Crystal structure and thermoelectric properties of Nb5+ - and Ln 3+ -(rare earth: La3+, Nd3+, Sm3+, and Gd3+) doped SrO (SrTiO3)n (n=1,2) ceramics, which were fabricated by conventional hot-pressing, were measured to clarify the effects of Ti4+ - and Sr2+ -site substitution on the thermoelectric properties. The thermal conductivities are very close between the n=1 and 2 phases either doped with Nb5+ or Ln3+ and decreased by ∼60% at room temperature and ∼30% at 1000 K as compared to that of SrTiO3, which is likely due to an enhanced phonon scattering at the SrO/ (SrTiO3)n (n=1,2) interfaces. The density of states effective mass md (1.8-2.4 m0) and consequently the Seebeck coefficient S in Nb5+ -doped samples are fairly smaller than those reported for SrTiO 3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a1g (Ti 3 dxy) orbital as the conduction band bottom, which should be induced by the distortion of TiO 6 octahedra in (SrTiO3)n layers. However, in the Ln3+ -doped SrO (SrTiO3)2, the TiO 6 octahedra were found to be restored, in contrast to the Nb 5+ -doped, with a gradually increasing O-Ti-O bond angle in the (100) plane at high temperatures, which would lead to the formation of triply degenerate Ti 3d- t2g (dxy, dyz, and d xz) orbitals to cause a significant enhancement in md (∼7.5 m0 at 1000 K) and consequently in S. Accordingly, the maximum dimensionless figure of merit ZT∼0.24 obtained in 5% -Gd3+ -doped SrO (SrTiO 3) 2 at 1000 K is about 70% larger than that of Nb-doped SrO (SrTiO3)2 (ZT1000 K ∼0.14).
Bibliographical noteFunding Information:
We all the authors are deeply indebted to Dr. Hideki Hyuga and Dr. Hideki Kita at National Institute of Advanced Industrial Science and Technology (AIST), Japan, for their technical support in hot-pressing process and to Dr. Katsuhiko Inaba (Rigaku Co.) for his great job of high temperature XRD measurements. Meanwhile, the first author would like to express his heartfelt gratitude to Nature COE program and Global COE program of Nagoya University for their financial support during the research.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)