Single-crystal X-ray diffraction reveals the crystal structure of a conductive hydrocarbon

Advances in high-pressure experimental techniques in recent years have spurred significant interest in the behaviour of light elements under ultra-high pressures. This interest intensified in 2015, following the report of superconductivity in H₃S with a transition temperature (T_c) of 203 K at 200 GPa [1].

Alongside hydrogen-rich systems, carbon-rich materials are considered promising candidates for high-T_c superconductivity, as they can support high-frequency phonon modes and exhibit strong electron-phonon coupling. A recent theoretical study proposed that benzene (C₆H₆) – a hydrogen and carbon “alloy” – may transition into a metallic state within the narrow pressure range of 180–200 GPa, driven by band gap reduction resulting from enhanced electron-phonon interaction [2].

In practice, however, probing organic crystals at such high pressures remains technically challenging due to the intrinsic fragility of organic molecules. Despite extensive experimental efforts to observe metallic behaviour in polycyclic aromatic hydrocarbons (PAHs), most studies have instead reported irreversible chemical transformations at pressures significantly below those theoretically required to induce a metallic state.   

This study combined chemical and physical compression to tune the electronic properties of PAH molecules at more accessible pressures. Increasing the number of π-electrons – a strategy known as chemical pre-compression – reduces the band gap at ambient pressure. For example, while the band gap energy of benzene (C₆H₆) is 6 eV at ambient conditions, that of C₄₈H₂₀, formed by fusing two C₂₄H₁₂ molecules, is reduced to 2.2 eV (Figure 1).
 

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Fig. 1: Optical band gap energy derived from the absorption spectrum for four PAH molecules, plotted against the number of π-electrons. The green line serves as a visual guide. 

A single crystal of C₄₈H₂₀ was synthetized and subjected to physical compression using a diamond anvil cell (DAC), reducing intermolecular distances, enhancing intermolecular interaction, and further narrowing the band gap. A continuous closing of the band gap energy from 2.2 eV down to 0.7 eV was observed at 33.6 GPa, accompanied by a change in the optical colour of the crystal from red to black.

Electrical resistance measurements as a function of pressure and temperature suggest that a semi-metallic character emerges in the pressure range 23.0–38.0 GPa. High-pressure Raman spectroscopy measurements indicated a possible structural transition above 23.0 GPa, while high-pressure crystallographic data provided a complete structural view. To date, no other single-crystal data on PAHs have provided such detailed insight into pressure-induced structural and electronic changes.  

A single-crystal of C₄₈H₂₀ was loaded into a specially designed DAC for single-crystal X-ray diffraction at ID15B. Using a monochromatic X-ray beam - focused to approximately 1.0 × 1.0 µm², data were collected while rotating the detector in 0.5° angular steps over a -36° to 36° range. Full structural solutions and refinements were achieved at pressures of 3.3, 10.3, 19.2 and 26.2 GPa (Figure 2), all revealing a monoclinic structure with space group P2₁/c. At 26.2 GPa, the unit cell volume had decreased by 20% compared to ambient pressure. 

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Fig. 2: a) Optical images of a single crystal of dicoronylene (C₄₈H₂₀) at various pressures, showing colour changes from red to opaque black, and then completely black (left). Corresponding optical micrograph and (0kl) reciprocal space images generated from single-crystal X-ray diffraction (right). b) Charge density distribution of C₄₈H₂₀ at ambient pressure and at 26.2 GPa, calculated from the crystal structures obtained via X-ray diffraction data. c) Pressure–volume equation of state (EOS) derived from high-pressure single-crystal X-ray diffraction (SCXRD) experiments and density functional theory (DFT) calculations. Solid and dashed lines represent fits to the volume data using the third-order Birch-Murnaghan EOS.

Intermolecular distances were determined from the refined structures. Notably, the nearest-neighbour C-C distance decreased from 3.474(3) Å at ambient pressure to 2.724(7) Å at 26.2 GPa. These results suggest that a semi-metallic state emerges when intermolecular C-C distances fall below 2.8 Å, with irreversible chemical changes occurring below ~2.6 Å. Electron localization function (ELF) analysis showed that the van der Waals space between C₄₈H₂₀ molecules shrinks due to both distance compression and molecular rotation. 

The pressure-induced changes in this hydrocarbon reveal a tuneable band gap and altered electric conductivity, consistent with the emergence of a semi-metallic state. The chemically pre-compressed, single-phase single crystal of C₄₈H₂₀ is stable under ambient conditions, making it an ideal system for detailed structural characterization before physical compression. These findings open new perspectives for the study of large PAHs under pressure, particularly regarding potential transitions to metallic and superconducting states.

Principal publication and authors
Narrowing band gap chemically and physically: Conductive dense hydrocarbon, T. Nakagawa (a), C. Zhang (a), K. Bu (a), P. Dalladay-Simpson (a), M. Vrankić (b), S. Bolton (c), D. Laniel (c), D. Wang (a), A. Liang (a), H. Ishii (d), N. Hiraoka (d), G. Garbarino (e), A. D. Rosa (e), Q. Hu (a), X. Lü (a), H-k. Mao (a,f), Y. Ding (a),  Commun. Mater. 6, 98 (2025); https://doi.org/10.1038/s43246-025-00814-2
(a) Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (P.R. China)
(b) Ruđer Bošković Institute, Zagreb (Croatia)
(c) Centre for Science at Extreme Conditions (CSEC), University of Edinburgh, Edinburgh (UK)
(d) National Synchrotron Radiation Research Center (NSRRC), Taiwan (R.O. China)
(e) ESRF
(f) Shanghai Key Laboratory of Material Frontiers Research in Extreme Environments (MFree), Shanghai Advanced Research in Physical Sciences (SHARPS), Shanghai (P.R. China)

References
[1] A.P. Drozdov et al., Nature 525, 73-76 (2015).
[2] X. Wen et al., J. Am. Chem. Soc. 133, 9023-9035 (2011).