A and B site doping of a phonon-glass perovskite oxide thermoelectric

By tuning the A site cation size it is possible to control the degree of octahedral distortion and ultimately structural symmetry in the new perovskite solid solution La 0.5 Na 0.5− x K x TiO 3 , affording a rhombohedral-to-cubic transition as x increases above 0.4. The La 3+ and K + cations are distributed randomly across the A site leading to significant phonon disorder in cubic La 0.5 K 0.5 TiO 3 ( Pm 3&cmb.macr; m ) which produces a phonon-glass with a thermal conductivity of 2.37(12) W m −1 K −1 at 300 K; a reduction of 75% when compared with isostructural SrTiO 3 . This simple cation substitution of Sr 2+ for La 3+ and K + maintains the flexible structural chemistry of the perovskite structure and two mechanisms of doping for the introduction of electronic charge carriers are explored; A site doping in La 1− y K y TiO 3 or B site doping in La 0.5 K 0.5 Ti 1− z Nb z O 3 . The phonon-glass thermal conductivity of La 0.5 K 0.5 TiO 3 is retained upon doping through both of these mechanisms highlighting how the usually strongly coupled thermal and electronic transport can be minimised by mass disorder in perovskites. Precise control over octahedral distortion in A site doped La 1− y K y TiO 3 , which has rhombohedral ( R 3&cmb.macr; c ) symmetry affords lower band dispersions and increased carrier effective masses over those achieved in B site doped La 0.5 K 0.5 Ti 1− z Nb z O 3 which maintains the cubic ( Pm 3&cmb.macr; m ) symmetry of the undoped La 0.5 K 0.5 TiO 3 parent. The higher Seebeck coefficients of A site doped La 1− y K y TiO 3 yield larger power factors and lead to increased thermoelectric figures of merit and improved conversion efficiencies compared with the mechanism for B site doping. The effect of structural symmetry is investigated in phonon-glass electron-crystal (PGEC) La 1− y K y TiO 3 and La 0.5 K 0.5 Ti 1− z Nb z O 3 thermoelectric oxides