Structure, Stability and Properties of Covalent C 34 , C 20 , and C 22 Crystals
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ABSTRACT Topological arguments indicate that an infinite number of covalent carbon crystals may exist with either diamond-like (sp 3), graphitic-like (sp 2)- or mixed (sp3/sp 2 ) bonding structure. We investigate the structural, elastic and electronic properties of three prototypical structure: C34 , C20 and C22 , respectively. All of them form a face-centered cubic lattice. Their properties have Montecarlo and molecular dynamics simulations based on the Tersoff been calculated by both potential. Both the sp3 -bonded C34 and the sp 2-bonded C20 are found to have a cohesive energy per atom very close to that of diamond. A comparison of elastic and electronic properties to those ones of diamond and graphite are also presented and discussed.
INTRODUCTION Atomic-scale porous materials, molecular sieves and intercalation compounds constitute one of the prominent research areas in modern materials science [1]. Good mechanical properties, chemical inertness and easy interfacing are often demanded for solid membranes and the functional components of ionic devices. All of these properties could in principle be met by carbon-based materials. We undertook a theoretical search for novel fiiily covalent carbon crystals, in either the sp 3 or sp2 or mixed sp3/sp 2 bond configurations, which combine a large nanoscale porosity with high stability and good mechanical properties. In this brief report we describe a few basic examples of such hypothetical carbon crystals with a theoretical calculation of their stability, their elastic constants and electronic structure.
THE HOLLOW DIAMOND fcc-C 34 With the aid of topological arguments we discovered three new infinite series of 4-fold coordinated (sp3 bonded) lattices having Nn(n+1)(2n+ 1)/3 carbon atoms per unit cell, where n is any natural number and N = 17 (face centered cubic lattices, Fd3m space group), 20 (hexagonal lattices, P6/mmm space group) and 23 (simple cubic lattices, Pm3n space group). They are formed by periodic arrays of fullerenic cavities so that we call them hollow diamonds (HDs).[2]In the first elements (n = 1) the fullerenic cavities are coalescing so as to form a periodic space filling with two or three kinds of polyhedra. The smallest HD, fcc-C 34 , is generated by two C28 and four C20 polyhedra per unit cell; hex-C 40 is generated by two C26 , two C24 and three C20 polyhedra per unit cell; sc-C 46 is generated by six C24 and two C20 polyhedra per unit cell. As regards nanoporosity the cavities of fcc-C 34 form a sort of tetrahedral zeolitic network; those of hex-C 40 form a hexagonal array of parallel tubes (made of C24 cages) along the c-axis passing through an orthogonal non-intersectingstack of planar hexagonal labyrinths (made of C26 cages); those of sc-C 46 form three orthogonal non-intersecting square arrays of tubes. The larger 157 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
elements (n >_ 2) of each series are formed by the same arrays of fullerenes covalently interconnected by diamond-like structure. The three series tend
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