Superhard sp²-sp³ hybridized B₂C₃N₂ with 2D metallicity

Ternary boron-carbon-nitrogen (B-C-N) compounds are considered to possess hardness comparable to diamond and thermal stability comparable to c-BN. Explorations for desirable B-C-N phases have been continuous. However, the nonconductive properties of most B-C-N compounds narrow the applications of these compounds. Herein, we propose a sp2-sp3 hybridized phase of t-B2C3N2, which consists of diamond-like BC blocks connected with single N-N bonds. Elastic constants and phonon dispersion curves confirm that t-B2C3N2 is mechanically and dynamically stable. The structure processes 2D metallicity in a strong 3D network. Furthermore, hardness and electron-phonon calculations reveal that t-B2C3N2 is superhard and superconductive with a superconducting critical temperature reaching 2.3 K.