Thermoelectric properties of electron doped SrO(SrTiO3)n (n=1,2) ceramics

Abstract

Crystal structure and thermoelectric properties of Nb5+- and Ln3+-(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 SrTiO3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a1g (Ti 3dxy) orbital as the conduction band bottom, which should be induced by the distortion of TiO6 octahedra in (SrTiO3)n layers. However, in the Ln3+-doped SrO(SrTiO3)2, the TiO6 octahedra were found to be restored, in contrast to the Nb5+-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 dxz) 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(SrTiO3)2 at 1000 K is about 70% larger than that of Nb-doped SrO(SrTiO3)2 (ZT1000 K∼0.14).