Crystal lattice and unit cell Chemistry Notes
Crystal Lattice or Space lattice :
The regular arrangement of constituent particles (lattice points) of crystalline solids in three-dimensional network is called crystal lattice or space lattice. Only 14 crystal lattices are possible. These are called Bravais lattice which are named in honour of french mathematiceian Bravais.
Some properties of a crystal lattice areas follow :
- Each point in a lattice is called lattice point
- Each point in a crystal lattice represents a constituent particle. These constituent particles may be atoms, molecules or ions.
- If lattice points are attached by straight lines then it can represent geometry of lattice,
Unit cell :
- The smallest unit of whole crystal structure which when repeated over again and again equally in different directions again form crystal structure, is called unit cell. There are following characteristics of unit cell:
- The dimensions of three edges of unit cell are represented by a, b and c which may or may not be perpendicular to each other. It is represented in figure 1.9
- The angles B and y between edges are also represented in figure 1.9. Unit cell is characterised by six parameters a,b,c, c.B and y.
Types of Unit Cells :
Primitive or simple cubic unit cell If constituent particles in unit cell are present only at corners then it is called primitive unit cell. Here, the total number of lattice points is 8. (Figure 1.10)
Centred unit cell :
When one or more constituent particles in a unit cell are also present at other places besides corners then it is called centred unit cell. These unit cells are of three types.
Body centred unit cell Here, constituent particles (atoms, molecules or ions) are present at corners and centre of body of unit cell. It is called body centred unit cell. The total number of lattice points are 9. (fig. 1.11)
Face-centred unit cell: Here, constituent particles are also present at centre of each faces besides corners. It is called face-centred unit cell. Here, the total number of lattice points are 14. (Figure 1.12)
End-centred unit cell Here, constituent particles are present at centres of two opposite faces besides the corners. Here, the total number of lattice points are 10. (Figure 1.13)