The first-principles and BTE investigation of phonon transport in 1T-TiSe2

Abstract

Through the first-principles density functional theory and the phonon Boltzmann transport equation, we investigated the phonon transport characteristics inside 1T-TiSe₂. The calculation results of the lattice thermal conductivity (κ_l) show that the κ_l of TiSe₂ is extremely low (1.28 W (m K)⁻¹, 300 K) and decreases with the shrinkage of the sample size. Moreover, the results also prove the isotropic nature of thermal transport. By decomposing the contribution of the thermal conductivity according to the frequency, the κ_l of the single-layer TiSe₂ is primarily attributed to the acoustic phonons and a small portion of optical phonons, with the frequency range of 0–4.5 THz. The calculation of the scattering rate further illustrates the competition of different scattering modes in this frequency range to verify the change in thermal conductivity of different sample sizes. The high scattering rate and low group velocity lead to the low thermal conductivity of the optical phonon mode in TiSe₂. In addition, reducing the size of the system can significantly limit the thermal conductivity by eliminating the contribution of long mean free path phonons. When the characteristic length of the single-layer TiSe₂ is about 14.92 nm, κ_l reduces to half. Our results also show that TiSe₂ has an extremely high Grüneisen parameter (about 2.62). Further decomposition of the three-phonon scattering phase space and scattering rate demonstrates that in the range 0–4.5 THz, the absorption process is the main conversion form of phonons. We conclude that, due to the high Grüneisen parameter, the high anharmonicity in TiSe₂ leads to the extremely low κ_l. This study provides κ_l related to the temperature, frequency, and MFP, and deeply discusses the phonon transport in TiSe₂, which has great significance to further adjust the thermal conductivity to develop highly efficient thermoelectric materials and promote the application of devices based on TiSe₂.