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The speculation of superdiamonds
Chien-Min J Sung

Diamond has the highest bond energy per unit volume of all known materials, and hence it is assumed to possess the highest hardness. The hardness of diamond comes from its small atoms that form four covalent bonds. To make a structure harder than diamond, its atoms must be smaller than carbon, and/or these atoms must form at least four covalent bonds. The first consideration would be to strike off all elements with period number higher than 2. The second criterion would be to eliminate all elements lighter than carbon. Hence, only carbon, nitrogen, oxygen, fluorine, and neon are possible candidates for superdiamond. However, in order to become a superdiamond, these elements must form monatomic structures with coordination number higher than 4. Moreover, no lone pair electrons are allowed, so all their valence electrons must be involved in single covalent bonds.

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