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

Aliovalent doping is a way to optimize the electrical properties of semiconductors, but its impact on the phonon structure and propagation is seldom considered properly. Here we show that aliovalent doping can be much more effective in reducing the lattice thermal conductivity of thermoelectric semiconductors than the commonly employed isoelectronic alloying strategy. We demonstrate this in the heavy-band NbFeSb system, finding that a reduction of 65% in the lattice thermal conductivity is achieved through only 10% aliovalent Hf doping, compared with the four times higher isoelectronic Ta alloying. We show that aliovalent doping introduces free charge carriers and enhances screening, leading to the softening and deceleration of optical phonons. Moreover, the heavy dopant can induce the avoided crossing of acoustic and optical phonon branches, 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. Aliovalent doping affects the electrical properties of semiconductors, but its effect on phonons is unclear. Now, strong softening and deceleration of phonons, causing a significant reduction in lattice thermal conductivity, is reported for Hf-doped NbFeSb.