Graphane Crystals Predicted

Graphanes are sheets of molecular carbon saturated  by hydrogen, a material that is related to graphene, which consists of pure carbon sheets and the subject of the 2010 Nobel Prize in Physics. Recent calculations by scientists from Cornell University, SUNY Stony Brook, and Moscow State University have led to the discovery of new ways in which types of graphane can be compacted in crystals. The scientists became interested in graphane while calculating the high-pressure behavior of benzene (C6H6).  At a critical pressure, benzene underwent a phase transformation, to saturated CH structures that were more stable. The researchers eventually identified five different graphane stackings at pressures up to 300 GPa.

While three-dimensional graphanes remain insulating or semiconducting at these pressures, there is a remarkable inversion in stability of the five crystals studied.  Two stacking 'polytypes' that are not the most stable at ambient pressure (one based on unusual chair cyclohexane net, the other on a boat) are significantly stabilized under increasing pressure relative to stackings of simple chair sheets.  The researchers suggest that the explanation may lie in the balance on intra- and inter-sheet contacts in the extended arrays of carbon atoms.  

[1] Wen, X.D., L. Hand, V. Labet, T. Yang, R. Hoffmann, N.W. Ashcroft, A. Oganov, and A. Lyakhov,Graphane sheets and crystals under pressureProc. Natl. Acad. Sci. 108, 6833-6837 (2011).  


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