Anomalous thermal transport in semiconductors induced by aliovalent doping

As an effective method for thermal management technologies, doping or substitution has been extensively utilized to reduce the lattice thermal conductivity of various materials. Intensive studies have been conducted about the phonon mechanism of isoelectronic alloying since the 1950s. Very recently, the specific role of aliovalent doping was elucidated in the half-Heusler NbFeSb system. Here, we have theoretically and experimentally investigated the mechanism of reducing thermal conductivity through aliovalent doping by combining first-principles calculations and neutron diffraction studies for the case study of the TiCoSb half-Heusler system. The softening of the acoustic branches induced by aliovalent doping can effectively reduce the phonon group velocities. Moreover, the introduction of compensating defects, resulting from changes in the Fermi level, plays a vital role in decreasing the relaxation time of phonons, as demonstrated by the analysis of neutron powder diffraction. Due to these two factors, doping with adjacent elements results in a significant reduction in lattice thermal conductivity (for instance, Ni doping at the Co site in the TiCoSb half-Heusler system), especially in the low-temperature range. Our findings provide valuable insight into the phonon scattering mechanism in aliovalent-doped materials and demonstrate the role of compensating defects in heat transport, which is applicable to other doped semiconductor systems.