Lone-Pair-Driven Structure Dimensionality: the Way to Ultralow Thermal Conductivity in PbmBi2S3+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 PbBi2S4 by mechanical alloying combined with spark plasma sintering. A detailed structural analysis of PbBi2S4 (m = 1 member of the series PbmBi2S3+m) and of the compounds Bi2S3 (m = 0) and Pb3Bi2S6 (m = 3) shows that the low dimensionality of their frameworks is induced by the stereochemical activity of Bi3+ and Pb2+ 6s2 lone pairs (L) and is mainly governed by the presence of BiS3L 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.