Enhancement of thermoelectric performance in rare earth-doped Sr3Ti2O7 by symmetry restoration of TiO6 octahedra

Abstract

Aiming at the realization of the enhancement of thermoelectric performance through structural modification, the present work has clarified the significant effects of rare earth (RE=Gd, Sm, Nd, and La) doping at Sr-sites in Sr3Ti2O7, both on the structural restoration of distorted TiO6 octahedra and on the Seebeck coefficient, especially at high temperatures. The preferential substitution of RE 3+ at the nine-coordinate Sr-sites can facilitate the degeneration of the conduction band (Ti 3d-t 2g ) orbital, owing to its special capability in restoring TiO6 octahedra to a higher state of symmetry and thus enhance the density of states (DOS) effective mass of the carriers, which gives rise to a rather large increase in the Seebeck coefficient. The present findings have affirmed the effectiveness of structural restoration in enhancing the Seebeck coefficient by Sr-site-doping, which will help establish a useful solution for Ti-based thermoelectric oxides with inherently distorted TiO6 octahedra to achieve high thermoelectric performance.