s-d Coupling-Induced Dynamic Off-Centering of Cu Drives High Thermoelectric Performance in TlCuS

Area of Science:

• Solid-state chemistry
• Materials science
• Condensed matter physics

Background:

• Thermoelectric materials are crucial for energy harvesting and cooling.
• Understanding microscopic structure-property relationships is key to designing efficient thermoelectrics.
• Layered metal sulfides offer potential for thermoelectric applications.

Purpose of the Study:

• To investigate the impact of local structural distortion on the thermoelectric properties of TlCuS.
• To elucidate the mechanism behind structural distortion and its effect on thermal conductivity.
• To evaluate the thermoelectric performance of TlCuS and its potential for practical applications.

Main Methods:

• Powder X-ray diffraction (XRD) to assess average crystal structure.
• Synchrotron X-ray pair distribution function (PDF) analysis to probe local structure.
• Analysis of electronic structure and lattice dynamics to understand thermoelectric mechanisms.

Main Results:

• Synchrotron XRD revealed dynamic off-centering distortions in CuS4 tetrahedra, not apparent from average structure analysis.
• These distortions, driven by a second-order Jahn-Teller mechanism and s-d orbital coupling, induce significant lattice anharmonicity.
• Ultralow lattice thermal conductivity (κlat ≈ 0.35-0.23 W m⁻¹ K⁻¹) was achieved due to phonon blocking.
• A high thermoelectric figure-of-merit (zT) of ~1.3 was obtained for TlCu0.98S at 573 K.

Conclusions:

• Local structural distortions are critical for enhancing thermoelectric properties in TlCuS.
• The interplay between dynamic distortions, anharmonicity, and ultralow κlat is a promising strategy for thermoelectric material design.
• Metal sulfides, exemplified by TlCuS, show significant potential as high-performance thermoelectric materials.