Chemistry · Ch 6 — Solid State
Schottky Defect
Schottky Defect
Schottky defect arises when equal numbers of cations and anions go missing from the crystal lattice. Because a cation vacancy and an anion vacancy always appear together in matched pairs, this defect leaves the crystal's overall stoichiometry unchanged. It is seen in ionic solids where the cation and the anion are of almost similar size -- NaCl is the standard example.
A large number of Schottky defects in a crystal noticeably lowers its density, since mass is removed from the lattice (as missing ions) while the overall unit-cell volume stays essentially the same. This effect can even be used to measure the extent of the defect: the theoretical density of vanadium monoxide (VO), calculated purely from its unit cell's edge length, works out to 6.5 g/cm^3, but the actual, experimentally measured density is only 5.6 g/cm^3 -- a shortfall that indicates approximately 14% Schottky defect in the VO crystal. …
What this figure shows. A grid of alternating pink '+' spheres (Na+) and green '-' spheres (Cl-) arranged in a checkerboard-like ionic lattice, with a visible gap running down the middle of the grid where several adjacent +/- sphere pairs are simply absent -- illustrating equal numbers of missing cations and anions -- with a legend keying pink-plus to Na+ and green-minus to Cl-, captioned 'Schottky Defect'. Crucially, the missing spheres in the gap always come in matched pairs -- for every absent pink Na+ site there is an absent green Cl- site nearby -- rather than the lattice simply running short of one ion type, which is exactly the visual signature of a defect that removes ions from …