Optic phonon bandwidth and lattice thermal conductivity: The case ofLi2X(X=O, S, Se, Te)

We examine the lattice thermal conductivities $({\ensuremath{\kappa}}_{l})$ of $\mathrm{L}{\mathrm{i}}_{2}X(X=\mathrm{O},\phantom{\rule{0.16em}{0ex}}\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te})$ using a first-principles Peierls-Boltzmann transport methodology. We find low ${\ensuremath{\kappa}}_{l}$ values ranging between 12 and 30 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ despite light Li atoms, a large mass difference between constituent atoms, and tightly bunched acoustic branches, all features that give high ${\ensuremath{\kappa}}_{l}$ in other materials including BeSe (630 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$), BeTe (370 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$), and cubic BAs (3170 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$). Together these results suggest a missing ingredient in the basic guidelines commonly used to understand and predict ${\ensuremath{\kappa}}_{l}$. Unlike typical simple systems (e.g., Si, GaAs, SiC), the dominant resistance to heat-carrying acoustic phonons in $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Se}$ and $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Te}$ comes from interactions of these modes with two optic phonons. These interactions require significant bandwidth and dispersion of the optic branches, both present in $\mathrm{L}{\mathrm{i}}_{2}X$ materials. These considerations are important for the discovery and design of new materials for thermal management applications and give a more comprehensive understanding of thermal transport in crystalline solids.