Isotope effect on four-phonon interaction and lattice thermal transport: An atomistic study of lithium hydride

Despite decades-long studies of how isotope-disorder scattering affects heat conduction, the effect of isotope-induced frequency shift on phonon-phonon interactions and lattice thermal transport remains largely unexplored and less understood. In this work, by using the phonon Boltzmann transport equation and machine-learning-assisted classical and path-integral molecular-dynamics simulations, we theoretically investigate the interplay between the intrinsic phonon-phonon scatterings and the isotopic compositions in lithium hydride (LiH). We reveal that the hydrogen isotopes notably impact thermal transport primarily via three-phonon interaction, while four-phonon scattering is less sensitive. Due to the stronger temperature dependence of higher-order processes, the isotope effect on thermal conductivity is increasingly suppressed by four-phonon scattering as temperature rises (from 73% to 35% at 700 K). In addition, we observe a more pronounced isotope effect on thermal diffusivity, which is attributed to the variation of heat capacity. Our work provides a promising avenue for manipulating thermal transport via isotope engineering, and it highlights the crucial role of four-phonon interaction.