrahedra
Each edge shared between two tetrahedra
2. 3-D network of cubes :
▫
▫
▫
four ONa8 cubes
Each corner common to 4 cubes
Each edge common to 2 cubes

9. 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.

10. • Arrangement of cubes shows MF8 coordination
in fluorites.
• Anions at the centre of cube i.e. , in voids.
• Cations at corners and face centres.

11. 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

12. 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.

13. 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.

14. 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.

15. 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 nonbridged.
• Exact structure cannot be determined.

16. 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

17. • Attaractive force given by
F=(Z+ Z- e2 / r2 )
Potential energy V=(- Z+ Z- e2 / r )
• Repulsive energy V=(B/ rn )
B=Born exponent
5<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.

18. 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.