rh.-B12 as host of interstitial atoms: Review of a large family with illustrative study of B12{CN2} from first-principles
Résumé
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 Rm 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 (dC-N = 1.38 Å) versus short ones in (ionic) calcium cyanamide CaCN2 (dC-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 μB 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.