Carbon exhibits a wide variety of behaviors that depend on the surrounding environment and processing techniques, reflecting different kinds of chemical bond nature. Carbon forms have been calssified by the type of sp hybridization, sp1, sp2 and sp3. Recently, several metastable forms were synthesized experimentally and also a few hypothetical forms were proposed on the base of theoretical calculation. These carbon forms represent some modified sp2 and/or spa bonds. This suggests that

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... ion of carbon atom is allowed to extend from those defined by an discrete integer and geometric symmetry to those by fraction and asymmetry.