Multilayer ceramic capacitors (MLCCs) are typically made of the ferroelectric perovskite BaTiO₃. Sintering of MLCCs is often carried out in a reducing atmosphere to avoid oxidation of base metal electrodes at high temperatures. This however can lead to oxygen loss and reduction of Ti⁴⁺ to Ti³⁺, generating undesired n-type electronic conductivity. Acceptor doping (such as Mg²⁺ doping at the Ti⁴⁺ site) is required to minimise oxygen loss and reduction of BaTiO₃ during high temperature sintering in a low oxygen partial pressure. Acceptor doping however creates oxygen vacancies, which can become mobile at relatively high temperatures and/or under a high voltage, leading to resistance degradation and fatal capacitor failures for high-voltage, high-temperature applications.

This raises an important fundamental question of how to suppress migration of oxygen vacancies in perovskite oxides. Rare earth oxides are often used as dopants to suppress resistance degradation in BaTiO₃, but the mechanisms remain unclear.   

Here we demonstrate a general mechanism to suppress migration of oxygen vacancies in La- and Mg-doped BaTiO₃. The new doping mechanism gives low bulk electrical conductivity with an activation energy of about 1.6 eV (half of the optical bandgap of 3.2 eV), indicating both the oxide ion conductivity and electronic conductivity are suppressed. Our results also reveal how subtle differences in starting compositions dramatically influence the electrical conduction mechanisms and electrical conductivity in La- and Mg-doped BaTiO₃.