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. 2021 Sep 1;12(1):5202.
doi: 10.1038/s41467-021-25471-0.

Carbon clusters formed from shocked benzene

Affiliations

Carbon clusters formed from shocked benzene

D M Dattelbaum et al. Nat Commun. .

Abstract

Benzene (C6H6), while stable under ambient conditions, can become chemically reactive at high pressures and temperatures, such as under shock loading conditions. Here, we report in situ x-ray diffraction and small angle x-ray scattering measurements of liquid benzene shocked to 55 GPa, capturing the morphology and crystalline structure of the shock-driven reaction products at nanosecond timescales. The shock-driven chemical reactions in benzene observed using coherent XFEL x-rays were a complex mixture of products composed of carbon and hydrocarbon allotropes. In contrast to the conventional description of diamond, methane and hydrogen formation, our present results indicate that benzene's shock-driven reaction products consist of layered sheet-like hydrocarbon structures and nanosized carbon clusters with mixed sp2-sp3 hybridized bonding. Implications of these findings range from guiding shock synthesis of novel compounds to the fundamentals of carbon transport in planetary physics.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Experimental configuration of laser-driven shock compression experiments on benzene.
A 100 μm-thick droplet of benzene was placed between an ablator window of coated z-cut sapphire and a rear single crystal [100] LiF window and held in place with a Teflon o-ring. XRD and SAXS were collected using a series of seven detectors place at a range of angles and distances from the target (see the “Methods” section). For clarity, not all of the seven detectors are shown. Shock states in liquid benzene from several literature sources (Dick, Nellis, Lysne, and Dattelbaum) obtained using traditional gas gun-driven plate impact techniques are shown in the pressure–volume plane in the plot. A “cusp” or deviation in the Hugoniot in the PV plane is observed at 13.8 GPa and is purported to be due to a shock-driven chemical reaction with a volume change of −12.5%.
Fig. 2
Fig. 2. SAXS profiles from benzene products formed within 20 ns of shock input.
The SAXS profiles correspond to input conditions of P = 55 ± 5 GPa, T = 4940 ± 710 K (panel A) and P = 27 ± 4 GPa, T = 2790 ± 455 K (panel B). A SAXS data (symbols) after subtraction of the static intensity (representative static measurement shown in gray): run 239 (red), run 303 (green), and run 237 (blue). Runs 237 and 303 are offset vertically for clarity. Thick solid lines are fits primarily consisting of a power-law (P1, thin solid lines) representing larger length scales and additional contributions from a Guinier–Porod level (G2, Rg2, P2, dashed lines) corresponding to a ~1 nm length scale, and a constant background (dotted lines). In the case of run 303, the addition of the Guinier–Porod level did not significantly improve the fit. B SAXS data (purple symbols) for lower pressure measurement (run 292), a 2-level model (purple lines), and intensity from Guinier-Porod levels with a log-normal Rg distribution (red line). The inset shows the modeled log-normal distribution of product sizes, with a mean Rg of 50 Å, and shaded ranges associated with the parameter errors.
Fig. 3
Fig. 3. X-ray diffraction data is shown for benzene before and after shockwave compression.
Static (A) and dynamic (B) XRD patterns were collected on CSPAD detectors located x = 6 cm from the benzene sample. (color map corresponds to recorded intensity) Diffraction lines from polycrystalline Au coating are observed in the static image. C Upon shockwave compression to 55 GPa, diffraction lines are observed from crystalline products formed from shocked benzene. The integrated pattern (in arbitrary units) in (C) shows diffraction peaks at this condition (P = 55 ± 5 GPa, T = 4940 ± 710 K). The color scale is relative intensity and is arbitrary to the CSPAD detector signal.
Fig. 4
Fig. 4. Diffraction patterns (in arbitrary units) of solid products formed from shocked benzene at 55 GPa, 4940 K.
A Expanded region between d = 2.5 and 3.8 Å shows the measured low-angle peaks (solid lines) and fits (dashed lines) relative to the estimated range of d-spacings for graphitic layered carbon structures at the same PT conditions (gray shaded region) and the range of predicted d-spacings for graphate I–IV and H18 at 50 GPa and ambient T (red shaded region). B Full range of the measured diffraction exhibiting multiple peaks that do not index to graphite or diamond forms, particularly in the regions of 2.4–2.8 and 3.4–3.6 Å−1. Dotted lines correspond to estimated graphite reflections, dashed lines to cubic diamond reflections at 55 GPa and 5000 K. Calculated diffraction patterns for graphate-I, and -II from Wen et al. , and H18 calculated at 50 GPa and ambient T are shown for comparison.
Fig. 5
Fig. 5. The products of shock-driven benzene on the principal Hugoniot form from dimerization and polymerization, followed by condensation into clusters with the sheet-like structure of sp2sp3 character.
A Carbon phase diagram showing diamond, graphite and liquid regions from several works (see legend)–,,,. A region of transition from sp2–sp3 hybridized allotropes is proposed to occur in the diamond region by Blank et al. . P–T states along the principal reactants and products Hugoniots, and the calculated P–T states from shocked benzene (this work) are overlaid on the phase diagram (errors as reported in Table 1 and Supplementary Note 1). B Schematic of the proposed mechanism of cluster formation from benzene. Shocked liquid benzene undergoes dimerization and polymerization addition reactions to form clusters with disordered, hydrogenated layered carbon structures with sp2–sp3 character. C Transmission electron micrograph of recovered carbon from PBX 9502, and explosive that samples a similar P–T condition to the experiments described here, showing the transition from layered-sp2 to sp3-diamond-like structures within the structure. Recovery of products from the laser-driven shock compression experiments of benzene was not feasible.

References

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