The design of exotic superhard materials

Diamond has the highest bond energy per unit volume of all known materials, and hence it is assumed to possess the highest hardness. Diamond's hardnes s comes from its small atoms that each of them forms four covalent bonds. T o make a structure harder than diamond, its atoms must be smaller than carb on, and/or these atoms form at least four covalent bonds. The first conside ration would rule out all elements with period number higher than 2. The se cond criterion would eliminate all elements lighter than carbon. Hence, onl y C, N, O, F, and Ne are possible candidates of superdiamond. However, in o rder to beat diamond in hardness, these elements must form mono-atomic stru ctures with coordination number higher than 4. Moreover, no lone pair elect rons are allowed, so all of their valence electrons must be used to form si ngle covalent bonds. The number of valence electrons in simple cubic carbon is less than the coo rdination number of 6. As a result, the bonds may turn metallic, so it is u nlikely harder than diamond. Potential superdiamond structures include diam ond-like nitrogen, simple cubic oxygen or fluorine, and body-centered cubic (BCC) neon. If these elements can form single covalent bonds that involve all their valence electrons, they could become superdiamond. Otherwise, dia mond's hardness for materials may be as insurmountable as speed of light fo r moving objects. The above hypothetical structures of superdiamond may be synthesized by aim ing collimated beams. of single ions from specific directions at a common c enter. Such a technique was developed by Nobel Laureate YT. Lee decades ago . The possible instantaneous formation of the predicted hypothetical struct ures, even though they are metastable, may be studied in situ at real times by laser strobe light flashed at femtoseconds (10(-15) S). Such femtochemi stry has already been invented by Dr. Ahmed Zewail, the latest Nobel Laurea te of chemistry. (C) 2001 Elsevier Science B V. All rights reserved.