Low Thermal Conductivity of RE-Doped SrO(SrTiO₃)₁ Ruddlesden Popper Phase Bulk Materials Prepared by Molten Salt Method

The SrO(SrTiO₃)₁ (Sr₂TiO₄) Ruddlesden Popper (RP) phase is a natural superlattice comprising of alternately stacking perovskite-type SrTiO₃ layers and rock salt SrO layers along the crystallographic c direction. This paper discusses the properties of the Sr₂TiO₄ and (La, Sm)-doped Sr₂TiO₄ RP phase synthesized via molten salt method, within the context of thermoelectric applications. A good thermoelectric material requires high electrical conductivity, high Seebeck coefficient and low thermal conductivity. All three conditions have the potential to be fulfilled by the Sr₂TiO₄ RP phase, in particular, the superlattice structure allows a higher degree of phonon scattering hence resulting in lowered thermal conductivity. In this work, the Sr₂TiO₄ RP phase is doped with Sm and La respectively, which allows injection of charge carriers, modification of its electronic structure for improvement of the Seebeck coefficient, and most significantly, reduction of thermal conductivity. The particles with submicron size allows excessive phonon scattering along the boundaries, thus reduces the thermal conductivity by fourfold. In particular, the Sm-doped sample exhibited even lower lattice thermal conductivity, which is believed to be due to the mismatch in the ionic radius of Sr and Sm. This finding is useful as a strategy to reduce thermal conductivity of Sr₂TiO₄ RP phase materials as thermoelectric candidates, by employing dopants of differing ionic radius.