Understanding Micro and Atomic Structures of Secondary Phases in Cu‐Doped SnTe

Highly efficient thermoelectric materials require, including point defects within the host matrix, secondary phases generating positive effects on lowering lattice thermal conductivity (kappa(L)). Amongst effective dopants for a functional thermoelectric material, SnTe, Cu doping realizes the ultra-low kappa(L) approaching the SnTe amorphous limit. Such effective kappa(L) reduction is first attributed to strong phonon scattering by substitutional Cu atoms at Sn sites and interstitial defects in the host SnTe. However, other crystallographic defects in secondary phases have been unfocused. Here, this work reports micro- to atomic-scale characterization on secondary phases of Cu-doped SnTe using advanced microscopes. It is found that Cu-rich secondary phases begin precipitation approximate to 1.7 at% Cu (x = 0.034 where Sn1-xCuxTe). The Cu-rich secondary phases encapsulate two distinct solids: Cu2SnTe3 (F4 over bar 3m$F\bar{4}3m$) has semi-coherent interfaces with SnTe (Fm3 over bar m$Fm\bar{3}{\rm{m}}$) such that they minimize lattice mismatch to favor the thermoelectric transport; the other resembles a stoichiometric Cu2Te model, yet is so meta-stable that it demonstrates not only various defects such as dislocation cores and ordered/disordered Cu vacancies, but also dynamic grain-boundary migration with heating and a subsequent phase transition approximate to 350 degrees C. The atomic-scale analysis on the Cu-rich secondary phases offers viable strategies for reducing kappa(L) through Cu addition to SnTe.