3.13.4 Conductivity in SrTiO₃

Perovskite oxides SrTiO₃ and BaTiO₃ have two cation sub-lattices onto which aliovalent ions can substitute. These oxides can also be easily reduced in reducing atmospheres leading creation of mobile defects. Typically, Ba/Ti or Sr/Ti is approximately very close to 1. Maximum deviation occurs at temperatures lower than 1200°C and is of the order of ∼100ppm.

To promote n-type conductivity in these materials, solutes such La³⁺, Nb⁵⁺, and Ta⁵⁺ are added to BaTiO₃, giving rise to shallow donor levels in the bandgap. The defect reaction upon doping with La or Nb is expressed as

and

Occupancies are determined by the relative atomic sizes.

On the other hand, undoped materials often contain impurities such as Fe³⁺ or Al ³⁺ substituting Ti⁴⁺ acting as acceptors. Consider lightly acceptor doped SrTiO₃ with Al₂O₃ as impurity with the following defect reaction:

In such a situation, either SrO is accepted or TiO₂ is rejected to maintain the stoichiometric cation ratio.

The above equation is ionically compensated reaction. It follows that [Al_Ti^'] = 2[V₀^•] and thus

Electronic compensation is also possible according to

It follows that [Al_Ti^'] = n_h and thus

Experiments show that Equation (3) , i.e. ionic compensation is predominant mechanism.

If you are interested, you can refer to the work of Choi and Tuller, Journal of American Ceramic Society, 71, 201, 1988 for discussion on defects and conduction in Ba-doped SrTiO₃ under various conditions.