Abstract
Four sp2–sp3 hybrid carbon allotropes are proposed on the basis of first principles calculations. These four carbon allotropes are energetically more favorable than graphite under suitable pressure conditions. They can be assembled from graphite through intralayer wrinkling and interlayer buckling, which is similar to the formation of diamond from graphite. For one of the sp2–sp3 hybrid carbon allotropes, mC24, the electron diffraction patterns match these of i-carbon, which is synthesized from shock-compressed graphite (H. Hirai and K. Kondo, Science, 1991, 253, 772). The allotropes exhibit tunable electronic characteristics from metallic to semiconductive with band gaps comparable to those of silicon allotropes. They are all superhard materials with Vickers hardness values comparable to that of cubic BN. The sp2–sp3 hybrid carbon allotroes are promising materials for photovoltaic electronic devices, and abrasive and grinding tools.
Highlights
Carbon adopts a wide range of allotropes, such as graphite, diamond, fullerene, nanotubes, graphdiyne, and amorphous carbon, because of its ability to form sp, sp2, and sp3-hybridized bonds
Four sp2–sp[3] hybrid carbon allotropes are proposed on the basis of first principles calculations
For one of the sp2–sp[3] hybrid carbon allotropes, mC24, the electron diffraction patterns match these of i-carbon, which is synthesized from shock-compressed graphite
Summary
Carbon adopts a wide range of allotropes, such as graphite, diamond, fullerene, nanotubes, graphdiyne, and amorphous carbon, because of its ability to form sp-, sp2-, and sp3-hybridized bonds. Studies have investigated the nature of the conversion mechanisms between different allotropes under pressure among various carbon configurations with different hybridizations, such as from sp[2] to sp[3]. Four sp2–sp[3] hybrid carbon allotropes are proposed on the basis of first principles calculations. The four sp2–sp[3] hybrid carbon allotropes are dynamically stable and energetically more favorable than graphite under appropriate pressures. The transition mechanism from graphite to a sp2–sp[3] hybrid allotrope involves the intralayer wrinkling and interlayer buckling of the graphite sheets; this is similar to the formation of diamond from graphite and would explain the coproduction of i-carbon and diamond. Sp2–sp[3] hybrid carbon allotropes show superhardness, and high Young’s, bulk, and shear moduli
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