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

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 d_xy) 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- t_2g (d_xy, d_yz, 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).