Bonding Heterogeneity in Mixed-Anion Compounds Realizes Ultralow Lattice Thermal Conductivity

Crystalline materials with intrinsically low lattice thermal conductivity (κ_lat) pave the way towards high performance in various energy applications, including thermoelectrics. Here we demonstrate a strategy to realize ultralow κ_lat using mixed-anion compounds. Our calculations reveal that locally distorted structures in chalcohalides MnPnS₂Cl (Pn = Sb, Bi) derives a bonding heterogeneity, which in turn causes a peak splitting of the phonon density of states. This splitting induces a large amount of scattering phase space. Consequently, κ_lat of MnPnS₂Cl is significantly lower than that of a single-anion sulfide CuTaS₃ with a similar crystal structure. Experimental κ_lat of MnPnS₂Cl takes an ultralow value of about 0.5 W m⁻¹ K⁻¹ at 300 K. Our findings will encourage the exploration of thermal transport in mixed-anion compounds, which remain a vast unexplored space, especially regarding unexpectedly low κ_lat in lightweight materials derived from the bonding heterogeneity.