The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr3Ti2O7 Ceramics

La-doping mechanisms and thermoelectric properties of Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics sintered under air and flowing 5% H2 at 1773 K for 6 hours have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (~1 at.%) based on La3+ + ½O2- → Sr2+ which resulted in insulating samples when processed in air. In contrast, electronic donor-doping (La3+ + e- → Sr2+) and oxygen loss [O2- → ½ O2 (g) + 2 e-] are the dominant mechanism(s) in 5% H2- sintered ceramics with a solution limit of ~5 at.%. The increased solubility limit is attributed to the formation of Ti3+ during reduction which compensates for the extra positive charge associated with La on the A-site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H2-sintered samples, an insulating surface layer formed associated with SrO volatilisation and oxygen up-take (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that 2 were phase mixtures containing highly conductive perovskite-based (Sr,La)TiO3- (ST). This highlights the superior power factor properties of reduced perovskite-type ST phases compared to reduced RP-type Sr3Ti2O7 and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases.