Symmetry breaking driven ultralow thermal conductivity in simple diamond-like crystal

Achieving ultralow lattice thermal conductivity (κ_ι) in simple crystals remains challenging without complex chemistry or micro/nanostructure engineering. In this work, we demonstrate that symmetry breaking is an effective approach to control thermal transport in diamond-like A(X-Y)₂ (A=Si, Ge; X/Y=C, B, N) simple crystals, ultimately leading to an ultralow κ_ι. In particular, Ge(B-N)₂ exhibits an ultralow κ_ι of 1.52 Wm^-1K^-1 at 300 K and 0.85 Wm-1K-1 at 800 K, comparable to that of amorphous materials. Based on a Wigner theory of thermal transport, we decompose κ_ι into the population (particle-like) κ_p and the coherence (wave-like) κ_c contributions. By introducing symmetry breaking, the uniform bond-strength network is redistributed, which enhances anharmonic phonon scattering and shortens phonon lifetimes, leading to a reduction of up to 64% in room temperature κ_p in Ge(B-N)₂ compared with symmetry-preserved Ge(C-C)₂. With the strong suppression of particle-like phonons, the wave-like contributions become increasingly important, with the κ_c/κ_p ratio reaching ~90% at 800 K in Ge(B-N)₂. This study provides a novel strategy for designing materials with tailored thermal conductivity, which has potential applications in the development of thermoelectric materials.