Two-Dimensional-Like Phonons in Three-Dimensional-Structured Rhombohedral GeSe-Based Compounds with Excellent...

Area of Science:

• Materials Science
• Solid-State Physics
• Thermoelectrics

Background:

• The coupling of charge and phonon transport hinders thermoelectric performance.
• Developing novel materials with decoupled transport is crucial for efficient thermoelectrics.

Purpose of the Study:

• To design and synthesize new narrow-gap semiconductors with decoupled charge and phonon transport.
• To investigate the structure-property relationships in GeSe_0.65Te_0.35 (GST) based materials.
• To enhance thermoelectric performance through structural engineering and doping.

Main Methods:

• Synthesis of layered hexagonal (H-GST) and non-layered rhombohedral (R-GST) phases of GeSe_0.65Te_0.35.
• Characterization of structural, thermal, and electrical transport properties.
• Analysis of phonon behavior using low-temperature specific heat and phonon spectra calculations.
• Sb doping to stabilize the rhombohedral phase at elevated temperatures.

Main Results:

• R-GST exhibits a 3D network structure with higher weighted mobility compared to H-GST.
• R-GST shows exceptionally low lattice thermal conductivity (∼0.5 W m^-1 K^-1 at 523 K) due to 2D-like phonon transport induced by off-centering Ge atoms and ferroelectric instability.
• Sb doping (1 mol %) suppressed phase transitions, enabling higher operating temperatures.
• A peak thermoelectric figure of merit (ZT) of 1.1 was achieved at 623 K in Sb-doped R-GST, a significant improvement over GeSe.

Conclusions:

• The rhombohedral GeSe_0.65Te_0.35 structure facilitates decoupled charge and phonon transport, leading to enhanced thermoelectric properties.
• Off-centering atoms and resulting ferroelectric instability are key to achieving low lattice thermal conductivity.
• This study demonstrates a viable strategy for designing high-performance thermoelectric materials by exploring compounds with off-centering atoms.