Optical phonon dominated heat transport: A first-principles thermal conductivity study of BaSnS2

Acoustic phonons with long mean free paths have long been believed to control the lattice thermal conductivity κL in solids dominantly. In this study, however, we demonstrate an optical phonon dominated κL in BaSnS2. By solving the Peierls-Boltzmann transport equation, we predict a low diagonal lattice thermal conductivity κL(D) of 0.34Wm−1K−1 at 850 K, which is less than half the κL(D) of SnS at the same temperature. Further calculations following the Allen-Feldman model suggest the additional off-diagonal lattice thermal conductivity κL(OD) contributed by wavelike tunneling phonons. The κL(OD) becomes pronounced at the high temperature (0.17Wm−1K−1 at 850 K) and leads to a deviation of the temperature dependence of κL from T–1 to T–0.76, suggesting the potential lattice anharmonicity in BaSnS2. Further analyses indicate BaSnS2 has over 68% of κL contributed by optical phonons. We show this uncommon optical phonon dominated κL is due to the relatively high group velocities of optical phonons in BaSnS2. The phonon mode visualization suggests these relatively high-velocity optical phonons correspond to the antiphase vibrations in BaSnS2 monolayers, which is originated from the unique permutation of SnS3 tetrahedra. Finally, by investigating the mode-resolved group velocity, relaxation time, and Grüneisen parameter, we attribute the intrinsic low κL of BaSnS2 to the soft lattice and the relatively high lattice anharmonicity induced by the Ba-S weak bonding and Sn(II) lone-pair electrons. Our study explicitly analyzes the microscopic mechanism of optical phonon dominated heat transport in BaSnS2 and suggests it worthy of further experimental studies as an intrinsic low-κL material.