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 (κL), 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). Pt3Bi4S9–xSex (x = 3, 6)
achieved an intrinsic ultralow κL 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 (ΘD = 170.4 K) in Pt3Bi4Se9.
Owing to the suppressed κL 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.