Unveiling the microscopic origin of anomalous thermal conductivity in amorphous carbon

Amorphous carbons pose fundamental questions, including incomprehensible phase transformations, microtopology identification, and origins of anomalous properties. However, transition pathways from low-density, high-sp² structures to high-density, high-sp³ forms remain poorly understood, within which thermodynamical behaviors are practically unexplored. Here, we investigated the phase transition and thermal transport properties of a recently reported amorphous carbon phase, amorphous diaphite (a-DG). The continuous transformation pathway of a-DG is characterized by distinctive heterogeneous microstructural evolutions across a wide density range. We observed anomalous thermal conductivities in a-DG, which initially decrease and then increase with the density and sp³/sp² ratio, deviating markedly from previously reported trends. This anomaly originates from changes in phonon mean free path and phonon lifetime at the medium-frequency range (7 to 30 terahertz), dictated by the unique two-stage microtopological transition. These findings challenge conventional views of thermal conductivity in amorphous carbons, and, in a wider context, our mechanistic understanding provides fundamental insights into phase transitions and thermodynamic mechanisms of other amorphous materials.