The physical mechanism of extremely low thermal conductivity of BiCuTeO and BiCuSeO revealed by inelastic neutron and Raman spectroscopy

Layered structure BiCuSeO-based compounds have extremely low thermal conductivity about similar to 0.5-0.8 W/m.K, and the corresponding physical origin has been extensively studied by the first-principles calculations. Here we experimentally revealed the physical mechanism of extremely low thermal conductivity in BiCuSeO and BiCuTeO through inelastic neutron and Raman scattering spectroscopy. Generalized phonon density of states (PDOS) characterized by inelastic neutron scattering reveals that the average acoustic-phonon velocities of BiCuSeO and BiCuTeO are as low as 2104 and 1547 m/s, respectively, which are lower than most of normal materials (similar to 3000 m/s), and strong anharmonic effect in BiCuSeO and BiCuTeO. Strongly anharmonic effect is also verified by the large Gruneisen constant of specific optical-phonon mode of BiCuSeO and BiCuTeO (similar to 6.7 in BiCuTeO). The calculated thermal conductivities of BiCuSeO and BiCuTeO by phenomenological thermal conductivity formula, under approximation of the relaxation-time as minimum quasi-particle lifetime of optical-phonon mode, are close to experimental values. Our work sheds more light on the physical mechanism of extremely low thermal conductivity in these compounds. (C) 2020 Elsevier B.V. All rights reserved.