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2D layered transition dichalcogenides have attracted tremendous attention for their excellent properties and multifarious applications. In particular, NbSe2 and TaSe2 are the canonical systems to study superconductivity and charge density waves. Here, we perform a comparative study of the thermal transport properties of 2D NbSe2 and TaSe2 for both 1T and 2H phases based on first-principles calculations. Usually, the lattice thermal conductivity () is smaller with larger average atom mass. However, it is contrary for the comparison between TaSe2 and NbSe2, despite the heavier Ta than Nb. The abnormally larger of TaSe2 originates from the weakened phonon–phonon scattering due to the combination of large phonon bandgap and bunching of the acoustic phonon branches, which is caused by the larger mass difference. On one hand, the large bandgap hinders the acoustic–optical phonon scattering. On the other hand, the bunching of the acoustic phonon branches hampers Umklapp process by weakening the high frequency acoustic–acoustic phonon scattering. The special characteristics of phonon transport are further conformed by mode level analysis and scattering channels of phonon–phonon scattering. Moreover, lower κL of 1T phase for both Nb and Ta selenides compared to 2H phase are also reported, which stems from the stronger anharmonicity.
Wang et al. (Fri,) studied this question.