Lone-Pair-Driven Structure Dimensionality: the Way to Ultralow Thermal Conductivity in Pb_mBi₂S_3+m Sulfides

Understanding the mechanisms that connect heat transport with crystal structures is fundamental to develop materials with optimized electrical and thermal properties for thermoelectric applications. In this work, we synthesized a series of bulk Cl-doped PbBi 2 S 4 by mechanical alloying combined with spark plasma sintering. A detailed structural analysis of PbBi 2 S 4 ( m = 1 member of the series Pb m Bi 2 S 3+ m ) and of the compounds Bi 2 S 3 ( m = 0) and Pb 3 Bi 2 S 6 ( m = 3) shows that the low dimensionality of their frameworks is induced by the stereochemical activity of Bi 3+ and Pb 2+ 6s 2 lone pairs (L) and is mainly governed by the presence of BiS 3 L chains of tetrahedrons. By combining experiments with the ab initio band structure and phonon calculations, we discuss the structure-thermoelectric property relationships and clarify the interesting crystal chemistry in this system. We demonstrate that the ultralow thermal conductivity of these sulfides originates from the prominent 1D character induced by the bismuth chains in these frameworks, leading to weak interchain interactions compared to their strong intrachain bonds.