A-site sub-stoichiometry and oxygen vacancies as the origin of the electrical properties of Sr2−yLuNb1−xTixO6−δ perovskite-like materials

Abstract

Aliovalent substitution of Nb⁵⁺ by Ti⁴⁺ in Sr₂LuNbO₆ is limited to 10% of Nb atoms. A full structural determination by NPD confirms this and reveals that the structure is better described as a superstructure of the simple cubic perovskite (as previously reported) with the monoclinic cell 2^1/2a_p × 2^1/2a_p × 2a_p and β ≈ 90° (S.G. P2₁/n). The substituted materials present both oxygen-vacancies induced by charge compensation and Sr-deficiency. Therefore, their formula should be given as Sr_2−yLuNb_1−xTi_xO_6−δ. Electrical properties can be fully understood considering these compositional defects. The parent compound Sr₂LuNbO₆ presents low electrical conductivity in air, which improves by more than one order of magnitude upon Ti substitution. In any case, the title oxides show low electrical conductivity in a wide oxygen partial pressure (pO₂) range (10⁻²⁵ atm ≤ pO₂ ≤ 10⁻¹ atm). At high pO₂ the conductivity increases with pO₂ due to oxygen-vacancy annihilation and hole creation, according to a general p-type semiconducting mechanism; A-site substoichiometry and Ti-substitution are the origin of this behaviour. In the low pO₂ region, the conductivity increases as the oxygen partial pressure decreases. Reduction of cations, Nb⁵⁺ or Ti⁴⁺, supports n-type conduction by electrons and oxygen vacancy creation. For the intermediate pO₂ range a low ionic conduction contribution is observed. Although the estimated ionic conductivity is not high in the substituted compounds, the strategy seems to be valid since a significant enhancement of ionic conduction is observed upon aliovalent substitution.