Semiconductor-to-semimetal transition on Te doping in a new semiconducting material Ba_2.75Pb_1.1Sb_4.1Se₁₀

Semiconducting antimony-containing chalcogenides are an emerging class of materials for their applications in the thermoelectric and optoelectronic fields. A series of new polycrystalline antimony-containing chalcogenides Ba_2.75Pb_1.1Sb_4.1Se_10-δTe_δ (δ = 0 to 4) have been prepared by heating the elements at 1073 K. The parent monoclinic selenide, Ba_2.75Pb_1.1Sb_4.1Se₁₀, adopts the P2₁/c space group, as determined by a single-crystal X-ray diffraction study. The selenide structure consists of 2∞[Pb_1.1Sb_4.1Se₁₀]^5.5- layers in which Pb and Sb sites are disordered. The PXRD studies show that a maximum of 40% of Se atoms in the Ba_2.75Pb_1.1Sb_4.1Se₁₀ structure can be substituted by Te atoms. The refined unit cell volume of the polycrystalline Ba_2.75Pb_1.1Sb_4.1Se_10-δTe_δ increases almost linearly with δ values following Vegard's law. Optical bandgap measurements indicate the semiconducting nature of the δ = 0, 1, and 2 samples, whereas no bandgap transition was found for the δ = 3 and 4 compositions. Ba_2.75Pb_1.1Sb_4.1Se₁₀ exhibits an ultra-low thermal conductivity (κ_tot) of ∼0.24 W m^-1 K^-1 at 773 K. The substitution of the Te atoms at the Se sites of the Ba_2.75Pb_1.1Sb_4.1Se₁₀ structure significantly improves electrical conductivity values. The δ = 3 and 4 compositions of the Ba_2.75Pb_1.1Sb_4.1Se_10-δTe_δ series show the semimetal-like temperature dependence of electrical conductivity. The zT value of the δ = 2 composition is almost zero at 773 K due to its poor electrical conductivity. A further increase in Te concentration in Ba_2.75Pb_1.1Sb_4.1Se_10-δTe_δ leads to an enhancement of the zT value (= 0.09 for δ = 4) at 773 K.