Giant phonon softening and avoided crossing in aliovalence-doped heavy-band thermoelectrics

TL;DR

Aliovalent doping in thermoelectric semiconductors significantly reduces lattice thermal conductivity by softening phonons and inducing avoided crossings, outperforming isoelectronic alloying strategies.

Contribution

This study demonstrates that aliovalent doping can more effectively suppress phonon propagation and reduce thermal conductivity than traditional isoelectronic alloying in thermoelectric materials.

Findings

65% reduction in thermal conductivity with 10% Hf-doping

Aliovalent doping causes giant phonon softening and avoided crossing

Heavy dopants decelerate acoustic phonons

Abstract

Aliovalent doping has been adopted to optimize the electrical properties of semiconductors, while its impact on the phonon structure and propagation is seldom paid proper attention to. This work reveals that aliovalent doping can be much more effective in reducing the lattice thermal conductivity of thermoelectric semiconductors than the commonly employed isoelectronic alloying strategy. As demonstrated in the heavy-band NbFeSb system, a large reduction of 65% in the lattice thermal conductivity is achieved through only 10% aliovalent Hf-doping, compared to the 4 times higher isoelectronic Ta-alloying. It is elucidated that aliovalent doping introduces free charge carriers and enhances the screening, leading to the giant softening and deceleration of optical phonons. Moreover, the heavy dopant can induce the avoided-crossing of acoustic and optical phonon branches, further decelerating the acoustic phonons. These results highlight the significant role of aliovalent dopants in regulating the phonon structure and suppressing the phonon propagation of semiconductors.