GATE MT 2012 Q9. Which one of the following is an equilibrium defect?

Correct answer is option A.

• Formation of a vacancy leads to ‘missing bonds’ and distortion of the lattice. Hence, the potential energy (Internal energy & Enthalpy) of the system increases.
• Work required for the formation of a point defect →  Enthalpy of formation {DHf = (H crystal with one vacancy- H perfect crystal)} [kJ/mol or eV/defect].
• Though it costs energy to form a vacancy, its formation leads to increase in configurational entropy (the crystal without vacancies represents just one state, while the crystal with vacancies can exist in many energetically equivalent states, corresponding to various positions of the vacancies in the crystal → ‘the system becomes configurationally rich’).
•  At some temperature above zero Kelvin there is an equilibrium concentration/number of vacancies (at ‘low’ temperatures no vacancies may be stable).  These type of vacancies are called Thermal Vacancies (and will not leave the crystal on annealing at a temperature at which these are stable→ Thermodynamically stable).