Thermal insulation enhanced by the dopant-induced phonon softening discovered in thermoelectric Heusler compounds

Suppression of phonon transports is a key requirement to achieve high thermoelectric performance, which has been often realized by doping an element to enhance the phonon scattering. However, detailed origins behind the dopant-induced low lattice thermal conductivity ( κ ph ) have not yet been comprehensively understood. Here, we demonstrate a κ ph reduction mechanism induced by the phonon softening in the element-doped Fe ₂VAl thermoelectric Heusler compounds. Inelastic X-ray scattering experiments revealed that overall optical phonons are softened with increasing Ti-doping concentration in marked contrast to the Ta-doped Fe ₂VAl. By means of photoelectron spectroscopy, we found that the observed phonon softening is originating from the rigid-band shift of the electronic density of states and the consequent deviation of the pseudo-gap position from the Fermi level, which causes the energetic instability of the electronic system. This instability is accompanied by the reduction of the filled bonding states, giving rise to the phonon softening. This mechanism of κ ph reduction caused by the close correlation between electronic and phononic systems will shed new light on achieving low- κ ph and thereby developing unconventional high-performance thermoelectric materials.