Superdense carbon allotropes

Superdense carbon allotropes are proposed configurations of carbon atoms that result in a stable material with a higher density than diamond.

Few hypothetical carbon allotropes denser than diamond are known. All these allotropes can be divided at two groups: the first are hypothetically stable at ambient conditions; the second are high-pressure carbon allotropes which become quasi-stable only at high pressure. According to the SACADA[1] database, the first group comprises the structures, called hP3,[2] tI12,[2] st12,[3] r8,[4] I41/a,[4] P41212,[4] m32,[5] m32*,[5] t32,[5] t32*,[5] H-carbon[6] and uni.[7] Among them st12 carbon was proposed as far as 1987 in the work of R. Biswas et al.[3]

MP8,[8] OP8,[8] SC4,[9] BC-8[10] and (9,0)[11] carbon allotropes belong to the second group - they are hypothetically quasi-stable at the high pressure. BC-8 carbon is not only a superdense allotrope but also one of the oldest hypothetical carbon structures - initially it was proposed in 1984 in the work R. Biswas et al.[10] The MP8 structure proposed in the work J. Sun et al.,[8] is almost two times denser than diamond - its density is as high as 7.06 g/cm3 and it is the highest value reported so far.

Band gaps

All hypothetical superdense carbon allotropes have dissimilar band gaps compared to the others. For example, SC4[9] is supposed to be a metallic allotrope while st12, m32, m32*, t32, t32* have band gaps larger that 5.0 eV.[5][3]

Carbon tetrahedra

These new materials would have structures based on carbon tetrahedra, and represent the densest of such structures. On the opposite end of the density spectrum is a recently theorized tetrahedral structure called T-carbon. This is obtained by replacing carbon atoms in diamond with carbon tetrahedra. In contrast to superdense allotropes, T-carbon would have very low density and hardness.[12][13]

References
  1. Hoffmann, R.; Kabanov, A.; Golov, A.; Proserpio, D. (2016). "Homo Citans and Carbon Allotropes: For an Ethics of Citation". Angewandte Chemie. 55 (37): 10962–10976. doi:10.1002/anie.201600655. PMC 5113780. PMID 27438532.
  2. Zhu, Qiang; Oganov, Artem; Salvadó, Miguel; Pertierra, Pilar; Lyakhov, Andriy (2011). "Denser than diamond: Ab initio search for superdense carbon allotropes". Physical Review B. 83 (19): 193410. Bibcode:2011PhRvB..83s3410Z. doi:10.1103/PhysRevB.83.193410.
  3. Biswas, R.; Martin, R. M.; Needs, R. J.; Nielsen, O.H. (1987). "Stability and electronic properties of complex structures of silicon and carbon under pressure: Density-functional calculations". Physical Review B. 35 (18): 9559–9568. Bibcode:1987PhRvB..35.9559B. doi:10.1103/PhysRevB.35.9559. PMID 9941381.
  4. Mujica, A.; Pickard, C. J.; Needs, R. J. (2015). "Low-energy tetrahedral polymorphs of carbon, silicon, and germanium". Physical Review B. 91 (21): 214104. arXiv:1508.02631. Bibcode:2015PhRvB..91u4104M. doi:10.1103/PhysRevB.91.214104. S2CID 59060371.
  5. Li, Z.-Z.; Wang, J.-T.; Xu, L.-F.; Chen, C. (2016). "Ab initio prediction of superdense tetragonal and monoclinic polymorphs of carbon". Physical Review B. 94 (17): 174102. Bibcode:2016PhRvB..94q4102L. doi:10.1103/PhysRevB.94.174102.
  6. Strong, R. T.; Pickard, C. J.; Milman, V.; Thimm, G.; Winkler, B. (2004). "Systematic prediction of crystal structures: An application to sp3-hybridized carbon polymorphs". Physical Review B. 70 (4): 045101. Bibcode:2004PhRvB..70d5101S. doi:10.1103/PhysRevB.70.045101.
  7. Ohrstrom, L.; O’Keeffe, M. (2013). "Network topology approach to new allotropes of the group 14 elements". Z. Kristallogr. 228 (7): 343–346. doi:10.1524/zkri.2013.1620. S2CID 16881825.
  8. Sun, J.; Klug, D. D.; Martoňák, R. (2009). "Structural transformations in carbon under extreme pressure: Beyond diamond". The Journal of Chemical Physics. 130 (19): 194512. Bibcode:2009JChPh.130s4512S. doi:10.1063/1.3139060. PMID 19466848.
  9. Scandolo, S.; Chiarotti, G. L.; Tosatti, E. (1996). "SC4: A metallic phase of carbon at terapascal pressures". Physical Review B. 53 (9): 5051–5054. Bibcode:1996PhRvB..53.5051S. doi:10.1103/PhysRevB.53.5051. PMID 9984087.
  10. Biswas, R.; Martin, R. M.; Needs, R. J.; Nielsen, O.H. (1984). "Complex tetrahedral structures of silicon and carbon under pressure". Physical Review B. 30 (6): 3210. Bibcode:1984PhRvB..30.3210B. doi:10.1103/PhysRevB.30.3210.
  11. Ning, X.; Li, J.-F.; Huang, B.-L.; Wang, B.-L. (2015). "Low-temperature phase transformation from nanotube to sp3 superhard carbon phase". Chinese Physics B. 24 (6): 066102. Bibcode:2015ChPhB..24f6102X. doi:10.1088/1674-1056/24/6/066102.
  12. Sheng, Xian-Lei; Yan, Qing-Bo; Ye, Fei; Zheng, Qing-Rong; Su, Gang (2011). "T-Carbon: A Novel Carbon Allotrope". Physical Review Letters. 106 (15): 155703. arXiv:1105.0977. Bibcode:2011PhRvL.106o5703S. doi:10.1103/PhysRevLett.106.155703. PMID 21568576. S2CID 22068905.
  13. "New carbon allotrope could have a variety of applications". Phys.Org. April 22, 2011. Retrieved 2011-06-10.
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