Rational Design of Multi-Polyhedral Units-Induced Hybrid Bonding and Quadruple-Valley Band Structure in...

Area of Science:

• Materials Science
• Solid State Physics
• Chemistry

Background:

• Thermoelectric materials convert heat energy into electrical energy, crucial for waste heat recovery and solid-state cooling.
• The pavonite crystal structure, a class of layered materials, has shown potential for thermoelectric applications but often suffers from low efficiency.
• Optimizing thermoelectric performance requires decoupling electrical and thermal transport properties, a significant challenge in materials design.

Purpose of the Study:

• To synthesize and characterize a novel high-performance thermoelectric compound based on the pavonite structure.
• To investigate the relationship between the unique quasi-superlattice structure and the resulting electronic and thermal transport properties.
• To explore strategies for optimizing the thermoelectric figure of merit (ZT) through carrier concentration tuning.

Main Methods:

• Crystal structure analysis of Ag$_{0.5}$CdBi$_{4.5}$Se$_{8}$ using X-ray diffraction and electron microscopy.
• Measurement of electronic transport properties, including Seebeck coefficient and electrical conductivity, as a function of temperature and carrier concentration.
• Thermal transport measurements to determine lattice thermal conductivity.
• Compositional tuning via Sb doping and Se-excess to control carrier concentration.

Main Results:

• A unique quasi-superlattice structure composed of five distinct polyhedral units was identified in Ag$_{0.5}$CdBi$_{4.5}$Se$_{8}$.
• The material exhibits a quadruple-valley conduction band structure, enhancing carrier transport and Seebeck coefficient.
• Extremely low lattice thermal conductivity (0.24 W m$^{-1}$ K$^{-1}$ at 823 K) was achieved due to hybrid bonding and strong acoustic-optical phonon coupling.
• A peak thermoelectric figure of merit (ZT) of 0.96 at 823 K was obtained at an optimal carrier concentration of 2.52 × 10$^{20}$ cm$^{-3}$.

Conclusions:

• The multipolyhedral integration in Ag$_{0.5}$CdBi$_{4.5}$Se$_{8}$ effectively decouples electron and phonon transport, leading to high thermoelectric performance.
• This study demonstrates the potential of rational design strategies based on structural complexity for developing advanced thermoelectric materials.
• The achieved ZT value surpasses most previously reported pavonite derivatives, highlighting the significance of this structural approach.