Influence of the optical-acoustic phonon hybridization on phonon scattering and thermal conductivity

We predict a marked effect of optical-acoustic phonon hybridization on phonon scattering and lattice thermal conductivity ($\ensuremath{\kappa}$), and illustrate it in the case of ${\mathrm{Fe}}_{2}{\mathrm{Ge}}_{3}$. This material presents very low-lying optical phonons with an energy of 1.8 meV at the Brillouin zone center, which show avoided crossings with longitudinal acoustic (LA) phonons, due to optical-acoustic phonon polarization hybridization. Because the optical phonons have nonvanishing scattering rates, even a small amount of hybridization with the optical phonon can increase the scattering rates of LA phonons by much more than one order of magnitude, causing the contribution of these phonons to $\ensuremath{\kappa}$ to vanish. At low temperatures, the contributions of all LA phonons are eliminated, and thus the avoided crossing leads to a reduction of thermal conductivity by more than half. The scattering rates are very sensitive to the optical-acoustic phonon hybridization strength, characterized by the gap at the avoided crossing point and varied with the wave-vector direction. Our work presents a different reduction mechanism of $\ensuremath{\kappa}$ in systems with optical-acoustic phonon hybridization, which can benefit the search for new thermoelectric materials.