When we talk about voids in the solid state, we're referring to the empty spaces between particles in a closely packed structure. This concept is critical in understanding the arrangement of atoms or molecules in a solid. There are three common ways to achieve close packing in solids: 1D, 2D, and 3D close packing.
In a 2D structure, atoms are arranged in square or hexagonal close packing, leaving empty spaces, or "voids", between them. In hexagonal packing, these voids take a triangular shape, hence the name "triangular voids".
Packing of Spheres in 2 dimension
When we look at these triangular voids in a hexagonal packing, we find that they show two orientations. One row has the triangle's apex pointing up, while the other row has it pointing down.
In 3D structures, such as CCP (Cubic Close Packing) and HCP (Hexagonal Close Packing), about 26% of the total space is not covered by spheres. These spaces are known as interstitial voids or interstices. The number of these voids depends on the number of spheres in the structure.
In a 3D structure, we typically find two types of interstitial voids:
- Tetrahedral voids: In a cubic close packed structure, the spheres of the second layer lie on top of the triangular voids of the first layer. Each sphere is in contact with three spheres of the first layer. By connecting the centers of these four spheres, we form a tetrahedron. The empty space left in the center of these spheres forms a tetrahedral void.
- Octahedral voids: Octahedral voids are located adjacent to tetrahedral voids. When the triangular voids of the first layer align with the triangular voids of the layer above or below it, a void is formed, enclosed by six spheres. This empty space, formed by combining the triangular voids of the two layers, is referred to as an octahedral void.
In a closely packed structure, the number of tetrahedral voids is twice the number of spheres (if the number of spheres is n, then the number of tetrahedral voids is 2n), and the number of octahedral voids equals the number of spheres.
Remember that learning is a never-ending journey. While we've touched on the basics of voids in solid states here, there's much more to explore in the world of solid state physics. Stay curious and keep learning!