Ultralow Thermal Conductivity of a Chalcogenide System Pt₃Bi₄Q₉ (Q = S, Se) Driven by the Hierarchy of Rigid...

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-Se (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.