Charge-balanced codoping enables exceeding doping limit and ultralow thermal conductivity

Abstract

Materials with low thermal conductivity are applied extensively in energy management, and breaking the amorphous limits of thermal conductivity to solids has attracted widespread attention from scientists. Doping is a common strategy for achieving low thermal conductivity that can offer abundant scattering centers in which heavier dopants always result in lower phonon group velocities and lower thermal conductivities. However, the amount of equivalent heavy-atom single dopant available is limited. Unfortunately, nonequivalent heavy dopants have finite solubility because of charge imbalance. Here, we propose a charge balance strategy for SnS by substituting Sn<sup>2+</sup> with Ag<sup>+</sup> and heavy Bi<sup>3+</sup>, improving the doping limit of Ag from 2% to 3%. Ag and Bi codoping increases the point defect concentration and introduces abundant boundaries simultaneously, scattering the phonons at both the atomic scale and nanoscale. The thermal conductivity of Ag<sub>0.03</sub>Bi<sub>0.03</sub>Sn<sub>0.94</sub>S decreased to 0.535 W·m<sup>−1</sup>·K<sup>−1</sup> at room temperature and 0.388 W·m<sup>−1</sup>·K<sup>−1</sup> at 275 ℃, which is below the amorphous limit of 0.450 W·m<sup>−1</sup>·K<sup>−1</sup> for SnS. This strategy offers a simple way to enhance the doping limit and achieve ultralow thermal conductivity in solids below the amorphous limit without precise structural modification.