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Rhombohedral boron (Rh-alpha) 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 B-12{E-E-E} generic formulation. This paper was also a good opportunity to make a short review of rh-alpha boron interstitial compounds. Preliminary energy calculations within quantum density functional theory DFT show enhanced cohesion versus B-12 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 B-12{CN2} is proposed and studied for its electronic structure. After full unrestricted geometry optimization within B-12 space group R (3) over barm 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 angstrom) versus short ones in (ionic) calcium cyanamide CaCN2 (d(C-N) = 1.23 angstrom) explained by the bonding of N with one of the two B12 boron substructures forming a "3B center dot center dot center dot N-C-N center dot center dot center dot 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 mu(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, B-12{CN2} is stabilized due to its magnetic character leading to a strong chemical bonding visualized by the SP COOP. The present conceptual view of B-12 as a host of interstitials extends the family of compounds to potential monoand di-atomic insertions and should enhance research among the communities of solid state chemists and physicist to prepare new compounds with targeted properties.


Etourneau, Jean;  Matar, Samir F.

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