Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
J Am Chem Soc. 2012 May 2;134(17):7530-8. doi: 10.1021/ja301582d. Epub 2012 Apr 10.
It is well established that graphite can be transformed into superhard carbons under cold compression (Mao et al. Science 2003, 302, 425). However, structure of the superhard carbon is yet to be determined experimentally. We have performed an extensive structural search for the high-pressure crystalline phases of carbon using the evolutionary algorithm. Nine low-energy polymorphic structures of sp(3)-hybridized carbon result from the unbiased search. These new polymorphic carbon structures together with previously reported low-energy sp(3)-hybridized carbon structures (e.g., M-carbon, W-carbon, and Cco-C(8) or Z-carbon) can be classified into three groups on the basis of different ways of stacking two (or more) out of five (A-E) types of buckled graphene layers. Such a classification scheme points out a simple way to construct a variety of sp(3)-hybridized carbon allotropes via stacking buckled graphene layers in different combinations of the A-E types by design. Density-functional theory calculations indicate that, among the nine low-energy crystalline structures, seven are energetically more favorable than the previously reported most stable crystalline structure (i.e., Cco-C(8) or Z-carbon) in the pressure range 0-25 GPa. Moreover, several newly predicted polymorphic sp(3)-hybridized carbon structures possess elastic moduli and hardness close to those of the cubic diamond. In particular, Z-carbon-4 possesses the highest hardness (93.4) among all the low-energy sp(3)-hybridized carbon structures predicted today. The calculated electronic structures suggest that most polymorphic carbon structures are optically transparent. The simulated X-ray diffraction (XRD) spectra of a few polymorphic structures are in good agreement with the experimental spectrum, suggesting that samples from the cold-compressed graphite experiments may consist of multiple polymorphic phases of sp(3)-hybridized carbon.
人们已经充分证实，石墨在冷压缩条件下可以转化为超硬碳（Mao 等人，《科学》，2003，302，425）。然而，超硬碳的结构仍有待实验确定。我们使用进化算法对高压下碳的晶相结构进行了广泛的搜索。无偏搜索得到了 9 种低能 sp3 杂化碳的多晶型结构。这些新的多晶型碳结构与之前报道的低能 sp3 杂化碳结构（如 M-碳、W-碳和 Cco-C8 或 Z-碳）一起，可以根据两种（或更多种）五个（A-E）类型的褶皱石墨烯层中堆叠的不同方式分为三组。这种分类方案指出了一种通过设计以不同组合堆叠褶皱石墨烯层来构建各种 sp3 杂化碳同素异形体的简单方法。密度泛函理论计算表明，在这 9 种低能晶态结构中，有 7 种在 0-25 GPa 的压力范围内比之前报道的最稳定的晶态结构（即 Cco-C8 或 Z-碳）更具能量优势。此外，几种新预测的多晶 sp3 杂化碳结构具有接近立方金刚石的弹性模量和硬度。特别是，Z-碳-4 具有所有预测的低能 sp3 杂化碳结构中最高的硬度（93.4）。计算的电子结构表明，大多数多晶型碳结构是光学透明的。几种多晶型结构的模拟 X 射线衍射（XRD）谱与实验谱吻合较好，表明冷压石墨实验中的样品可能由多种 sp3 杂化碳的多晶相组成。
