Polymorphic phases of sp3-hybridized carbon under cold compression
1Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
Journal of the American Chemical Society
|April 12, 2012
Summary
Researchers discovered new superhard carbon structures by computationally exploring cold-compressed graphite. These novel sp(3)-hybridized carbon allotropes exhibit high hardness and optical transparency, potentially explaining experimental findings.
Area of Science:
- Materials Science
- Solid State Physics
- Computational Chemistry
Background:
- Graphite transforms into superhard carbons under cold compression.
- The precise structure of these superhard carbons remains experimentally undetermined.
Purpose of the Study:
- To perform an extensive structural search for high-pressure crystalline phases of carbon.
- To classify and characterize newly predicted sp(3)-hybridized carbon structures.
Main Methods:
- Utilized an evolutionary algorithm for unbiased structural search.
- Employed density-functional theory calculations for energy and property analysis.
- Simulated X-ray diffraction (XRD) spectra for comparison with experimental data.
Main Results:
- Identified nine low-energy, sp(3)-hybridized carbon polymorphic structures.
- Classified structures based on stacking arrangements of buckled graphene layers.
- Seven new structures are more stable than Cco-C(8) (Z-carbon) between 0-25 GPa.
- Several predicted structures show elastic moduli and hardness comparable to cubic diamond.
- Z-carbon-4 exhibits the highest predicted hardness (93.4).
- Most structures are optically transparent.
- Simulated XRD spectra align with experimental data from cold-compressed graphite.
Conclusions:
- The classification scheme provides a design strategy for novel sp(3)-hybridized carbon allotropes.
- Newly predicted structures are energetically favorable and possess desirable mechanical properties.
- Experimental samples of cold-compressed graphite may contain multiple polymorphic phases of sp(3)-hybridized carbon.
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