Question

What is meant by voids in crystal structure?

Answer 1

Voids literally mean $gaps$ between the constituent particles. Voids in solid states mean the vacant space between the constituent particles in a closed packed structure. Close packing in solids can be generally done in three ways: 1D close packing, 2D close packing and 3D close packing. In 2 dimensional structures when the atoms are arranged in square close packing and hexagonal close packing, we see empty spaces left over between the atoms. These empty spaces are called voids and in the case of hexagonal packing, these voids are in triangular shapes and are known as the triangular voids.

The size and distribution of voids in materials plays a role in determining In the close packed crystals (FCC, HCP) there are two types of voids - $tetrahedral$ and $octahedral$ voids (identical in both the structures as the voids are formed between two layers of atoms).