The fate of aluminium in (Na,Bi)TiO3-based ionic conductors†
The formation of associated defects (e.g. [AlTi–VO]˙) upon acceptor doping is commonly seen as a reason for trapping of mobile vacancies in perovskite ionic conductors and electromechanical hardening in piezoelectric perovskites. In order to clarify the presence of associated defects in Al-doped (Na1/2,Bi1/2)TiO3 (NBT–Al) and Al-substituted ((Na,K)1/2Bi1/2)TiO3–BiAlO3 (NKBT–BA), we employ a combination of impedance spectroscopy, 27Al NMR spectroscopy, and electronic structure calculations. Our results indicate that associated defects between and oxygen vacancies can only be found in case of low acceptor doping concentrations. This suggests a decreased driving force for defect association at high doping concentrations as the reason for the non-linear dependence between acceptor concentration and oxygen ionic conductivity for NBT-based ceramics. Furthermore, the combination of experimental and theoretical techniques provides clear evidence for the successive occupation of the B-site, the A-site, and finally the formation of a secondary phase with increasing Al3+ content. Altogether, these results call for a new evaluation of the interaction between aliovalent dopants and O2− vacancies in acceptor-doped functional oxides, with implications for the design of ionic conductors as well as ferroelectric materials.