Difference Between BCC and FCC

Key Difference – BCC vs FCC
 

The terms BCC and FCC are used to name two different arrangements of crystalline structures. BCC stands for body-centred cubic structure whereas FCC stands for face-centred cubic structure. These are forms of cubic lattices. Therefore, these arrangements have spheres (atoms, molecule or ions from which the lattice is made of) arranged in cubic structures. The unit cell of BCC has spheres in the corners of a cube and one sphere in the centre of the cube. Since there are eight corners in a cube, the total number of spheres present in a BCC unit cell is 9. The unit cell of the FCC has spheres in each corner of a cube and also in the centre of each cubic face. Then the unit cell of FCC has 12 spheres. The key difference between BCC and FCC is that the coordination number of BCC is 8 whereas the coordination number of FCC is 12.

CONTENTS

1. Overview and Key Difference
2. What is BCC
3. What is FCC
4. Similarities Between BCC and FCC
5. Side by Side Comparison – BCC vs FCC in Tabular Form
6. Summary

What is BCC?

The term BCC stands for the body-centred cubic arrangement of spheres (atoms, molecule or ions from which the lattice is made of). In this arrangement, the spheres are located in each corner of a cube and one sphere in the middle of the cube. A unit cell of a lattice is the smallest unit that resembles the whole structure of the lattice. Since a cube has 8 corners, there is a total of 9 spheres in a BCC structure (eight in the corners plus on in the middle).

However, each sphere in the corner of the unit cell of BCC is a member of the neighbouring unit cell. That is because the lattice is made of many unit cells packed together. Since there are 8 spheres in a unit cell which are the corners of other unit cells, the coordination number of the BCC structure is known to be 8. Then, when considered the net total of spheres in a BCC unit cell, it has 2 spheres because one corner has 1/8^th of spheres. The eight corners together form one sphere, and there is one sphere in the middle, together with results in two spheres.

The packing of spheres in BCC arrangement is not tight. That means the packing of spheres in BCC is not a close packing as in FCC (face-centred cubic) or HCP  (hexagonal close packing). The packing factor of BCC is 0.68. Packing factor is the volume of spheres per the volume of the unit cell. Examples of metals that have the BCC structure include Lithium (Li), Sodium (Na), Potassium (K), Chromium (Cr) and Barium (Ba).

What is FCC?

The term FCC stands for the face-centered cubic arrangement of spheres. In this arrangement, the spheres are located in each corner of a cube (unit cell) and in the centres of each cubic face. Here also, each sphere in the corners is a member of the neighbouring unit cell. Apart from that, each sphere in the centre of the cubic face is shared with the adjacent unit cell.

The coordination number of FCC is 12. That is because there are 12 spheres per unit cell that are shared with other unit cells. The net total of spheres present in FCC unit cell is 4. It can be calculated as follows.

Total spheres in corners = (1/8) x 8 = 1

Total spheres in cubic faces = (1/2) x 6 = 3

Then the total spheres per unit cell = 1 + 3 = 4

The FCC structure has more packing of spheres than that of BCC (the spheres pack closer together). The packing factor of FCC structure is 0.74. This means the ratio between the volume occupied by spheres and the total volume of the unit cell is 0.74. Some examples of the metals having FCC structure are Aluminum (Al), Copper (Cu), Gold (Au), Lead (Pb) and Nickel (Ni).

What are the Similarities Between BCC and FCC?

• Both BCC and FCC are forms of arrangement of cubic lattices.
• The unit cell of both BCC and FCC structures is a cube.

What is the Difference Between BCC and FCC?

Summary – BCC vs FCC

BCC stands for the body-centred cubic arrangement. FCC stands for the face-centred cubic arrangement. These arrangements are used to describe the location of atoms, molecules or ions and the empty spaces present in a lattice structure. The difference between BCC and FCC is that the coordination number of BCC is 8 whereas the coordination number of FCC is 12.

Reference:

1.Primary Metallic Crystalline Structures.  Available here 
2.“FCC and BCC.” BINDT. Available here  

Image Courtesy:

1.’CCC crystal cell (opaque)’By  Daniele Pugliesi (CC BY-SA 3.0) via Commons Wikimedia  
2.’FCC crystal structure’By User:ARTE – Own work, (Public Domain) via Commons Wikimedia