  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.
