▎ 摘　　要

Materials which contract on heating (negative thermal expansion, NTE) are of significant interest for advanced applications. Graphene has shown NTE up to 1000 K, which motivates further improvements in two-dimensional carbon to attain superior performance. In this Communication, very large negative thermal expansion coefficients (alpha(T)) are reported for tri-graphene (TrG) and T-graphene (TG). Quasi-harmonic approximation calculations show that alpha(T )remains negative until 4200 K and 2900 K for TrG and TG, respectively. The high NTE for these systems is understood on the basis of the soft phonon modes, which induce rotation of the 3-membered and the 4-membered rings in TrG and TG, respectively, and ab initio molecular dynamics simulations. The local distortions for the 3-12 rings (in TrG) and 4-8 rings (in TG) have structural resemblance with the rigid-unit modes that are usually envisioned for bulk systems.