Nanostructures versus solid solutions: Low lattice thermal conductivity and enhanced thermoelectric figure of merit in Pb_9.6Sb _0.2Te_10-xSe_x bulk materials

The series of Pb_9.6Sb_0.2Te_10-xSe _x compounds with different Se content (x) were prepared, and their structure was investigated at the atomic and nanosized regime level. Thermoelectric properties were measured in the temperature range from 300 to 700 K. The Pb_9.6Sb_0.2Te_10-xSe_x series was designed after the refinement of the single-crystal structure of Pb _3.82Sb_0.12Te₄ (Pb_9.6Sb _0.3Te₁₀; S.G. Pm3̄m) by substituting isoelectronically in anion positions Te by Se. The Pb_9.6Sb _0.2Te_10-xSe_x compounds show significantly lower lattice thermal conductivity (κ_L) compared to the well-known PbTe_1-xSe_x solid solutions. For Pb_9.6Sb _0.2Te₃Se₇ (x = 7), a κ_L value as low as 0.40 W/m-K was determined at 700 K. High-resolution transmission electron microscopy of several Pb_9.6Sb_0.2Te _10-xSe_x samples showed widely distributed Sb-rich nanocrystals in the samples which is the key feature for the strong reduction of the lattice thermal conductivity. The reduction of κ_L results in a significantly enhanced thermoelectric figure of merit of Pb _9.6Sb_0.2Te_10-xSe_x compared to the corresponding PbTe_1-xSe_x solid solution alloys. For Pb_9.6Sb_0.2Te₃Se₇ (x= 7), a maximum figure of merit of ZT ≈ 1.2 was obtained at ∼650 K. This value is about 50% higher than that of the state-of-the-art n-type PbTe. The work provides experimental validation of the theoretical concept that embedded nanocrystals can promote strong scattering of acoustic phonons.