Symmetry Breaking Driven Ultralow Thermal Conductivity in Simple Diamond-Like Crystal

Achieving ultralow lattice thermal conductivity (\kappa_\rm l) 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\text-Y)_2 (A = Si, Ge; X/Y = C, B, N) simple crystals, ultimately leading to an ultralow \kappa_\rm l. In particular, Ge(B-N)_2 exhibits an ultralow \kappa_\rm l of 1.52 W\cdotm^-1\cdot K^-1 at 300 K and 0.85 W\cdotm^-1\cdot K^-1 at 800 K, comparable to that of amorphous materials. Based on Wigner theory of thermal transport, we decompose \kappa_\rm l into the population (particle-like) \kappa_\rm p and the coherence (wave-like) \kappa_\rm 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 \kappa_\rm 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 \kappa_\rm c/\kappa_\rm p ratio reaching \sim 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.