Vacancy and Schottky defects

A vacancy defect in a crystal is simply when one of the atoms is missing from a lattice site:

In ionic solids these are called Schottky defects:

Schottky defects retain the stoichiometry of the compound, since any any ions removed have to be balanced by counter-ions. Schottky defects are mostly found in compounds which have:

1. High ionic character

2. High coordination numbers

3. Similarly sized ions.