Role of phonon scattering by elastic strain field in thermoelectric Sr_1−xY_xTiO_3−δ

Perovskite-type SrTiO_3−δ ceramics are multifunctional materials with significant potential as n-type thermoelectric (TE) materials. The electronic and thermal transport properties of spark plasma sintered polycrystalline Sr_1−xY_xTiO_3−δ (x = 0.05, 0.075, 0.1) ceramics are systematically investigated from (15–800) K. The Sr_0.9Y_0.1TiO_3−δ simultaneously exhibits a large Seebeck coefficient, α > −80 μV/K and moderately high electrical resistivity, ρ ∼ 0.8 mΩ-cm at a carrier concentration of ∼10²¹ cm⁻³ at 300 K resulting in a high TE power factor defined herein as (α²σT) ∼ 0.84 W/m-K at 760 K. Despite the similar atomic masses of Sr (87.6 g/mol) and Y (88.9 g/mol), the lattice thermal conductivity (κ_L) of Sr_1−xY_xTiO_3−δ is significantly reduced with increased Y-doping, owing to the smaller ionic radii of Y³⁺ (∼1.23 Å, coordination number 12) compared to Sr²⁺ (∼1.44 Å, coordination number 12) ions. In order to understand the thermal conductivity reduction mechanism, the κ_L in the Sr_1−xY_xTiO_3−δ series are phenomenologically modeled with a modified Callaway's equation from 30–600 K. Phonon scattering by elastic strain field due to ionic radii mismatch is found to be the prominent scattering mechanism in reducing κ_L of these materials. In addition, the effect of Y-doping on the elastic moduli of Sr_1−xY_xTiO_3−δ (x = 0, 0.1) is investigated using resonant ultrasound spectroscopy, which exhibits an anomaly in x = 0.1 in the temperature range 300–600 K. As a result, the phonon mean free path is found to be further reduced in the Sr_0.9Y_0.1TiO_3−δ compared to that of SrTiO_3−δ, resulting in a considerably low thermal conductivity κ ∼ 2.7 W/m-K at 760 K. Finally, we report a thermoelectric figure of merit (ZT) ∼ 0.3 at 760 K in the Sr_0.9Y_0.1TiO_3−δ, the highest ZT value reported in the Y-doped SrTiO₃ ceramics thus far.