Monatomic structures of B, C, N, and O: first-principle study of relative stabilities and bulk moduli

Relative stabilities and bulk moduli of monatomic structures with coordinat ion numbers 4 (cubic diamond or cd), 6 (simple cubic or sc), 8 (body-center ed cubic or bcc), and 12 (face-centered cubic or fee) for elements B, C, N, and O were calculated based on the first-principles. The structure for eac h element may be stabilized by the geometrical matching between the coordin ated positions and the orbital shape. Thus, all elements favor loosely pack ed cd structure at low pressures, and they prefer moderately packed se stru cture at high pressures. However, denser phases with bee and fee structures may become more stable at much higher pressures. B has only three valence electrons that are insufficient to fill up all sp( 3) orbitals in a cd structure. Moreover, these three p-electrons can only f ill half of the three mutually perpendicular bonds in sc structure. As a re sult of reduced stabilities of cd and se structures, relative stabilities o f the four structures for B are less distinctive. C has four valence electrons that match perfectly with four tetrahedral coo rdinated atoms. Hence, its cd structure is greatly stabilized by the extens ive overlap of bonded electrons. As a result of this expanded region of sta bility, the equilibrium pressure between cd and sc structures is pushed up substantially. Both N and O have sufficient p-electrons to align with octahedrally coordin ated atoms. Hence, their sc structures are greatly stabilized and the equil ibrium pressures between cd and sc structures for N and O are much reduced relative to that for B and C. The electron density maps of cd coordination for N and O suggest that these monatomic structures may be isotropic superc onductors, Bulk modulus of a symmetric structure may be determined by the average conc entration of electrons in overlapped orbitals. C has the smallest atomic vo lume of the four elements studied, so it is uniquely capable to form the cd structure with the highest bulk modulus (447 GPa) of all structures. Altho ugh C's p-orbitals are not fully occupied for effective bonding in octahedr al coordination, its small atoms still make the bulk modulus of the sc stru cture the second highest (337 GPa) among various phases of the four element s studied. Except for diamond, the structures discussed above are all hypothetical. Ho wever, they may be synthesized by the bombardment of collimated beams of at oms focussed to a common center. Moreover, the structures so formed, even t hough they may be metastable, can be studied by the laser spectroscope fire d in femtosecond (10(-15) s) bursts. Such femtochemistry has been developed by Dr. Ahmed Zewail, the latest Nobel laureate. (C) 2001 Elsevier Science B.V. All rights reserved.