Impact of the lead lone pair on the local structure of the aikinite-bismuthinite family

We have recently identified the aikinite-bismuthinite series, Cu1–x□xPb1–xBi1+xS3 (0 <= x <= 1) as a potential new family of thermoelectric materials with ultralow thermal conductivity (< 0.5 W m-1 K-1). All materials in this series adopt closely related crystal structures, intermediate between those of aikinite (CuPbBiS3) and bismuthinite (Bi2S3). In mineral samples in this series the copper and lead cations are ordered, with the copper cations positioned in tetrahedral sites close to the Pb cations, and this results in the formation of a number of superstructures (e.g. x= 0.33 hammarite; x = 2/3 gladite, x = 0.83 pekoite). By contrast, according to X-ray diffraction data, synthetic samples in this series adopt a cation-deficient aikinite structure, which contains disordered vacancies at the copper site. Our computational and experimental work on the aikinite end member suggests that the interaction between the rattling copper cation and the lone pair of the lead cation is key to understanding the ultralow thermal conductivity of these materials. The large atomic displacement parameters for Cu+ and Pb2+ in aikinite may hint to an underlying distortion at the local scale, which may arise from uncorrelated lone pair stereochemical activity of the Pb2+ cations. Here, we wish to exploit powder neutron total scattering to carry out a pair distribution function (PDF) analysis to investigate the local structure around the copper and lead cations.