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 ${\ensuremath{\kappa}}_{\mathrm{L}}$ in solids dominantly. In this study, however, we demonstrate an optical phonon dominated ${\ensuremath{\kappa}}_{\mathrm{L}}$ in ${\mathrm{BaSnS}}_{2}$. By solving the Peierls-Boltzmann transport equation, we predict a low diagonal lattice thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{L}}(\mathrm{D})$ of $0.34\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 850 K, which is less than half the ${\ensuremath{\kappa}}_{\mathrm{L}}(\mathrm{D})$ of SnS at the same temperature. Further calculations following the Allen-Feldman model suggest the additional off-diagonal lattice thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{L}}(\mathrm{OD})$ contributed by wavelike tunneling phonons. The ${\ensuremath{\kappa}}_{\mathrm{L}}(\mathrm{OD})$ becomes pronounced at the high temperature ($0.17\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 850 K) and leads to a deviation of the temperature dependence of ${\ensuremath{\kappa}}_{\mathrm{L}}$ from ${T}^{\text{--}1}$ to ${T}^{\text{--}0.76}$, suggesting the potential lattice anharmonicity in ${\mathrm{BaSnS}}_{2}$. Further analyses indicate ${\mathrm{BaSnS}}_{2}$ has over 68% of ${\ensuremath{\kappa}}_{\mathrm{L}}$ contributed by optical phonons. We show this uncommon optical phonon dominated ${\ensuremath{\kappa}}_{\mathrm{L}}$ is due to the relatively high group velocities of optical phonons in ${\mathrm{BaSnS}}_{2}$. The phonon mode visualization suggests these relatively high-velocity optical phonons correspond to the antiphase vibrations in ${\mathrm{BaSnS}}_{2}$ monolayers, which is originated from the unique permutation of ${\mathrm{SnS}}_{3}$ tetrahedra. Finally, by investigating the mode-resolved group velocity, relaxation time, and Gr\"uneisen parameter, we attribute the intrinsic low ${\ensuremath{\kappa}}_{\mathrm{L}}$ of ${\mathrm{BaSnS}}_{2}$ 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 ${\mathrm{BaSnS}}_{2}$ and suggests it worthy of further experimental studies as an intrinsic low-${\ensuremath{\kappa}}_{\mathrm{L}}$ material.