Unravelling the Influence of the Local Structure on the Ultralow Thermal Conductivity of the Bismuthinite-Aikinite Series, Cu1-x□xPb1-xBi1+xS3.

Understanding the relationship between crystal structure, bonding and thermal transport is critical for the discovery of materials with ultralow thermal conductivities. Materials in the bismuthinite-aikinite series, Cu1-x□xPb1-xBi1+xS3 (0 ≤ x ≤ 1), in which a Bi3+ cation and a vacancy (□) are progressively substituted by a Pb2+ and a Cu+ cation, exhibit ultralow thermal conductivities (∼0.5 W m-1K-1 for x < 1). Here, we investigate the effect of decreasing the Pb2+ and Cu+ content on the crystal structure and properties of Cu1-x□xPb1-xBi1+xS3 (x = 0, 0.33, 0.6 and 0.83). These materials exhibit two-channel thermal transport, with non-propagating phonons being the dominant contribution. Neutron diffraction data reveal that intermediate compositions crystallize in the krupkaite structure (x = 0.5, P21ma), instead of the end-member aikinite structure (x = 0, Pnma). Pair distribution function (PDF) analysis reveals that the disordering of vacancies and cations deviates significantly from that expected for a statistical distribution and that, at a local level, copper-rich and copper-poor regions occur. Reducing the Pb2+ and Cu+ content results in lattice softening, which may be attributed to the increased concentration of vacancies in copper-poor regions. Moreover, the persistence of short Pb2+-Cu+ distances in the copper-rich regions is likely to facilitate the cooperative interaction between lone pairs and rattling Cu+ cations that leads to phonon scattering. These findings provide crucial insights into the effect of the local structure on the phonon transport and highlight the potential of local-structure design to achieve high thermoelectric performance in crystalline solids.