Ultralow Thermal Conductivity of a Chalcogenide System Pt<sub>3</sub>Bi<sub>4</sub>Q<sub>9</sub> (Q = S, Se) Driven by the Hierarchy of Rigid [Pt<sub>6</sub>Q<sub>12</sub>]<sup>12–</sup> Clusters Embedded in Soft Bi-Q Sublattice
Ruiqi Wang, Fei Liang, Xian Zhang, Chendong Zhao, Yuqiang Fang, Chong Zheng, Fuqiang Huang
Abstract
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 Pt 3 Bi 4 Q 9 (Q = S, Se). Pt 3 Bi 4 S 9– x Se 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. 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 Grüneisen parameters (γ) 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 (∼25 cm –1 ) and Debye temperatures (Θ D = 170.4 K) in Pt 3 Bi 4 Se 9 . Owing to the suppressed κ L and considerable power factor (PF), the ZT value of Pt 3 Bi 4 S 6 Se 3 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.