The creation of carbon allotropes is synthetically highly challenging, but their practical significance to science and technology cannot be underestimated. Here, we report the synthesis of a new carbon allotrope, diamondiyne, which is the second known one (after diamond) that forms covalent bonds in all three dimensions. Diamondiyne has a mixed sp–sp ³ hybridization, resulting in a carbo‐mer of diamond with an expanded diamond‐topology network. On the nanoscopic scale, two asymmetrically interpenetrated 3D networks form the tetragonal space group I 4 ₁ / amd with unit cell parameters a = b =  11.47 Å and c =  16.22 Å. Amorphous carbon films containing diamondiyne crystals are formed using a cascade reaction at a liquid–liquid interface. Just as for a covalent organic framework, molecular nodes are connected based on geometrical constraints to form an extended structure, and we show that the method is scalable in both the thickness and the lateral direction of the film. New carbon allotropes have historically found widespread use in materials science, and we look forward to what applications might emerge for diamondiyne.

The creation of carbon allotropes is synthetically highly challenging, but their practical significance to science and technology cannot be underestimated. Here, we report the synthesis of a new carbon allotrope, diamondiyne, which is the second known one (after diamond) that forms covalent bonds in all three dimensions. Diamondiyne has a mixed sp–sp ³ hybridization, resulting in a carbo‐mer of diamond with an expanded diamond‐topology network. On the nanoscopic scale, two asymmetrically interpenetrated 3D networks form the tetragonal space group I 4 ₁ / amd with unit cell parameters a = b =  11.47 Å and c =  16.22 Å. Amorphous carbon films containing diamondiyne crystals are formed using a cascade reaction at a liquid–liquid interface. Just as for a covalent organic framework, molecular nodes are connected based on geometrical constraints to form an extended structure, and we show that the method is scalable in both the thickness and the lateral direction of the film. New carbon allotropes have historically found widespread use in materials science, and we look forward to what applications might emerge for diamondiyne.