High-Figure-of-Merit Thermoelectric La-Doped A-Site-Deficient SrTiO3 Ceramics

The structure and thermoelectric (TE) properties of La-doped, A-site-deficient SrTiO3 (Sr1-3x/2LaxTiO3) ceramics sintered in air and N-2/5% H-2 have been investigated. Air sintered ceramics with 0.10 <= x < 0.30 appear cubic by X-ray diffraction (XRD) but exhibit superstructure consistent with a tetragonal cell (a(0)a(0)c(-)), according to electron diffraction (ED) studies. 0.30 <= x < 0.50 have additional short-range A-site vacancy (V-A) ordering, and x >= 0.50 are orthorhombic with an a(-)a(-)c(+) tilt system and long-range V-A ordering. 0.10 <= x <= 0.50 reduced in N-2/5% H-2 are oxygen-deficient and appear cubic in XRD patterns but exhibit superstructure compliant with an a(0)a(0)c(-) tilt system by ED. For x = 0.50, additional short-range V-A order is observed, and x > 0.50 are orthorhombic with an a(-)a(-)c(+) tilt system and long-range V-A ordering. x = 0.15 sintered in N-2/5% H-2 shows the largest dimensionless TE figure-of-merit ZT = 0.41 at 973 K reported for n-type SrTiO3-based ceramics, suggesting that the accommodation of La through formation of (V-Sr) coupled with reduction in N-2/5% H-2 represents a new protocol for the development of oxide-based thermoelectrics.