Interstitial Defects: Introduction and Exploration

A interstitial defect is when an atom occupies a lattice site in between the sites of the atoms making up the normal crystal. You may have a intrinsic interstitial defect, also called a self-interstitital in which an atom from the host crystal is occupying its own interstitial sites. This would be an intrinsic defect because no foreign atoms are needed to form it. You can also have a interstitial impurity which would be extrinsic because there's a impurity, or foreign atom, in the host crystal.

Self-interstitials

These defects also existed in the graphite piles at the Windscale facility. Indeed, when the carbon atom gets knocked out of the graphite lattice, it needs to go somewhere, and it could sometimes settle in a triangular interstitial location (or, more commonly, in-between the planes of carbon). Because interstitial sites are usually only a fraction of the size of host atoms, self-interstitials can often cause very large distortions in host crystals and can therefore be highly energetically unfavorable.

This energy depends on the crystal structure. Crystals with large interstitial sites (like SC and diamond) can more easily accommodate self-interstitials because there is less strain. For example, the cubic site in simple cubic is $r_{\mathrm{cubic}} = 0.72 r_{\mathrm{SC}}$. More closely packed structures like FCC or BCC will experience much more strain when a self-interstitial is present.

Interstitial Impurities

On the other hand, when impurity atoms are introduced to a host lattice, sometimes the impurity atoms are the right size to nestle into an interstitial site. In this case, we don't introduce a huge amount of strain energy to the lattice and the atoms can be accommodated.

The construction of ionic crystals in Section 5.12 really built on this concept, where we analyzed the size of impurity atoms (ions in that case) in interstitial sites in terms of ranges of stability. We introduced Pauling's rules for building intuition about crystal stability based on this.

Here, it is important to know that you can also have simple, non-charged impurities positions in interstitial sites as well. The most prominent example is carbon in iron, which will occupy various interstitial sites (and induced strain, which is important later) to form steel. This strained iron-carbon structure turns out to be one major reason that steels are so much stronger than pure iron - via a process called solution strengthening. We'll delve into this in Chapter 11.

NetLogo model 6.6.1 below is a molecular dynamics model of a crystal with a single interstitial impurity atom. Note, in this model, temperature (and therefore kinetic energy) is held constant at the value of the temp slider. This means that the velocities of the atoms are scaled each time step so that the average kinetic energy of the atoms results in the set temperature.

NetLogo model 6.6.1 above is a molecular dynamics model, like the one in the previous chapter (Section 5.2), but now there is am interstitial atom. It is the small red atom in the middle. You can increase and decrease its size with the corresponding buttons. The way this is modeled is that instead of a single σ parameter in the Lennard-Jones potential (Eq. 4.5.1), each atom has its own σ and the interatomic potential between two atoms is calculated using the average of their σ parameters. The relative size of the atom compared to the host lattice is displayed in the "interstitial size" monitor on the left of the model.

On the previous page, you were asked specific questions about what happens at different sizes of the substitutional atoms. Here, you will need to do a little more open-ended exploration yourself. At different sizes of the interstitial atom what changes in terms of the potential energy of the system and the lattice structure? Identify different types of behavior at different sizes ranges.