Question

Explain briefly seven types of unit cells.

Answer 1

Seven types of unit cells:

      Cubic

a = b = c

α = β = γ = 90°

   Rhombohedral

a = b = c

α = β = γ ≠ 90°

    Hexagonal

a = b ≠ c

α = β = 90°, γ = 120°

 Tetragonal

a = b ≠ c

α = β = γ = 90°

   Orthorhombic

a ≠ b ≠ c

α = β = γ = 90°

     Monoclinic

a ≠ b ≠ c

α = γ = 90°, β ≠ 90°

     Triclinic

a ≠ b ≠ c

α ≠ β ≠ γ ≠ 90°

1. Cubic – NaCl
2. Rhombohedral – Cinnabar Cubic
3. Hexagonal – ZnO
4. Tetragonal – TiO₂
5. Orthorhombic – BaSO₄
6. Monoclinic – PbCrO₄
7. Triclinic – H₃BO₃

They differ in the arrangements of their crystallographic axes and angles.

Corresponding to the above seven, Bravis defined 14 possible crystal systems as shown in the figure.