Localized vibration and avoided crossing in SrTi11O20 for oxide thermoelectrics with intrinsically low thermal conductivity

Abstract

Many state-of-the-art thermoelectric alloys achieve phonon glass and electron crystal (PGEC) behaviour via chemical bonding hierarchy (CBH), which can provide a rigid network for the electron conduction pathway and loosely bonded atoms for phonon scattering, the so-called rattling effect. Thermoelectric oxides share many superiorities, e.g., high stability, low cost and nontoxicity, but CBH is rarely seen in these materials, due to the simplicity in the chemical bonding dominated by the ionic metal–oxygen bond. In this paper, we attempt to construct rattling systems in the absence of CBH. A novel thermoelectric oxide SrTi₁₁O₂₀ is presented, in which the TiO₆ octahedra form a rigid network for the electron conduction pathway, while creating an irregular polyhedron environment to accommodate the loosely bonded Sr ions. The localized and anharmonic vibration of the Sr ions forms low-lying optical phonon bands that can strongly distort acoustic phonon branches through the avoided crossing effect. A high electrical conductivity and low thermal conductivity are thus simultaneously obtained in SrTi₁₁O₂₀ with high thermoelectric performance. This work opens a new avenue for realizing PGEC behaviour in thermoelectric oxides through the topological connectivity of metal–oxygen polyhedra.