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Role of graphite crystal structure on the shock-induced formation of cubic and hexagonal diamond

Since cubic diamond was first recovered from explosively shocked graphite samples in 1961, the shock-induced graphite to diamond phase transformation has been of great scientific and technological interest. Recent real-time x-ray diffraction results on different types of pyrolytic graphite under sho...

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Bibliographic Details
Published in:Physical review. B 2020-06, Vol.101 (22), p.1, Article 224109
Main Authors: Volz, Travis J., Turneaure, Stefan J., Sharma, Surinder M., Gupta, Y. M.
Format: Article
Language:English
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Summary:Since cubic diamond was first recovered from explosively shocked graphite samples in 1961, the shock-induced graphite to diamond phase transformation has been of great scientific and technological interest. Recent real-time x-ray diffraction results on different types of pyrolytic graphite under shock compression have reported hexagonal diamond and cubic diamond formation at comparable stresses. To resolve and understand these differences, synchrotron x-ray diffraction measurements were used to examine, in real time, the plate impact shock response of two grades of highly oriented pyrolytic graphite and as-deposited pyrolytic graphite-at stresses below and above their respective phase transformation stresses. The present results show that at their respective transformation stresses, crystallites in as-deposited pyrolytic graphite are compressed ∼ 30 % more along the c axis than crystallites in both highly oriented pyrolytic graphite types. This work establishes that the high-pressure phase of even ZYH-grade highly oriented pyrolytic graphite (a less oriented variety with mosaic spread 3 . 5 ∘ ± 1 . 5 ∘ ), at ∼ 50 GPa, is hexagonal diamond. In contrast, the high-pressure phase of as-deposited pyrolytic graphite (mosaic spread ∼ 45 ∘ ) in the present work, at ∼ 60 GPa, is cubic diamond. Analysis of ambient x-ray diffraction data demonstrates that the crystallites in the highly oriented pyrolytic graphite samples have the hexagonal graphite crystal structure with three-dimensional long-range order. In contrast, the crystallites in the as-deposited pyrolytic graphite samples have a turbostratic carbon crystal structure which lacks rotational/translational order between parallel adjacent graphene layers. The ambient results suggest that the observed high-pressure crystal structure of shocked graphite depends strongly on the initial crystal structure-shock compression along the c axis of hexagonal graphite (in highly oriented pyrolytic graphite) results in highly textured hexagonal diamond and shock compression of turbostratic carbon (in as-deposited pyrolytic graphite) results in nanograined cubic diamond. The present results reconcile previous disparate findings, establish the definitive role of the initial crystal structure, and provide a benchmark for theoretical simulations.
ISSN:2469-9950
2469-9969
DOI:10.1103/PhysRevB.101.224109