The void is an empty space inside the sphere. The unit cell and the type of packing of crystals are two main factors that determine the size and shape of the voids. Close packing in solids happen in 3 ways, I,e 1D close packing, 2D close packing and 3D close packing. In 2-dimensional packing structures, the atoms are arranged in between squares hence we see square type empty spaces. In the hexagonal close packing, we see triangular empty spaces left over between the atoms and these empty spaces are called voids.The size of the void can be expressed in terms of the radius of a sphere that can exactly fit in the void to the radius of the entire sphere. Mathematically we can express the radius ratio as follows:
Radius ratio= rR
Types of voids
Voids are divided into various groups. Namely, trigonal, tetrahedral and octahedral voids.
The trigonal void is formed with equal radii and touches each other in the structure.
From the above diagram, the radius ratio of a trigonal void is approximately 0.155 and has a coordination number of 3.
Tetrahedral voids are formed by four spheres lying close to each other and they are situated at the central point of the tetrahedron.
From the diagram above, the radius ratio of the tetrahedral void is approximately 0.225 and has a coordination number of four.
The octahedral void is formed at the centre when six spheres are linked together in the form of an octahedron.
Cubic voids are formed by close contact of eight spheres. It has a radius ratio of approximately 0.732 and has a coordination number of eight. They have a rank of one and are always positioned at the centre of the body.