The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe

It is a general consensus that phonon anharmonicity increases near structural phase transitions (PT). Increased phonon anharmonicity leads to lower lattice thermal conductivity. However, the lattice thermal conductivity in germanium telluride (GeTe) increases near the ferroelectric PT. We use first-principles calculations coupled with the temperature-dependent effective potential method [1] and the Boltzmann transport equation (BTE) to elucidate this unexpected phenomenon. We find that, although anharmonicity increases at the PT in GeTe, the phonon group velocities increase as well, leading to an overall increase of the lattice thermal conductivity. The increased anharmonicity strongly affects the phonon spectral functions, leading to a giant softening of the peaks of the soft phonon power spectra at the PT. To account for these effects, we implement a novel method of calculating lattice thermal conductivity, which uses the information from the entire phonon power spectra. Using this approach, we find that the BTE underestimates the lattice thermal conductivity of GeTe at the phase transition.

[1] O. Hellman and I. A. Abrikosov, Phys. Rev. B 88, 144301 (2013)