Colossal enhancement of the thermoelectric power factor in stress-released orthorhombic phase of SnTe

Abstract

At normal conditions, tin telluride (SnTe) adopts a cubic NaCl-type structure, but under applied pressure above 1.5–2 GPa, it transforms to a distorted crystal structure with an orthorhombic symmetry. Electronic properties of this high-pressure phase, including potential thermoelectricity, remain unexplored to date. Here, we measure the thermoelectric power (the Seebeck coefficient) and electrical resistivity of undoped single crystals of SnTe under applied high pressure up to 9 GPa, i.e., across the above phase transition. We establish that the high-pressure polymorph of SnTe is a p-type semiconductor and estimate its bandgap value at 3 GPa as Eg ∼ 65 meV. In contrast to the NaCl-type phase, the orthorhombic phase is stable in a much wider pressure range up to about 20 GPa, and its energy gap only insignificantly decreases with pressure with a coefficient of dEg/dP ∼ −4 meV/GPa. We find that the thermoelectric power factor of SnTe can be significantly improved in its orthorhombic phase due to the enhancement of the Seebeck coefficient. Furthermore, we show that the high-pressure phase preserves on the pressure releasing down to 0.3 GPa, and its thermopower grows progressively up to about 100 μV/K due to the bandgap expansion to Eg ∼ 105 meV. This results in a colossal rising of the thermoelectric power factor to about 8 mW/(K2m). Probably, this enhancement is contributed by structural distortions in the orthorhombic phase. We discuss how one could fabricate and optimize the orthorhombic polymorph of SnTe for potential use in various technologies, including thermoelectric applications.