🔬 materials science

Single-crystal X-ray diffraction of BC8 carbon near terapascal pressures

This study reports the first direct crystallographic identification of the long-predicted BC8 carbon phase as a single crystal formed from molten carbon under terapascal pressures, resolving decades of uncertainty regarding carbon structures beyond diamond.

Original author: Liang Sun, Bo Chen, Zhiyu He, Pin Yang, Zanyang Guan, Guo Jia, Fan Zhang, Yi Yang, Bo Xu, Pei Guo, Yonggang Liu, Wei Fan, Tao Xi, Yulong Li, Lingbiao Meng, Wei Kang, Zhiyong Xie, Yuqiong Tu, Cheng Ji

Published 2026-11-05

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CC BY 4.0

Original author: Liang Sun, Bo Chen, Zhiyu He, Pin Yang, Zanyang Guan, Guo Jia, Fan Zhang, Yi Yang, Bo Xu, Pei Guo, Yonggang Liu, Wei Fan, Tao Xi, Yulong Li, Lingbiao Meng, Wei Kang, Zhiyong Xie, Yuqiong Tu, Cheng Ji, Xiuguang Huang, Zhongjing Chen, Ji Yan, Jiamin Yang, Wenge Yang, Toshimori Sekine, Zongqing Zhao, Jiayu Dai, Ho-Kwang Mao

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the author. For technical accuracy, refer to the original paper. Read full disclaimer

Deep inside the cores of planets like Uranus and Neptune, and perhaps within the hearts of distant, exotic worlds, carbon exists under conditions that defy our everyday experience. Pressures there are so immense they are measured in terapascals—a unit so large it is difficult to visualize, representing forces millions of times greater than the weight of the atmosphere pressing down on us at sea level. Under such crushing weight, the familiar forms of carbon we know, like the soft graphite in a pencil or the hard diamond in a ring, are expected to change. For decades, scientists have used powerful computers to predict that at these extreme depths, carbon should rearrange itself into a new, even denser structure called BC8. This theoretical form is thought to be more tightly packed than diamond, yet despite forty years of searching, no one had ever seen it directly. The question remained: does this hidden phase of matter actually exist, or is it just a mathematical ghost?

A team of researchers has now answered that question by creating the first direct crystallographic evidence of BC8 carbon. To find it, they did not wait for nature to provide the conditions; instead, they recreated them in a laboratory using a high-powered laser. By firing a brief, intense pulse of light at a single crystal of diamond, they generated a shockwave that traveled through the material, compressing it to pressures between 1.1 and 1.2 terapascals. This process was so fast that it happened in a billionth of a second, but it was fast enough to melt the diamond into a liquid state and then, as the shockwave weakened, allow the carbon atoms to cool and re-form into a solid. The researchers captured this fleeting moment by firing a synchronized pulse of X-rays at the sample, which acted like a high-speed camera flash, freezing the atomic structure in place.

The resulting images were not the blurry, smeared patterns one might expect from such a chaotic event. Instead, the X-rays produced sharp, distinct spots, much like the clear reflections seen when light hits a cut gemstone. These spots are the signature of a crystal, and by measuring the angles and distances between them, the scientists could map out the exact arrangement of the carbon atoms. The pattern they found matched the predicted geometry of the BC8 structure perfectly. This was a significant breakthrough because previous experiments had only been able to guess at the structure based on broad, fuzzy signals, or they had failed to see any new phase at all. In this study, the researchers were able to prove that the carbon had indeed transformed into a single, coherent crystal of BC8, rather than remaining as diamond or turning into a disordered liquid.

The experiment also revealed something crucial about how this transformation happens. It turned out that simply squeezing carbon with pressure is not enough to force it into the BC8 shape; the material must also be heated and then cooled in a specific way. The diamond had to melt first, allowing the atoms the freedom to move around and find their new, tighter positions before locking back into place. This finding explains why earlier attempts to create BC8 had failed; those experiments often kept the carbon solid the entire time, trapping it in its diamond form. By guiding the carbon through a liquid state, the researchers unlocked a pathway that nature likely uses in the deep interiors of planets.

The density of this newly observed BC8 carbon was measured to be approximately 7.19 grams per cubic centimeter, a value that fits the theoretical predictions for this high-pressure phase. The researchers noted that the crystal structure showed slight distortions, likely due to the extreme heat and uneven pressure during the split-second experiment, but the core identity of the material was unmistakable. This discovery does more than just confirm a long-held theory; it provides a concrete benchmark for understanding the composition of carbon-rich planets and improves our knowledge of how materials behave under the most extreme conditions imaginable. It also opens the door to future studies, suggesting that if scientists can learn to capture and hold this material, they might one day be able to study its unique properties, such as whether it is harder than diamond or how it conducts electricity. For now, the decades-long quest to find the BC8 phase of carbon has moved from the realm of computer simulations into the reality of the laboratory.

Technical Summary: Single-crystal X-ray diffraction of BC8 carbon near terapascal pressures

Problem Statement
The existence of the BC8 phase of carbon, a denser tetrahedrally bonded structure predicted to be thermodynamically stable over diamond at ultrahigh pressures, has remained unresolved for nearly four decades. While theoretical studies and crystal-structure searches have consistently identified BC8 as the stable high-pressure allotrope, direct experimental evidence has been elusive. Previous attempts using ramp-compression and shock compression on microcrystalline diamond failed to detect BC8, often observing the persistence of diamond or melting without phase transformation. These failures suggest that pressure alone is insufficient to drive the diamond-to-BC8 conversion due to kinetic barriers; instead, the specific pressure-temperature (P-T) loading pathway is critical. Furthermore, the rapid evolution of P-T conditions in dynamic compression experiments, combined with the limitations of conventional polycrystalline diffraction, has hindered the unambiguous crystallographic identification of competing high-pressure phases.

Methodology
To address these challenges, the authors employed an in situ orientation-resolved single-crystal X-ray diffraction (SCXRD) technique combined with line-imaging VISAR diagnostics at the SG-IIIp laser facility. The experimental design utilized a 1-ns, 351-nm laser pulse to launch a decaying shock into a [100]-oriented single-crystal diamond sample, which was covered by a CH ablator and a thin Au interlayer. This decaying-shock profile was chosen to guide the material through a specific thermodynamic trajectory: descending from a high-temperature liquid carbon regime, passing through a region near the diamond-BC8-liquid triple point, and potentially entering the BC8 stability field.

A synchronized 1-ns, quasi-monochromatic 8.4-keV Cu He-α X-ray pulse probed the compressed sample, with diffraction patterns recorded on image plates. The pressure history was constrained using VISAR velocity data, mapping the sampled conditions (ranging from ~1.2 to 0.8 TPa in the reported shots) onto reference Hugoniots and theoretical phase diagrams. Unlike standard powder diffraction, the authors analyzed the diffraction records by mapping spots to scattering angles ( $2\theta$ ) and azimuthal angles ( $\phi$ ). They tested whether discrete Bragg spots could be indexed to a common crystal orientation matrix using provisional BC8 Miller indices, providing a more stringent criterion for phase identification than radial peak matching alone.

Key Results
The study reports the first direct crystallographic identification of BC8 carbon formed from molten carbon. Analysis of two independent experimental shots (242 and 366) revealed discrete, localized diffraction spots consistent with single-crystal domains rather than polycrystalline rings.

Crystallographic Indexing: In shot 242, a group of three reflections (indexed as (2̅00), (112), and (130)) was found to be compatible with a common orientation matrix with an angular root-mean-square (RMS) deviation of 1.64°. In shot 366, a similar group ((002), (12̅1̅), and (22̅0)) yielded a tighter angular RMS mismatch of 0.48°. These groups were further validated by leave-one-out tests, where the position of a third reflection was successfully predicted based on the other two.
Lattice Metrics and Density: The measured interplanar spacings across both shots aligned with the BC8 {200}, {112}, {220}, and {310} families. While deviations from ideal cubic spacings were observed (attributed to lattice distortion under extreme P-T conditions), the overall pattern supported the BC8 framework. Assuming 16 carbon atoms per unit cell, the fitted volumes yielded densities of $7.19 \pm 0.45$ g cm⁻³ (shot 242) and $7.20 \pm 0.46$ g cm⁻³ (shot 366).
Thermodynamic Consistency: The inferred pressure window for BC8 crystallization was 830–1000 GPa. The measured density exceeds the liquid carbon reference trends in this window, consistent with a denser solid coexisting with liquid. This ordering aligns with a positive entropy of fusion and a positive BC8 melting-curve slope, consistent with prior thermodynamic constraints.

Significance and Claims
The paper claims to resolve the decades-long quest for a post-diamond carbon structure by providing the first direct crystallographic evidence of the BC8 phase. The authors emphasize that their success relied on a specific high-temperature pathway—crystallization from the melt during shock decay—which overcame the kinetic barriers that prevented BC8 formation in previous subsolidus experiments. By extending orientation-resolved crystallographic identification to near-terapascal pressures on nanosecond timescales, the study establishes an experimental benchmark for dense carbon models relevant to planetary interiors and inertial confinement fusion. The authors conclude that this work demonstrates how dynamic loading pathways can make predicted high-pressure phases experimentally accessible, though they note that further measurements with improved resolution are needed to clarify the microscopic origins of the observed lattice distortions.

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