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 (kappa(L)), which embeds rigid clusters into the soft matrix to induce large lattice anharmonicity, and accordingly discover a new series of chalcogenides Pt(3)Bi(4)Q(9) (Q = S, Se). Pt3Bi4S9-xSex (x = 3, 6) achieved an intrinsic ultralow kappa(L) down to 0.39 W/(m K) at 773 K, which is considerably low among the Bi chalcogenide thermoelectric materials. Pt(3)Bi(4)Q(9) contains the rigid cubic [Pt(6)Q(12)](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 Gr & uuml;neisen parameters (gamma) 1.97 of Pt(3)Bi(4)Q(9), 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 (similar to 25 cm(-1)) and Debye temperatures (Theta(D) = 170.4 K) in Pt3Bi4Se9. Owing to the suppressed kappa(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.