Thermal expansion induced reduction of lattice thermal conductivity in light crystals

Combining first-principles calculations with quasiharmonic approximation and Boltzmann transport equation we take light ionic crystals LiH and NaH as examples to systematically investigate the lattice thermal properties. Remarkably, thermal expansion (TE) effect plays a significant role in heat transport of these light solids, which makes the lattice thermal conductivity κL reduce about 40% compared with the value excluding the TE effect. As a result, the calculated κL of LiH is 14.67 (12.98)Wm−1K−1 at 300 (327) K, which is well consistent with the experimental value of 14.70 (12.47)Wm−1K−1 at the same temperature. Our analyses reveal that the TE effect lead to the reduction of phonon frequency and group velocity, the enhancement of phonon scattering processes and scattering rate, and consequently the reduction of κL. These results support that TE effect is a considerable factor in calculations of κL in light materials.