The key difference between lattice site and interstitial site is that the lattice site is the position of constituent particles in the crystal lattice, whereas the interstitial site is a position between regular positions in the array of constituents of the crystal that can be occupied by other particles.
Crystal lattices are arrangements of particles (such as atoms, ions or molecules) in crystals. Crystals are solid material which has the particles in a highly ordered manner. There are different terms we can discuss regarding a crystal lattice: lattice site, interstitial site, void, crystal defects are among those terms.
CONTENTS
1. Overview and Key Difference
2. What is a Lattice Site
3. What is an Interstitial Site
4. Side by Side Comparison – Lattice Site vs Interstitial Site in Tabular Form
5. Summary
What is a Lattice Site?
Lattice site is the position of constituent particles, atoms, molecules, or ions in a crystal lattice. Therefore, a lattice site contains a series of points that has an arrangement of a specific pattern with high symmetry. We can observe a lattice site only through a microscope because they are minute and are invisible to the naked eye.
Lattices sites are occupied by atoms, ions or molecules of the crystal; these are either of the same type or different types. Moreover, if the lattice sites contain atoms of the same type, then we call it a monatomic crystal lattice, and if there are different types of atoms, then it is a polyatomic crystal lattice. Often, monoatomic crystal lattices are simple compared to polyatomic crystals. Polyatomic lattices are composite lattices. For instance, table salt or NaCl is a composite lattice, and its lattice sites are occupied by sodium (Na) and chlorine (Cl) atoms.
Figure 01: Face-centered Cubic Crystal Lattice with Lattice Sites in Blue Colour
Furthermore, there can be interstitials. An interstitial is an atom that occupies a crystal lattice in a position where regular constituents are not assigned. That means; interstitial atoms do not occupy lattice sites. Consequently, these atoms do no mix with substitutional atoms of the crystal. Moreover, if there is a vacancy in a lattice, we call it an empty lattice site. We can also create a vacancy by removing a particle from a lattice site. Then this removed atom will accommodate a nearby atomic site, which is easy to accommodate. Introducing this type of vacant points to the crystal lattice increases the entropy of the crystal.
What is Interstitial Site?
Interstitial site is a position between the regular positions in an array of constituent particles that can be occupied by other particles. Most of the times, crystals have cubic close-packed or hexagonal close-packed structures. There are sites or “holes” that atoms can occupy in these structures (atoms other than the constituents of the crystal). These are called interstitial sites and they have either tetrahedral or octahedral coordination geometry. The atoms that accommodate these sites are interstitials or interstitial atoms. We can observe one octahedral hole and two tetrahedral holes per each packing structure.
Figure 02: An Interstitial Site in a Crystal
Furthermore, interstitial atoms can jump from one interstitial site to another, which we can call as diffusion of interstitials. However, regular lattice sites are not involved in this diffusion mechanism. This ability is very important in creating semiconductors.
What is the Difference Between Lattice Site and Interstitial Site?
Lattice site and interstitial site are two different positions in a crystal lattice. The key difference between lattice site and interstitial site is that the lattice site is the position of constituent particles in the crystal lattice, whereas the interstitial site is a position between regular positions in the array of constituents of the crystal that can be occupied by other particles.
Below infographic provides more facts on the difference between lattice site and interstitial site.
Summary – Lattice Site vs Interstitial Site
Lattice site and interstitial site are two different positions in a crystal lattice. The key difference between lattice site and interstitial site is that the lattice site is the position of constituent particles in the crystal lattice, whereas the interstitial site is a position between regular positions in the array of constituents of the crystal that can be occupied by other particles.
Reference:
1. Tan, T.y. “Compound Semiconductors: Diffusion.” Encyclopedia of Materials: Science and Technology, 2001, pp. 1425–1441., doi:10.1016/b0-08-043152-6/00263-1.
Image Courtesy:
1. “Face-centered cubic crystal lattice -(111)” By Jcwf – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Sites interstitiels cubique a faces centrees” By Christophe Dang Ngoc Chan (cdang) – Own work (CC BY-SA 3.0) via Commons Wikimedia
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