Final answer:
Vacancies are thermodynamically stable due to entropy contribution to the crystal's Gibbs free energy, while dislocations create substantial strain that requires energy to maintain, making them less stable.
Step-by-step explanation:
Why are vacancies thermodynamically stable defects, whereas dislocations aren't? Vacancies are a type of point defect and are inherent to the crystalline structure due to thermodynamic equilibrium. Because the formation of vacancies increases the entropy of the crystal, which is favorable in terms of Gibbs free energy (ΔG = ΔH - TΔS), this stability is generated. Dislocations, on the other hand, are line defects that significantly disrupt the crystal structure along a one-dimensional line. They are less stable because they create a substantial strain field within the crystal, which requires energy to maintain.
During the process of crystal formation, a certain number of vacancies can be thermodynamically favored. Vacancies can maintain electrical neutrality, such as in Schottky defects, where a coupled pair of vacancies - one cation and one anion - maintain the electrical neutrality of an ionic solid. Conversely, line defects like dislocations can weaken the structure along a line, affecting the material's mechanical properties. The study of dislocations is critical for structural materials like metals, where controlled dislocation movement can enhance qualities like ductility and malleability. Chemical etching can reveal these dislocations, which might result in pits visible under small magnifications.