A unit cell is defined as the basic
structural part in the composition of materials. When many unit cells repeat in
a three-dimensional space, a crystal is obtained. The structure of a crystal is
same as that of a repeating unit cell. Hence crystal structure may be
classified as
(1) monoatomic
crystal, (2) diatomic crystal,
and (3) multiatomic
crystal.
Molecular
Crystal: -
Diatomic and multiatomic crystals
are also known as molecular crystals. There exist thousands of varieties of
crystal structures. Many of them have complex nature. Complex crystal structure
has two or more types mixed together, such as in
·
Sulphur, · Gallium, · Phosphorus, · Uranium etc.
Q.2 Define Bravais crystal system.
Ans. Bravais Crystal System: -
Crystals have unit cells of various
geometries. The geometries are defined in terms of their linear dimensions a, b
and c, and angular dimensions a, b and g. Dimensions a, b and c are along the x, y and z axes
respectively; and angles a, b and g between xy, yz and zx axes respectively. These are shown in Fig.
(1).
Bravais has classified 14 space lattices into 7 crystal systems. These are displayed in Table.
Crystals have inherent symmetry of different types. Different crystals exhibit with varieties of symmetry. The cubic crystal is most symmetric while the triclinic is least symmetric. Symmetry decreases as we move from cubic crystal system towards triclinic crystal system in Table.
Table : Bravais Crystal System
