aliovalent doping
/ AY-lee-oh-VAY-lent /
To dope a crystal is to sprinkle in a controlled amount of a foreign atom. Aliovalent doping is the special, powerful case where that foreign atom carries a DIFFERENT charge from the host ion it replaces (Latin alius = other, valent = valence). Contrast it with isovalent doping (same charge, e.g. Sr2+ for Ba2+), which changes little; aliovalent doping forces the crystal to react, because a mismatched charge cannot simply sit there — the lattice must balance the books.
The dopant can be a donor (higher charge than the host, giving the site a net positive effective charge) or an acceptor (lower charge, net negative). Either way the crystal must compensate. A donor is balanced either by creating a cation vacancy or by releasing an electron; an acceptor by creating an oxygen vacancy or by releasing a hole. Which route wins depends on the material and the atmosphere. Classic acceptor example: dope ZrO2 with Y3+ (lower than Zr4+) and the crystal makes oxygen vacancies (ionic compensation) — the recipe for a solid electrolyte. Classic donor example: dope BaTiO3 with La3+ on the Ba2+ site or Nb5+ on the Ti4+ site and, under the right conditions, the compensation is electronic (free electrons), turning an insulator into an n-type semiconductor for a PTC thermistor.
Aliovalent doping is the master lever of the electroceramics industry: it stabilizes the cubic phase of zirconia and fills it with oxygen vacancies (YSZ for fuel cells and sensors), makes barium titanate semiconducting, sets the carrier type and count in varistors and thermistors, and tunes the colour of gemstones and phosphors. An honest caveat: whether the crystal compensates with ions or electrons is not free to choose — it shifts with temperature and oxygen partial pressure, which is exactly what a Brouwer diagram maps.
Replacing two Zr4+ with two Y3+ in zirconia leaves a charge deficit of 2, which the crystal cancels by removing one O2- to make an oxygen vacancy: 2 Y_Zr' + V_O•• per formula unit of Y2O3 dissolved. The added vacancies both stabilize the cubic structure and carry the ionic current — one dopant, two jobs.
Aliovalent means 'other-valence': the dopant's charge mismatch is the whole point, because it forces a useful compensating defect into being.
Isovalent doping (same charge) creates no compensating defect and mostly just tweaks the lattice; only aliovalent doping generates the vacancies or carriers that switch on ionic and electronic function.