Lone Pair Rotation and Bond Heterogeneity Leading to Ultralow Thermal Conductivity in Aikinite

Understanding the relationship between the crystal structure, chemical bonding, and lattice dynamics is crucial for the design of materials with low thermal conductivities, which are essential in fields as diverse as thermoelectrics, thermal barrier coatings, and optoelectronics. The bismuthinite-aikinite series, Cu_1-□Pb_1-Bi₁₊S₃ (0 ≤ x ≤ 1, where □ represents a vacancy), has recently emerged as a family of n-type semiconductors with exceptionally low lattice thermal conductivities. We present a detailed investigation of the structure, electronic properties, and the vibrational spectrum of aikinite, CuPbBiS₃ (x = 0), in order to elucidate the origin of its ultralow thermal conductivity (0.48 W m^-1 K^-1 at 573 K), which is close to the calculated minimum for amorphous and disordered materials, despite its polycrystalline nature. Inelastic neutron scattering data reveal an anharmonic optical phonon mode at ca. 30 cm^-1, attributed mainly to the motion of Pb²⁺ cations. Analysis of neutron diffraction data, together with ab-initio molecular dynamics simulations, shows that the Pb²⁺ lone pairs are rotating and that, with increasing temperature, Cu⁺ and Pb²⁺ cations, which are separated at distances of ca. 3.3 Å, exhibit significantly larger displacements from their equilibrium positions than Bi³⁺ cations. In addition to bond heterogeneity, a temperature-dependent interaction between Cu⁺ and the rotating Pb²⁺ lone pair is a key contributor to the scattering effects that lower the thermal conductivity in aikinite. This work demonstrates that coupling of rotating lone pairs and the vibrational motion is an effective mechanism to achieve ultralow thermal conductivity in crystalline materials.