a Schottky defect
/ SHOT-key /
How can an ionic crystal make vacancies without ever unbalancing its charge? A Schottky defect is the answer: it is a matched SET of vacancies — the right number of cation vacancies AND anion vacancies together — so that the crystal stays neutral and keeps its ratio of sites. Picture removing one Na+ and one Cl- from the middle of a salt crystal and rebuilding those two ions onto the surface: two empty seats appear inside, the crystal is very slightly bigger, and no net charge has moved.
The stoichiometry of the compound fixes the ratio. In a 1:1 compound like NaCl or MgO a Schottky defect is one cation vacancy plus one anion vacancy; in a 1:2 compound like a fluorite MO2 it is one cation vacancy plus two anion vacancies. In Kroger-Vink notation the Schottky reaction for MgO is written nil ⇌ V_Mg'' + V_O••, read as 'nothing on the left becomes a magnesium vacancy plus an oxygen vacancy', and you can check it balances: the two negative effective charges of V_Mg'' cancel the two positive of V_O••, mass is conserved (the ions went to the surface), and one cation site plus one anion site preserve the 1:1 ratio. Their equilibrium number follows a Boltzmann factor with the Schottky formation energy, n proportional to exp(-E_S/2kT).
Schottky disorder is the dominant intrinsic (self-generated, impurity-free) defect type in most close-packed rock-salt oxides and halides — MgO, NaCl, KCl, NiO — where making a roomy interstitial would cost too much. It sets the floor on how many vacancies a pure crystal carries at a given temperature, and therefore the intrinsic diffusion and conductivity of the pure material, before any doping is added.
Rock-salt MgO is a classic Schottky crystal: heat it and matched pairs of V_Mg'' and V_O•• appear throughout the bulk. This intrinsic vacancy population is why nominally pure MgO still sinters and still conducts a trickle of ionic current at high temperature.
Schottky means vacancies only, in a charge- and ratio-balanced set; no interstitials are involved.
A Schottky defect is not a single vacancy but a stoichiometric set of them. Counting only the cation vacancy and forgetting its partner anion vacancies is the most common bookkeeping error.