Imperfections in Crystal Structure: Stoichiometric Point Defects

Stoichiometric point defects are intrinsic defects that form in crystal lattices of pure compounds without altering their overall chemical formula and electrical neutrality.

In ionic crystals, like sodium chloride, Schottky defects arise from the absence of equal and oppositely charged ions forming lattice vacancies.

For instance, an MX₂ lattice with a Schottky defect exhibits one vacant cation and two vacant anions, ensuring electrical neutrality.

The number of Schottky defects is calculated from the N ions and the thermodynamic probability of the formation of these defects.

Frenkel defects are commonly found in crystals like silver bromide, where smaller cations move to interstitial sites between lattice points of larger anions.

The number of Frenkel defects is calculated from the N ions and N_i interstitial sites.

Frenkel defects do not change the observed density due to the consistent number of ions, while Schottky defects result in a lower observed density than that calculated from X-ray diffraction and unit cell data.

In both defects, vacancies provide a more open pathway for enhanced ion diffusion, resulting in improved electrical conductivity.