Vacancies, interstitials and radiation damage

The formation of an interstitial, on the other hand, involves the transfer of a molecule from a general surface position to an interstitial position, breaking in the process two hydrogen bonds and replacing the Van der Waals interaction with a half-population of 

Because of the open, four-coordinated structure of ordinary ice, the energy involved in the creation of an interstitial molecule is not extreme. This is doubly clear from the existence of Ice VII, in which the energy penalty for the creation of a whole network of interstitials in a self-clathrate structure is more than balanced by the energy gained from the decrease in volume once the pressure exceeds 22 kbar (see chapter 3), though the phase transition I VII 

although vacancies and interstitials exist in thermal equilibrium in ice, their concentrations are small even at the melting point and for this reason the density of ice crystals, measured by macroscopic means, agrees closely with the value derived from X-ray determinations of lattice constants. 

To produce larger concentrations of defects we must resort to a non-equilibrium situation such as exists in a crystal bombarded by 

---