Carbon clusters formed from shocked benzene

The study addresses how liquid benzene reacts under shock compression and what solid products form on nanosecond timescales. Understanding shock-driven organic chemistry is important for models of prebiotic synthesis, planetary processes, and materials synthesis, yet real-time observations of early reaction steps and product structures have been limited due to optical opacity and complex mixtures. Benzene, with its aromatic stability and π-stacked solid phases, undergoes phase transformations under high pressure; theory suggests polymerization to saturated networks and possible graphane-like phases at high P. Under shock loading, benzene exhibits a Hugoniot cusp near 13 GPa associated with reaction and significant volume decrease. Prior optical studies inferred opacity changes and possible carbon particle formation, but lacked in situ structural identification. The goal here is to resolve, in situ, the product morphology and crystal structure of shock-driven benzene reaction products using femtosecond x-ray free-electron laser (XFEL) techniques, and to relate them to transformation mechanisms and phase space of carbon.

Prior experimental shock studies of benzene reported a cusp on the Hugoniot near ~13–14 GPa with reaction over ~180 ns and ~12.5% volume decrease. Optical diagnostics observed irreversible opacity and color changes suggestive of carbon particle formation, and temperature measurements refined EOS under shock; quasi-isentropic loading suggested benzene remained liquid up to 13 GPa. Ultrafast (300 ps) shocks up to 18 GPa showed benzene largely unreactive. Theoretically, ReaxFF simulations indicated ring opening and formation of aliphatic chains, H2 formation, and increased C–C bonding at high P–T; ab initio simulations found dimerization pathways above 18 GPa leading toward polymerization. Static high-pressure studies predicted stability of graphane-like (hydrogenated graphene) polymorphs and a quasi-layered mixed sp2–sp3 phase (H18). Shock compression of graphite produces diamond/lonsdaleite at higher pressures, but the transformation pathways and product mixtures can differ from molecular precursors like benzene.

• Samples and setup: High-purity liquid benzene (ρ0 = 0.876 g/cm3) was loaded into a 100 µm-thick droplet cell between an Al/Au-coated z-cut sapphire ablator window and a [100] LiF rear window, sealed with a Teflon o-ring. Cartridge loading allowed multiple samples.
• Shock generation and timing: Laser-driven shock experiments were conducted at the MEC endstation of the LCLS. A 20 J, 5–20 ns Nd:Glass (λ = 527 nm) long-pulse laser with a 250 µm phase-plated, flat-top spot launched planar shocks via ablation of the coated sapphire. Experiments were timed so the x-ray probe intercepted the shock after it had traversed ~90% of the benzene layer, ensuring principal Hugoniot states. Timing jitter was 30–50 ps. VISAR measured particle velocity at the benzene–LiF interface; impedance matching to the LiF Hugoniot determined shock states. Input pressure variation at fixed laser power was <3%.
• Conditions: Five experiments at nominal input pressures of 27 ± 4 GPa and 55 ± 5 GPa. Reaction under these conditions is prompt (τ < 1 ns; k ~ 10^3 µs−1). Shock durations were 10–20 ns.
• X-ray probe: XFEL pulses of ~50 fs duration, ~11 keV (λ = 1.127 Å), ~25 µm spot, ~10^12 photons/pulse.
• Detectors: Seven CSPAD detectors arranged for wide-angle XRD and SAXS. XRD CSPADs at ~6 cm covered q ≈ 1.6–6.0 Å−1 with azimuthal coverage ~22° at 2 Å−1 to 70° at 5 Å−1. SAXS CSPAD arrays at 0.9 m and 2.5 m covered q ≈ 0.02–0.5 Å−1.
• Data processing: XRD images azimuthally integrated and calibrated using Dioptas; q = 4π sinθ/λ. Crystallite size estimates used the Scherrer equation L = 0.92 λ / FWHM. SAXS data were dark-corrected and static-background subtracted (empty ablator/LiF equivalent). SAXS fits used an empirical Guinier–Porod model under dilute conditions: I(Q) = G exp(−Q^2 Rg^2/3) for Q ≤ Q1 and I(Q) = B / Q^P for Q ≥ Q1, with continuity constraints relating Q1, B, P, Rg. For some cases, a log-normal distribution of Guinier–Porod contributions was used, with width correlated to mean per Brownian coagulation kinetics.
• Controls: Shocked LiF/sapphire assemblies without benzene showed none of the product peaks, attributing observed diffraction to benzene reaction products.
• Equation of state and temperature estimation: Equilibrium EOS for liquid benzene (SESAME framework; zero-T fit to shock data; thermal part via generalized Tarasov model) and for decomposition products (thermochemical modeling using Ross perturbation theory with exp-6 potentials and ideal mixing) were constructed to match Rankine–Hugoniot data in their domains. Experimental shock temperature at a given pressure was estimated as the mean of reactant and product equilibrium temperatures with uncertainty taken as half their difference, yielding conservative error bars.

• Rapid solid product formation: Liquid benzene transforms to solid products within the shock duration (10–20 ns), with reaction times τ < 1 ns under 27 ± 4 and 55 ± 5 GPa input pressures.
• Shock state temperatures: Estimated T ≈ 2790 ± 455 K at 27 ± 4 GPa and T ≈ 4940 ± 710 K at 55 ± 5 GPa (from EOS bracketing procedure).
• SAXS morphology:
  • At 55 GPa: Scattering displays a power-law dependence with Porod exponent P ≈ 4, indicating smooth-surfaced products with characteristic sizes exceeding the SAXS window (R > 15 nm). Minor Guinier–Porod contributions indicate features with Rg ≈ 5–15 Å (~0.5–1.5 nm). In some runs (e.g., run 303) the Guinier–Porod level did not improve fits significantly.
  • At 27 GPa: SAXS exhibits a feature near q ≈ 0.04 Å−1 consistent with a characteristic size ~4 nm. Modeling with a log-normal distribution of Guinier–Porod levels yields a mean Rg ≈ 50 Å (~5 nm), consistent with coagulation-limited growth.
• XRD structure:
  • Prominent low-q reflections at q ≈ 1.8–1.9 Å−1 (d ≈ 3.49–3.34 Å):
    • Run 237: single peak at q = 1.88 Å−1 (d = 3.34 Å).
    • Run 239: single peak at q = 1.80 Å−1 (d = 3.49 Å).
    • Run 303: two peaks at q = 1.81 Å−1 (d = 3.47 Å) and q = 1.88 Å−1 (d = 3.34 Å).
  • These peaks lie in the region of the graphite (002) interlayer reflection but indicate an expanded layer spacing relative to compressed graphite at similar P–T, consistent with hydrogenated layered structures (graphane/graphate-like).
  • Multiple higher-q peaks do not index to pure graphite or diamond; several are consistent with hydrogenated graphite (graphane/graphate polymorphs) and a mixed sp2–sp3 layered phase (H18). Some reflections overlap with cubic/hexagonal diamond positions but cannot be uniquely assigned.
• Product identity and bonding: Evidence indicates layered, sheet-like hydrogenated carbon/hydrocarbon structures and nanosized carbon clusters with mixed sp2–sp3 hybridization, rather than dominant diamond, methane, and hydrogen products.
• Mechanistic interpretation: Data support a pathway of benzene dimerization and polymerization to hydrogenated graphite-like sheets, followed by densification toward sp3-bonded, diamond-like structures. The observed variations and mixtures suggest kinetically trapped, non-equilibrium intermediates within the diamond stability region.
• Contrast with shocked graphite: Unlike graphite-to-diamond transitions (e.g., q ~ 2.2 Å−1 graphite (002) under 19 GPa), benzene products show expanded interlayer spacings (1.8–1.9 Å−1) and different transformation pathways, likely due to greater compression energy and temperature rise for molecular benzene at the same pressure.

The findings directly address the longstanding question of what solid products form when benzene is shock-compressed and on what timescales. In situ XFEL XRD and SAXS reveal that, within tens of nanoseconds, benzene does not primarily convert to diamond or revert to graphite; instead, it forms layered hydrogenated carbon structures and mixed sp2–sp3 bonded clusters. The low-q XRD peaks around 1.8–1.9 Å−1, expanded relative to graphite (002), and the higher-q reflections match hydrogenated graphite/graphate and H18-like motifs, consistent with polymerized, layered hydrocarbons transitioning toward sp3 bonding. SAXS shows smooth-surfaced larger clusters at higher pressure (R > 15 nm) and smaller 4–5 nm clusters at lower pressure, indicating pressure-dependent growth/coagulation dynamics. These observations imply kinetic trapping of non-equilibrium intermediates, influenced by the high temperatures and entropy rise along benzene's Hugoniot, which differ from solid carbon shock behavior. The results inform shock synthesis strategies for novel carbon/hydrocarbon materials and enhance understanding of carbon transport and phase transformations under planetary impact conditions, where similar P–T states occur.

This work provides the first in situ XFEL-based structural identification of shock-driven products from liquid benzene, showing rapid formation of crystalline solid products comprised of layered hydrogenated carbon sheets and mixed sp2–sp3 carbon clusters, rather than dominant diamond or graphite. The combined SAXS and XRD data support a mechanistic pathway of dimerization/polymerization to hydrogenated layered structures with subsequent densification toward sp3-rich, diamond-like material, with products reflecting kinetically trapped non-equilibrium states. These insights have implications for shock-driven synthesis and planetary carbon chemistry. Future research should perform systematic scans across a broader range of pressures, temperatures, and pulse durations to map product phase space, quantify kinetics and growth mechanisms, improve temperature determinations, and explore recovery strategies for ex situ characterization of transient phases.

• Product recovery from laser-driven shock experiments was not feasible, limiting ex situ validation and detailed microscopy of the transient phases.
• Diffraction peak overlap and texture prevented unambiguous indexing of some reflections; certain peaks could be consistent with multiple phases (e.g., diamond vs graphate/H18).
• Temperature estimates rely on equilibrium EOS bracketing of inherently non-equilibrium states, introducing uncertainty.
• Limited number of shock states (primarily ~27 and ~55 GPa) constrains mapping of the full product phase space.
• SAXS fitting assumes dilute conditions and uses empirical Guinier–Porod models; morphology inference is model-dependent.
• Measurement window is restricted to nanoseconds, potentially missing slower transformations beyond the shock duration.