an extrinsic defect
An extrinsic defect is one that owes its existence to a foreign atom — an impurity that crept in, or a dopant deliberately added. Unlike intrinsic defects, extrinsic ones do not vanish when you cool the crystal; their number is fixed by how much impurity you put in, not by temperature. They are how a ceramist takes control: choose the dopant, choose its amount, and you dial in the defect population you want.
The powerful case is an aliovalent impurity — a substitutional atom whose charge differs from the host ion it replaces. Because the charge no longer matches, the crystal must compensate by creating a partner defect to keep neutral. Put Ca2+ on a Zr4+ site in zirconia and the calcium is two charges short (Ca_Zr''); the crystal balances it by making one doubly-positive oxygen vacancy: CaO becomes Ca_Zr'' + V_O•• + O_O^x. Put Al3+ on a Mg2+ site in MgO and the aluminium is one charge too many (Al_Mg•); the crystal compensates with a magnesium vacancy, giving 2 Al_Mg• + V_Mg''. The dopant valence thus DICTATES which compensating defect appears — vacancies, interstitials, or electronic carriers.
This is the engine of functional ceramics. Extrinsically created oxygen vacancies make stabilized zirconia an oxygen-ion conductor; extrinsic electronic carriers from donor or acceptor doping tune semiconducting and thermistor ceramics; extrinsic defects set the colour of doped gemstones and phosphors. At low and moderate temperatures a real ceramic sits in the extrinsic regime, where the impurity-fixed defect concentration is flat and independent of temperature — only at high temperature does the intrinsic thermal term take over.
The workhorse solid electrolyte YSZ is entirely extrinsic: 8 mol% Y2O3 in ZrO2 fixes the oxygen-vacancy concentration at roughly 4 percent of the anion sites, essentially independent of temperature, giving a large and reliable pool of mobile V_O•• for the fuel cell to use.
Extrinsic defect populations are set by composition, so they are stable and designable — the basis of every doped electroceramic.
Adding a dopant never creates net charge. Every aliovalent substitution is automatically balanced by a compensating defect; the whole design skill is choosing which compensation you get.