Impact of crystal structure on the thermoelectric properties of n-type SrTiO3

We present a detailed first-principles study of the electrical and thermal transport, and the thermoelectric figure of merit zT, of the oxide perovskite SrTiO₃ in the orthorhombic Pnma, tetragonal I4/mcm and cubic Pmm phases. Analysis of the lattice thermal conductivity shows that the “particle-like” contribution, κ_p, is highest in the Pmm phase due to larger phonon group velocities. We also find that all three phases show significant heat transport through glass-like interband tunnelling. On the other hand, we predict the cubic and orthorhombic phases to show superior n-type conductivity, due to significantly stronger polar-optic phonon scattering and shorter electron lifetimes in the tetragonal phase. Due to its superior electrical properties, we predict that the Pmm phase will attain a 25% larger high-temperature zT than the I4/mcm phase, while we predict the best zT can be obtained for the Pnma phase due to its favourable electrical properties and low κ_latt. This work provides new insight into the impact of structure type on the thermoelectric performance of oxide perovskites, and indicates targeting particular structure types, e.g. through chemical doping, could provide a facile route to optimising the zT of SrTiO₃ and related systems.