Stability of sp3 Hybridized Amorphous Carbon and its Transformation to Nanodiamond

YF Zhao and C Zhao and X Wang and F Ding, SMALL METHODS, 9 (2025).

DOI: 10.1002/smtd.202500294

A newly synthesized carbon allotrope, fully sp3-hybridized amorphous carbon (sp3-AC), has generated widespread research interest, especially in terms of structural stability and evolution. Here, the molecular simulations with machine learning force field to explore the origin of its stability and the structural transformation from sp3-AC to diamond is performed. It is found that the diffusion of carbon in the covalently bonded sp3-AC is extremely slow. Although the sp3-AC exhibits a higher energy than diamond, the nucleation of diamond within it remains thermodynamically unfavorable. The transformation from sp3-AC to diamond is impeded by an exceptionally high nucleation energy barrier of approximate to 17 eV, contributing to its kinetic stability. Analysis indicates that diamond nucleation within sp3-AC induces a localized density reduction, which may serve as the underlying mechanism that inhibits nucleation and suppresses continuous growth. As a result, annealing the sp3-AC at high temperature and high pressure leads to a specific type of composite, where randomly oriented diamond nanocrystals are evenly distributed in sp3-AC. The specific composite of sp3-AC- nanodiamond serves as an intermediate between sp3-AC and diamond, which reveals the underlying mechanism of recent experimental observation of sp3-AC-nanodiamond composite.

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