A and B site doping of a phonon-glass perovskite oxide thermoelectric
2018
Daniels, L. M. | Ling, S. | Savvin, S. N. | Pitcher, M. J. | Dyer, M. S. | Claridge, J. B. | Slater, B. | Corà, F. | Alaria, J. | Rosseinsky, M. J.
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 La₀.₅Na₀.₅₋ₓKₓTiO₃, affording a rhombohedral-to-cubic transition as x increases above 0.4. The La³⁺ and K⁺ cations are distributed randomly across the A site leading to significant phonon disorder in cubic La₀.₅K₀.₅TiO₃ (Pm3̄m) 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 SrTiO₃. This simple cation substitution of Sr²⁺ for La³⁺ 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 La₁₋yKyTiO₃ or B site doping in La₀.₅K₀.₅Ti₁₋zNbzO₃. The phonon-glass thermal conductivity of La₀.₅K₀.₅TiO₃ 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 La₁₋yKyTiO₃, which has rhombohedral (R3̄c) symmetry affords lower band dispersions and increased carrier effective masses over those achieved in B site doped La₀.₅K₀.₅Ti₁₋zNbzO₃ which maintains the cubic (Pm3̄m) symmetry of the undoped La₀.₅K₀.₅TiO₃ parent. The higher Seebeck coefficients of A site doped La₁₋yKyTiO₃ yield larger power factors and lead to increased thermoelectric figures of merit and improved conversion efficiencies compared with the mechanism for B site doping.
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