Engineering Physics

2.7 Structure and packing fractions of body-centred cubic structure [BCC]

For this unit cell, atoms are present at the corners of the cube and one atom is completely present at the centre of the unit cell. The centre of the unit cell is defined as the intersecting point of two body diagonals [AD and BE as shown in Fig. 2.13]. A corner atom is shared by eight unit cells so that the contribution of a corner atom to a unit cell is 1/8. Therefore, the number of atoms per unit cell = (1/8)×8+1 = 2. The centre atom is surrounded by eight corner atoms, so the coordination number is 8. The surfaces of unit cell corner atoms may not touch, but they are in contact with the centre atom i.e., the surfaces of atoms are in contact along a body diagonal of ...