Mixed conductivity, nonstoichiometric oxygen, and oxygen permeation properties in Co-doped Sr₃Ti₂O_7-δ

Electrical conductivity and oxygen permeation rates in Co-doped Sr ₃Ti₂O_7-δ with Ruddesden-Popper type structures were investigated. The effects of metal dopants (M) in the Ti site of Sr₃Ti₂(M)O_7-δ on the mixed conductivity were also studied. Doping of Sr₃Ti₂O_7-δ with Co was found to be effective for improving the electrical conductivity as well as the oxygen permeation rate, which could be assigned to the increased oxygen vacancy concentration by doping Co³⁺ into Ti⁴⁺ sites. The nonstoichiometric oxygen of these oxides was measured by using a thermal gravimetric method. The creation of oxygen vacancies, which is compensated with Co³⁺ doping, leads to higher oxide ion conductivity. The oxygen permeation rate monotonously increased with increasing amounts of Co in the Ti site. Sr₃Ti_0.8Co_1.2O _7-δ exhibited high oxide ion conductivity and a large oxygen permeation rate. The highest oxygen permeation rate achieved a value of 2.02 cc min^-1 cm^-2 at 1273 K for Sr₃Ti _0.8Co_1.2O_7-δ. Neutron diffraction analysis and redox titration suggests that the oxygen diffusion occurs through oxygen vacancies in the perovskite block, but not through excess oxygen in the rock salt block.