New 3D and 2D Octacarbon C8 and isoelectronic B4N4 having peculiar mechanic and magnetic properties. First-principles identifications.

Cohesive energies, energy-volume equations of states EOS, electron localization ELF maps, elastic constants, and band structures are reported for original octacarbon C8 three dimensional 3D and two-dimensional 2D chemical systems based on density functional theory calculations. Specifically, tetragonal C8 is identified cohesive with hardness close to experimentally identified cubic Ia-3 C8; both exhibiting comparable hardness to diamond. Also, isoelectronic and isostructural B4N4 is calculated with a slightly lower hardness due to the ionocovalent B-N bonding and a bandgap with the same magnitude as Diamond. 2D-C8 on the other side is proposed with interpenetrating two carbon hexagonal substructures, identified from energy calculations as stable in a ferromagnetic ground state. Critical pressure for the collapse of magnetization PC=12 GPa let assign a soft ferromagnetic behavior alike Invar alloys. Electronic band structures analyses identify specific bands differentiating magnetic carbon substructure (C1) from nonmagnetic semi-conducting honeycomb-like C26 layers. These observations let propose spin chemistry perspectives once such multilayered carbon 2D compounds are grown as stand-alone or on selected substrates as thin or thick films