CRYSTAL STRUCTURE

2.  Submitted to –
Mr.Gyanrao
Dhote
 Submitted by -
Kamini Malviya

5. A unit cell of the crystal is an
imaginary parallel-sided region from
which the entire crystal can be built
up.
Usually the smallest unit cell
which exhibits the greatest
symmetry is chosen. If repeated
(translated) in 3 dimensions, the
entire crystal is recreated.

6. •The simplest repeating unit in a crystal is
called a unit cell.
•Opposite faces of a unit cell are parallel.
•The edge of the unit cell connects equivalent
points.
•Not unique. There can be several unit cells of
a crystal.
•The smallest possible unit cell is called
primitive unit cell of a particular crystal
structure.
•A primitive unit cell whose symmetry matches
the lattice symmetry is called Wigner-Seitz
cell.
The Unit Cell Concept

7. •Each unit cell is defined in terms of lattice points.
•Lattice point not necessarily at an atomic site.
• For each crystal structure, a conventional unit
cell, is chosen to make the lattice as symmetric as
possible. However, the conventional unit cell is not
always the primitive unit cell.
•A crystal's structure and symmetry play a role in
determining many of its properties, such as
cleavage (tendency to split along certain planes
with smooth surfaces), electronic band structure

9. Symmetry elements: Four 3-fold
rotation axes along cube diagonals
a = b = c
 =  =  = 90o
3 Bravais lattices

10. Only one Bravais lattice
Symmetry element:
One 6-fold rotation
axis
a = b  c
= 120o
 =  = 90o

11. Symmetry element: One 4-
fold rotation axis
a = b  c
=  =  = 90o
Two Bravais
lattices

12. One Bravais lattice
Symmetry element: One
3-fold rotation axis
a = b  c
= 120o
 =  = 90o

13. Four Bravais lattices
Symmetry element: Three mutually
perpendicular 2-fold rotation axes
a  b  c
 =  =  = 90o

14. Two Bravais lattices
Symmetry element: One 2-fold rotation
axis
a  b  c
 =  = 90o,   90o

15. One Bravais lattice
Symmetry element:
None
a  b  c
      90o