Aliovalent

Aliovalent refers to a type of substitution in a crystal lattice where a dopant ion has a different valence than the ion it replaces. When an aliovalent dopant enters a host lattice, it introduces a net charge imbalance. To maintain overall electrical neutrality, the material compensates through the formation of defects (such as vacancies or interstitials) or by changing the oxidation state of neighboring ions. This charge compensation can also generate free electronic carriers, such as electrons or holes, and can thereby alter the material’s electrical, ionic, and catalytic properties.

Common contexts and examples:

- Oxide ion conductors: Doping CeO2 with trivalent ions like Gd3+ or Sm3+ creates oxygen vacancies that enhance oxide-ion conductivity, a principle used in solid oxide fuel cells (Ce1−xGdxO2−x/2).

- Stabilized zirconia: Y2O3-doped ZrO2 (yttria-stabilized zirconia) relies on Y3+ replacing Zr4+ to generate oxygen vacancies, stabilizing the cubic phase and enabling high ionic conductivity.

- Semiconductor doping: In silicon, trivalent boron substitutes for tetravalent silicon to create acceptor levels (p-type), while pentavalent phosphorus substitutes create donor levels (n-type).

- Perovskites and other oxides: Aliovalent substitutions (for example, Sr2+ or Ba2+ in certain oxide lattices) are used to tune valence states, mixed valency, and properties such as magnetism, conductivity, or catalytic activity.

Distinction: Isovalent doping substitutes an ion with the same valence and does not inherently require charge compensation. Aliovalent doping is a key strategy for engineering electrical, ionic, and catalytic behavior in a wide range of materials.