Cation vacancies in perovskites doped with La and Gd

Whereas doping CaTiO₃ or SrTiO₃ with ∼0·05 formula units (f.u.) of trivalent rare earth ions substituted for Ca can yield majority positron annihilation lifetime τ₁ increases of up to 30% due to the presence of cation vacancies, similar doping of BaTiO₃ produces increases of only ∼10%. The difference is attributed to hole trapping around cation vacancies in BaTiO₃. The absence of significant changes to τ₁ for La doped CaTiO₃ when oxidised samples are reduced by sintering in a hydrogenous atmosphere is attributed to the formation of oxygen vacancies that co-exist with the original cation vacancies. Solid state nuclear magnetic resonance showed that a significant fraction of the La ions in BaTiO₃ containing 0·04 f.u. of La were in cubic coordination, irrespective of the charge compensation scheme employed. Doping of sintered SrTiO₃ with La to produce up to 0·2 f.u. of Sr cation vacancies had no significant effect on Sr leaching in deionised water at 90°C.