rh.-B12 as host of interstitial atoms: Review of a large family with illustrative study of B12{CN2} from first-principles

Rhombohedral boron (Rh-α) considered as a matrix hosting triatomic linear interstitial elements (E) of the first period (B,C,N,O) and elements of the second period as (Si, P, S) as well as the fourth period (As), generates a relatively large family of solid state chemical systems with B₁₂{E-E-E} generic formulation. This paper was also a good opportunity to make a short review of rh-α boron interstitial compounds. Preliminary energy calculations within quantum density functional theory DFT show enhanced cohesion versus B₁₂ matrix structure upon embedding the {E-E-E} providing compounds with particular physical and chemical properties. Focusing exemplarily on linear {N–C–N} cyanamide known to combine with gallium arsenide giving GaAs:CN₂, as well as in forming calcium cyanamide CaCN₂, the sub carbonitride B₁₂{CN₂} is proposed and studied for its electronic structure. After full unrestricted geometry optimization within B₁₂ space group R$\bar{3}$m and subsequent discussion of the cohesive energies and the energy related properties, details are provided for original electronic and magnetic structures. Particularly we show an elongated N–C–N (d_C-N = 1.38 Å) versus short ones in (ionic) calcium cyanamide CaCN₂ (d_C-N = 1.23 Å) explained by the bonding of N with one of the two B₁₂ boron substructures forming a “3B⋯N–C–N⋯3B “-like complex illustrated by charge density and electron localization function (ELF) and computed from the overlap population (COOP). From energy-volume equation of state EOS in non spin-polarized NSP and spin polarized SP configurations the latter is found to be the ground state one, with a magnetic moment of 2 μ_B carried by central carbon and forming a torus like magnetic charge density. Site and spin projected electronic density of states DOS exhibit a small gap insulator. Furthermore, B₁₂{CN₂} is stabilized due to its magnetic character leading to a strong chemical bonding visualized by the SP COOP. The present conceptual view of B₁₂ as a host of interstitials extends the family of compounds to potential mono- and di-atomic insertions and should enhance research among the communities of solid state chemists and physicist to prepare new compounds with targeted properties.