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.