Lone-Electron-Pair Micelles Strengthen Bond Anharmonicity in MnPb₁₆Sb₁₄S₃₈ Complex Sulfosalt Leading to Ultralow Thermal Conductivity

Designing crystalline solids in which intrinsically and extremely low lattice thermal conductivity mainly arises from their unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ∼0.34 W m^-1 K^-1 at 300 K in the new complex sulfosalt MnPb₁₆Sb₁₄S₃₈. We attribute the ultralow lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a C_p value of 0.206 J g^-1 K^-1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g^-1 K^-1). Further analysis of the C_p data and sound velocity (ν = 1834 m s^-1) measurement yielded Debye temperature values of 161 and 187 K, respectively. The resulting Grüneisen parameter, γ= 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.