Investigation of microstructural details in low thermal conductivity thermoelectric Sn_1-xSb_xTe alloy

We report on the detailed microstructural features in a low thermal conductivity Sn_1-xSb_xTe (x = 0.04, 0.08, 0.15) alloy investigated by transmission electron microscopy and density functional theory calculation. A near theoretical minimum thermal conductivity is obtained for Sn_0.85Sb_0.15Te alloy composition containing distinct microstructures. The crisscross lines along {111} planes forming nano-scale structures have been identified as areas with Sb replacing both regular Sn sites and Te anti-sites. This leads to the modulation in the {111} inter-planar spacing (d₁₁₁) and results in superstructure spots in the electron diffraction pattern. The formation of such structures is supported by theoretical calculation. In general, two different phases are observed in the system, one with Sb replacing the regular Sn sites and another with crisscross lines where Sb replaces both the Sn and Te sites. Theoretical calculation further reveals that while the areas with Sb at the regular site give rise to large thermo-power, the areas with Sb substituting regular and anti-sites combination forming a superstructure contribute towards low lattice thermal conductivity and the combined effect increases the zT to ∼1.