Seven primitive unit cells and their possible variations as centered unit cell Chemistry Notes
Parameters of Unit cell → a, b, c : axial distances or core lengths, α.β.γ : axial angles
Crystals have seven types of primitive unit cells which are given as follow
→ Cubic : In this type of unit cell, three aixal distances are same and each axial angle is of 90°. Examples: NaCl, Cu, Au, Ag, Hg, KCl, zinc blende (ZnS). KCI, CaF2, diamond etc.
→ Tetragonal : In this type of unit cell, three axis are perpendicular to each other in which two axis are same but one axis is different. The axial angle between all faces is 90°
Examples: White tin, TiO2, SnO2, CaSO4, urea etc.
→ Orthorhombic : In this type of unit cell, three axial distances are different. Here each axial angle is of 90°
Examples: Orthorhombic sulphur, KNO3, K2SO4, BaSO4, PCO3, CuSO3, C – sulphur, I2 etc.
→ Monoclinic : In this type of unit cell, three axial distances are different. Two axial angle are equal to 90° and one axial angle is different from 90°.
Examples: Monoclinic sulphur. Na2 SO4 10H2O4, PbCrO4. CaSO4. 2H2O, β-sulphur etc.
→ Hexagonal : In this type of unit cell, two axes are same but third axis is different. Similarly in this, two axial angles are of 90° but one axial angle is of 120°.
Examples: Graphite, ZnO, CdS, Pbl2, HgS, Agl, SiC, Zn, Cd, Mg etc.
→ Rhombohedral or Trigonal : In this type of unit cell, three axis are of same length. Axial angles between them are also same but not equal to 90°.
Examples: Calcite (CaCO3), quartz. NaNO3, Cinnabar (HgS), Sb etc.
→ Triclinic : In this type of unit cell, three axis are of different length and three axial angle are also different and also not equal to 90°
Example : K2Cr2O7. CuSO4 5H2O.H3BO3 etc.
Above discussed crystal systems are represented in table 1.3 .
→ Note : Bravais lattice : French mathematician Bravais (1848) proposed that seven crystal systems have been divided in to 14 different types of crystal lattice on the basis of constituent particles present in unit cell. These 14 different types of crystal lattices are called Bravais lattices. 14 Different Bravais lattices are represented in table 1.4:
|Crystal System||Types of Lattices||Number of Lattices|
|Cubic||Primitive, Body centred, Face-centred||3|
|Tetragonal||Primitive, Body centred||2|
|Orthorhombic||Primitive, Body centred, Face-centred, End centred||4|
|Total = 14|
The structures of unit cell of 14 types of Bravais lattices ara shown as follows