Question

Why do vacancies and interstitials have a positive energy of formation? Why are they present at finite temperatures if they’re not energetically favored?

Answer 1

Solution: Naturally, a vacancy and interstitial can form simultaneously if an atom is displaced from a lattice site into a nearby interstitial site. Since, the newly formed interstitial and vacancy are in close proximity, the strain introduced into the lattice is less than in the case of isolated vacancies and interstitials. Hence, the formation energy of a vacancyinterstitial pair is reduced in comparison to the total formation energy required to produce an isolated vacancy and isolated interstitial.