Thermoelectric Material SnPb2Bi2S6: The 4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity

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.

Incommensurately Modulated Cu0.9Pb1.2Sb2.9Se6 in the Lillianite Structure Type

Samples with the nominal composition Cu0.9Pb1.2Sb2.9Se6 mainly contain a phase with incommensurately modulated lillianite-type structure with the respective composition. Single crystal diffraction with synchrotron radiation enabled a detailed refinement using the superspace group Cmcm(α00)00s with lattice parameters a = 4.16537(5), b = 14.0821(2), c = 19.8234(3) Å, and a modulation vector q = 0.6890(2)a* at room temperature. The structure is built up from tilted and distorted NaCl-type slabs that are interconnected by bicapped trigonal prisms, which mainly host Pb atoms, according to a 4L arrangement. Satellites up to the second order reveal positional and occupational modulation that mainly involves a sequence of Sb and Cu atoms and allows the Se substructure to adapt in a way that Sb and Cu feature predominantly octahedral and tetrahedral coordination, respectively. Above 523 K, satellite reflections disappear, and the crystal structure becomes more disordered with average coordination spheres of both Sb and Cu atoms corresponding to distorted octahedra. This phase transition leads to discontinuities in the evolution of lattice parameters and physical properties as functions of temperature. HRTEM investigations corroborate centrosymmetry and highlight atoms that are strongly affected by the modulation. Measurements of transport properties reveal a p-type semiconductor with a thermoelectric figure of merit up to 0.1 at 623 K. In accordance with B factor analysis, a small amount of substitution could increase zT significantly by optimizing the carrier concentration.

The crystal structure of the selenide-based synthetic sulfosalt CuPbSb3Se6

Single crystals of copper lead tri-anti-mony hexa-selenide, CuPbSb3Se6, were obtained as a minor phase during systematic studies of the formation conditions of selenide-based sulfosalts. The crystal structure is an unusual representative of the family of sulfosalts. Instead of the expected galena-like slabs with octa-hedral coordination, it features mono and double-capped trigonal-prismatic (Pb), square-pyramidal (Sb) and trigonal-bipyramidal (Cu) coordination. All metal positions are occupationally and/or positionally disordered.

A Tunable Structural Family with Ultralow Thermal Conductivity: Copper-Deficient Cu1-x□xPb1-xBi1+xS3

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi₅S₉ with an ultralow κ value of 0.6-0.4 W m^-1 K^-1 in the temperature range 300-700 K. In contrast to prior studies, we show that this synthetic sulfide does not exhibit the ordered gladite mineral structure but instead forms a copper-deficient disordered aikinite structure with partial Pb replacement by Bi, according to the chemical formula Cu_1/3□_2/3Pb_1/3Bi_5/3S₃. By combining experiments and lattice dynamics calculations, we elucidated that the ultralow κ value of this compound is due to very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints at substantial anharmonic effects that contribute to enhance phonon scattering. Importantly, we show that this aikinite-type sulfide, despite being a poor semiconductor, is a potential matrix for designing novel, efficient n-type thermoelectric compounds with ultralow κ values. A drastic improvement in the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The Cu_1-x□_xPb_1-xBi_1+xS₃ series provides a new, interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.