Ultralow Thermal Conductivity of a Chalcogenide System Pt₃Bi₄Q₉ (Q = S, Se) Driven by the Hierarchy of Rigid [Pt₆Q₁₂]^12– Clusters Embedded in Soft Bi‑Q Sublattice

Knowledge of structure–property relationships in solids with intrinsic low thermal conductivity is crucial for fields such as thermoelectrics, thermal barrier coatings, and refractories. Herein, we propose a new “rigidness in softness” structural scheme for intrinsic low lattice thermal conductivity (κ_L), which embeds rigid clusters into the soft matrix to induce large lattice anharmonicity, and accordingly discover a new series of chalcogenides Pt₃Bi₄Q₉ (Q = S, Se). Pt₃Bi₄S_9–xSe_x (x = 3, 6) achieved an intrinsic ultralow κ_L down to 0.39 W/(m K) at 773 K, which is considerably low among the Bi chalcogenide thermoelectric materials. Pt₃Bi₄Q₉ contains the rigid cubic [Pt₆Q₁₂]^12– clusters embedded in the soft Bi-Q sublattice, involving multiple bonding interactions and vibration hierarchy. The hierarchical structure yields a large lattice anharmonicity with high Grüneisen parameters (γ) 1.97 of Pt₃Bi₄Q₉, as verified by the effective scatter of low-lying optical phonons toward heat-carrying acoustic phonons. Consequently, the rigid-soft coupling significantly inhibits heat propagation, exhibiting low acoustic phonon frequencies (∼25 cm^–1) and Debye temperatures (Θ_D = 170.4 K) in Pt₃Bi₄Se₉. Owing to the suppressed κ_L and considerable power factor (PF), the ZT value of Pt₃Bi₄S₆Se₃ can reach 0.56 at 773 K without heavy carrier doping, which is competitive among the pristine Bi chalcogenides. Theoretical calculations predicted a large potential for performance improvement via proper doping, indicating the great potential of this structure type for promising thermoelectric materials.