Controlling the A-site deficiency and oxygen vacancies by donor-doping in pre-reductive-sintered thermoelectric SrTiO3 ceramics

Oxygen vacancies are intrinsic defects in ABO ₃ perovskite structures, and the reductive sintering of SrTiO ₃ ceramics combined with donor-doping can create mobile carriers and/or vacancies at A-site to improve their thermoelectric performance. By scanning and transmission electron microscopy analyses, A-site deficiency was detected in undoped and donor-doped SrTiO ₃ ceramics, reaching 19% of A-site vacancies by co-doping La, Ce and Nb respectively into A- and B-sites. Titania with mixed Ti ⁴⁺/Ti ³⁺ cations were commonly detected as intergranular phases of Ti ⁴⁺ _{1− m}Ti ³⁺ _mO _{2− m /2}, which amounts to 17 vol.% in the undoped SrTiO ₃ , thus revealing their inheritance from the pre-reductive nano-powders via liquid-phase sintering. Oxygen and A-site vacancies are both detectable by cathodoluminescence analysis from either the nano-powders or sintered phases, which reveal that mixed-donor-doping creates more vacancies at A-site, together with more oxygen vacancies to form Schottky pairs. The combination of pre-reductive sintering and mixed-donor-doping creates abundant vacancies as electronic defects in SrTiO ₃, while carrier transport were not hindered through the grain boundaries which enables zT value of an oxide to exceed 0.38 at 1000 K.