Physical insights on the low lattice thermal conductivity of AgInSe2

Uncovering the microscopic mechanism of low lattice thermal conductivity is essential for exploration and design of high-performance thermoelectrics. AgInSe2 exhibits high thermoelectric performance mainly due to its low thermal conductivity. Here, the origin of its intrinsic low lattice thermal conductivity is studied by temperature-dependent inelastic neutron scattering (INS), X-ray absorption fine structure (XAFS) spectra measurements, and first-principles calculations. A prominent avoided crossing feature and low-lying optical modes in the phonon dispersion of AgInSe2 are observed experimentally. These lattice dynamical features cause a local reduction of the phonon group velocity and strongly scatter heat-carrying acoustic phonons, contributing to its intrinsic low lattice thermal conductivity. In addition, both temperature-dependent phonon dispersions and phonon density-of-states measurements reveal strong anharmonicity or phonon-phonon interactions in AgInSe2. XAFS and phonon eigenvector analysis demonstrate the dominant role of Ag vibrations, which is closely associated with the avoided crossing, low-lying optical modes and large structural distortion, and thus dominates the reduction of lattice thermal conductivity of AgInSe2. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The low lattice thermal conductivity of AgInSe2 is attributed to the avoided crossing feature and low-lying optical modes in its phonon dispersion, which results in a local reduction of phonon group velocity and strong scattering of heat-carrying acoustic phonons. Additionally, strong anharmonicity or phonon-phonon interactions in AgInSe2 are revealed by temperature-dependent phonon dispersions and phonon density-of-states measurements. The dominant role of Ag vibrations in reducing the lattice thermal conductivity of AgInSe2 is demonstrated by XAFS and phonon eigenvector analysis, closely related to the avoided crossing, low-lying optical modes, and large structural distortion.