Interpenetrating graphene networks: Three-dimensional node-line semimetals with massive negative linear compressibilities

We investigated the stability and mechanical and electronic properties of 15 metastable mixed sp²-sp³ carbon allotropes in the family of interpenetrating graphene networks (IGNs) using density functional theory (DFT). IGN allotropes exhibit nonmonotonic bulk and linear compressibilities before their structures irreversibly transform into new configurations under large hydrostatic compression. The maximum bulk compressibilities vary widely between structures and range from 3.6 to 306 TPa^-1. We find all the IGN allotropes have negative linear compressibilities with maximum values varying from --0.74 to --133TPa^-1. The maximal negative linear compressibility of Z33 (--133TPa^-1 at 3.4 GPa) exceeds previously reported values at pressures higher than 1.0 GPa. IGN allotropes can be classified as either armchair or zigzag type, and these two types of IGNs exhibit different electronic properties. Zigzag-type IGNs are node-line semimetals, while armchair-type IGNs are either semiconductors or node-loop or node-line semimetals. Experimental synthesis of these IGN allotropes might be realized since their formation enthalpies relative to graphite are only 0.1--0.5 eV/atom (that of C₆₀ fullerene is about 0.4 eV/atom), and energetically feasible binary compound pathways are possible.