Hypothetical superhard carbon metal
- PDF / 367,639 Bytes
- 4 Pages / 593.28 x 841.68 pts Page_size
- 88 Downloads / 203 Views
Hypothetical superhard carbon metal M. A. Tamor and K. C. Hass Research Staff, Ford Motor Company, Dearborn, Michigan 48121-2053
(Received 9 April 1990; accepted 18 May 1990) We describe a hypothetical new phase of carbon. The proposed structure is a rigid three-dimensional network with sp2 (threefold) coordination only. Tight-binding calculations indicate that the proposed material is a metastable metal significantly harder than diamond. Although it is unclear at present whether such a material can be produced in significant quantity, we speculate that it (and related structures) plays a role in the chemical vapor deposition of diamond films.
I. INTRODUCTION
In this paper we describe the structure of a hypothetical new phase of carbon and present theoretical estimates of several of its properties. The structure, which we call H-6, consists entirely of threefold coordinated carbon in a rigid three-dimensional lattice. Total energy calculations based on a semi-empirical tight binding model predict that H-6 carbon is a metastable metal with a bulk modulus larger than that of any known material, including diamond. Although purely hypothetical at present, H-6 carbon may represent an important missing link in the understanding of the chemical vapor deposition of diamond films.1 The H-6 lattice is shown in Fig. l(a). The proposed structure is comprised of layers of carbon chains joined by bonds along the c (vertical) axis. All carbon atoms are bonded to three other carbon atoms in the flat sp2 configuration. The orientation of the chains rotates by 60° about the c-axis from layer to layer, resulting in hexagonal (H) symmetry (group P6222). With an inchain repeat interval of two atoms and three chain layers, the H-6 structure is so named because of its six atom unit cell [Fig. l(b)]. Unlike other proposed all-sp2 carbon networks,2 the H-6 structure is topologically related to diamond in that the lattice can be continuously deformed to diamond without breaking or crossing any existing bonds. Figure l(c) shows a top view of the unit cell and indicates the atomic motions associated with the continuous conversion to diamond. Several properties of H-6 carbon can be derived from simple structural arguments. An assumption of graphitic bond lengths and angles yields a density of 3.4 g/cm3, only slightly less than that of diamond (3.5 g/cm3) and much larger than that of graphite (2.25 g/cm3). Unless bond alternation occurs, 7r-elecJ. Mater. Res., Vol. 5, No. 11, Nov 1990
http://journals.cambridge.org
Downloaded: 16 Mar 2015
trons will be delocalized along the chains and the material will be metallic. Given that the covalent sp2 bonds are shorter and stronger than sp3 carbon-carbon bonds of diamond, it is likely that H-6 carbon will be at least comparable in hardness to diamond. A simple empirical formula for the bulk moduli of covalent solids, discussed recently by Liu and Cohen,3 predicts that an isotropic solid composed of 1.42 A bonds should have a bulk modulus of approximately 5.8 Mbar. While this formula is strictly int
Data Loading...
