Bond heterogeneity, lone pairs, and ultralow thermal conductivity: the story of aikinites

Large-scale production of thermoelectric devices requires the discovery of high-performance materials, which are low cost [PV1] and based on abundant elements in the Earth crust. In this context, sulfide minerals, such as CuPbBiS₃, are a promising class of thermoelectric materials. Of special interest are materials that can provide, with minor modifications, the p-type and n-type legs of the thermocouple, to ensure compatibility. To this goal we have investigated the structural, electronic and vibrational properties of aikinite and copper-deficient aikinite Cu_1–xPb_1–xBi_1+xS₃ (0 ≤ x ≤ 1). Our work encompasses synthesis, powder X-ray and neutron diffraction, electronic and thermal transport properties, inelastic neutron scattering, and first-principles molecular dynamics calculations. We have established that these materials are semiconductors that can be doped p- or n-type. We have assessed the bond heterogeneity and the role of Pb and Bi lone-pairs on the thermal transport and will discuss how these features lead to an exceptionally low thermal conductivity of 0.48 W m^-1 K^-1 at 573 K.