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

Abstract Achieving ultralow lattice thermal conductivity ( κ 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 - Y ) 2 ( A = Si, Ge; X / Y = C, B, N) simple crystals, ultimately leading to an ultralow κ l . In particular, Ge(B-N) 2 exhibits an ultralow κ l of 1.52 W·m −1 ·K −1 at 300 K and 0.85 W·m −1 ·K −1 at 800 K, comparable to that of amorphous materials. Based on Wigner theory of thermal transport, we decompose κ l 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.