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).
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)