a Schottky defect
/ SHOT-kee /
In an ionic crystal like table salt, you cannot simply remove one atom the way you can in a metal — every ion carries an electric charge, and pulling out a single positive sodium ion would leave the crystal with a net negative charge, which nature will not allow. A Schottky defect is the honest way ionic crystals make vacancies while keeping the books balanced: they remove a positive ion and a negative ion together, as a matched pair, so the overall charge stays neutral.
Take sodium chloride (NaCl), a rock-salt crystal of Na+ and Cl- ions. A Schottky defect is one empty sodium site plus one empty chlorine site, created together. The two missing ions typically migrate out to the surface. Charge stays balanced (one plus and one minus removed), and so does the ratio of the two elements (one of each), so the crystal's overall formula NaCl is preserved. Because it removes atoms without adding any, a Schottky defect very slightly lowers the crystal's density — the same mass sits in a slightly larger volume once you count the empty sites. In a compound with different charges, such as MgO or CaF2, the pair becomes a charge-balanced SET (for CaF2, one Ca2+ vacancy for every two F- vacancies).
Schottky defects are the dominant intrinsic (built-in, temperature-driven) point defect in the alkali halides and many oxides, and they are the carriers of ionic conduction in these materials. An ion can only move if there is an empty site of the right kind next door to hop into, so the number of Schottky vacancies directly sets how well a salt crystal conducts electricity by moving ions — the basis of solid electrolytes. Their equilibrium number rises with temperature by a Boltzmann law, just like metal vacancies, but with the formation energy shared over the pair.
In sodium chloride at high temperature, roughly one cation-anion vacancy pair forms per hundred thousand ion pairs. Each Schottky pair leaves one Na+ site and one Cl- site empty; a nearby Na+ can then hop into the sodium vacancy, carrying charge — which is exactly how solid NaCl conducts a tiny ionic current when hot.
A Schottky defect removes a cation and an anion together, keeping charge and stoichiometry balanced.
A Schottky defect is not a single vacancy — it is a charge-balanced set of vacancies. Removing just one ion would leave the crystal electrically charged, which is energetically forbidden; that constraint is why Schottky defects come in matched groups.