Enhancement of thermoelectric performance in rare earth-doped Sr ₃Ti₂O₇ by symmetry restoration of TiO ₆ octahedra

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 Sr ₃Ti ₂O ₇, both on the structural restoration of distorted TiO ₆ 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 TiO ₆ 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 TiO ₆ octahedra to achieve high thermoelectric performance.