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 (κ_L). Amongst effective dopants for a functional thermoelectric material, SnTe, Cu doping realizes the ultra-low κ_L approaching the SnTe amorphous limit. Such effective κ_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 ≈1.7 at% Cu (x = 0.034 where Sn_1−xCu_xTe). The Cu-rich secondary phases encapsulate two distinct solids: Cu₂SnTe₃ (F4‾3m) has semi-coherent interfaces with SnTe (Fm3‾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 ≈350 °C. The atomic-scale analysis on the Cu-rich secondary phases offers viable strategies for reducing κ_L through Cu addition to SnTe.