Enhancing the thermoelectric performance of β-Zn₄Sb₃ via progressive incorporation of Zn interstitials

Owing to the intrinsically low lattice thermal conductivity, β-Zn₄Sb₃ is regarded as a promising candidate for thermoelectric applications. To further improve its thermoelectric performance, it is essential to increase the carrier mobility of β-Zn₄Sb₃ for enhanced electrical transport. While incorporating light ions is a useful approach for this purpose, the excess Zn ions tend to segregate in as-grown samples. Here, we present a design strategy to stabilize the excess Zn ions in the interstitial sites by applying electric current and uniaxial stress, which triggers a progressive directional migration of Zn ions towards one terminal of the sample. In this Zn-enriched end, a homogeneous distribution of excess Zn interstitials was achieved, as confirmed by atom probe tomography and transmission electron microscopy at the atomic scale. As revealed by ab initio simulations, the additional electrons brought by Zn interstitials connect the separated (Sb₂)^4- dimers, enhancing the carrier mobility and thus the electrical conductivity. In consequence, the detached Zn-enriched section shows a much improved thermoelectric figure of merit, a very high maximum ZT of ∼1.5 at 618 K, and an average ZT of ∼1.1 between 300 and 618 K, outperforming the majority of thermoelectric materials in this temperature range.