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

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

AI summary · arXiv: 2611.06045v1 · cond-mat.mtrl-sci

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

Liang Sun, Bo Chen, Zhiyu He, Pin Yang, Zanyang Guan, Guo Jia, Fan Zhang, Yi Yang,

and 20 more authors

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 Submitted to arXiv on 04 Nov. 2026 3 figures License: CC BY 4.0

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The essentials

Orientation-resolved X-ray diffraction identifies BC8 carbon crystals formed from molten diamond during decaying-shock compression above 1.1 TPa.

Question

Can carbon form the predicted BC8 phase under terapascal conditions, and can it be identified crystallographically rather than inferred from powder peak matches? The study tests whether a decaying-shock pathway through molten carbon produces BC8.

Method

At the SG-IIIp laser facility, a 1-ns, 351-nm pulse drove shocks into [100]-oriented single-crystal diamond, while a synchronized 1-ns, 8.4-keV X-ray pulse recorded diffraction. The pressure in shot 242 fell from about 1.2 to 0.8 TPa during exposure; the researchers fitted orthorhombic lattice metrics to 12 reflections in shot 242 and seven in shot 366, then tested reflection groups against common orientation matrices and BC8 spacing relationships.

Findings

Result Three reflections in shot 242 were approximately consistent with one crystal orientation, with an angular RMS deviation of 1.64°.
See the passage in the paper
In shot 242, three highlighted reflections, including (2̅00), (112) and (130), were approximately compatible with a common crystal orientation, yielding an angular root-mean-square (RMS) deviation of 1.64° (Fig. 2, A and B).
Result Three reflections in shot 366 matched a common orientation matrix more closely, with an angular RMS mismatch of 0.48°.
See the passage in the paper
In shot 366, the three highlighted reflections, including (002), (12̅ 1̅)and (22̅ 0), were reproduced by a common orientation matrix with a smaller angular RMS mismatch of 0.48° (Fig. 2, C and D).
Result Across both experiments, measured spacings were compatible with several characteristic BC8 reflection families.
Result The two experiments yielded consistent diffraction-derived density estimates of approximately 7.19–7.20 g cm⁻³.
See the passage in the paper
Assuming 16 carbon atoms per conventional BC8 unit cell, the fitted volumes yield nearly identical densities of 7.19 ± 0.45 g cm⁻³ for shot 242 and 7.20 ± 0.46 g cm⁻³ for shot 366, with uncertainties reported at ±2σ.
Authors’ interpretation The authors interpret the crystallographic evidence as identifying BC8 carbon formed from molten carbon during decaying-shock compression.
See the passage in the paper
Taken together, our results establish the first direct crystallographic identification of BC8 carbon.

Limitations

The diffraction records integrate spatially and temporally varying conditions, limiting assignment of the sampled pressure-temperature state.
See the passage in the paper
Each record therefore integrates contributions from regions experiencing spatially and temporally varying conditions.
The microscopic origin of departures from ideal cubic spacings remains unresolved.
See the passage in the paper
Spatial and temporal variations in pressure, temperature and stress may contribute to these deviations, although their microscopic origin remains unresolved.
Some reflection assignments remain tentative or conditional rather than fully established.
See the passage in the paper
We retained this association as a candidate group, while treating additional reflection pairs as tentative assignments.

Why it matters

The authors present this as a crystallographic benchmark for the predicted post-diamond carbon phase and show that a high-temperature pathway from molten carbon can make BC8 experimentally accessible. They argue that the approach may aid identification of other high-pressure crystalline phases and inform models of planetary interiors and carbon ablators.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: BC8 carbon; ultrahigh pressure; shock compression; X-ray diffraction; crystallization pathway

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Comprehensive Summary
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Original abstract

The paper investigates whether carbon adopts the predicted BC8 structure at pressures near or above one terapascal (TPa). Proposed for decades as a dense, tetrahedrally bonded phase, BC8 may become more stable than diamond at ultrahigh pressures. However, earlier experiments had not provided direct crystallographic evidence of its formation. Previous work also showed that diamond can persist at extreme pressures, suggesting that pressure alone may not be sufficient: temperature and compression history could determine whether BC8 forms. The authors examined a pathway in which carbon first melts and then crystallizes during shock decay. At the SG-IIIp facility, they compressed [100]-oriented single-crystal diamond using a 1-nanosecond, 351-nanometer laser pulse. A synchronized 1-nanosecond pulse of quasi-monochromatic 8.4-keV X-rays probed the compressed material, and diffraction was recorded on image plates. Line-imaging VISAR measurements constrained the pressure history. In shot 242, the pressure fell from about 1.2 to 0.8 TPa during the X-ray exposure; shot 366 sampled a comparable, overlapping pressure range. Because pressure and temperature changed during the measurements, each diffraction record integrated over a range of conditions. The diffraction images contained numerous discrete spots, as well as tantalum calibration features and reflections from ambient diamond. The researchers mapped the spots to scattering angle and azimuth, fitted orthorhombic lattice metrics using provisional BC8 indices, and tested whether selected reflections could be explained by a single crystal orientation. This orientation-resolved analysis provides evidence beyond simply matching radial diffraction-peak positions. In shot 242, three reflections indexed as (200), (112), and (130) were approximately consistent with a common orientation, with an angular root-mean-square mismatch of 1.64 degrees. In shot 366, three reflections indexed as (002), (12̅1̅), and (22̅0) produced a smaller mismatch of 0.48 degrees. With the fitted lattice metrics held fixed, leave-one-out tests predicted each omitted reflection from the other two within the nominal two-degree criterion. The orientation evidence was therefore stronger in shot 366, while some other proposed reflection associations remained tentative or conditional. Across both experiments, the measured spacings were compatible with the BC8 reflection families {200}, {112}, {220}, and {310}. The fitted lattice metrics allowed symmetry-related spacings to deviate from their ideal cubic values, while the family relationships remained consistent with a distorted BC8 framework. Reflections forbidden in ideal diamond further supported the assignment. Assuming 16 carbon atoms per conventional BC8 unit cell, the authors calculated densities of 7.19 ± 0.45 g/cm³ for shot 242 and 7.20 ± 0.46 g/cm³ for shot 366, with uncertainties reported at two standard deviations. The combined representative density was 7.19 ± 0.45 g/cm³. The proposed crystallization window was 830–1000 GPa. Within uncertainty, the density was consistent with thermodynamic constraints from earlier work, including a comparison estimate of about 7.10 g/cm³ at 915 GPa. The authors interpret their findings as the first direct crystallographic identification of BC8 carbon formed from molten carbon during shock decay. They argue that the formation pathway, rather than pressure alone, may enable BC8 to crystallize. The observed structural distortions remain unexplained; variations in pressure, temperature, and stress could contribute. The experiment also averages over changing conditions and has limited spatial and temporal resolution, while some reflection assignments are less secure than others.

Has Carbon Finally Formed BC8 at Terapascal Pressures?

At pressures around a terapascal—one trillion pascals—carbon may take on a structure unlike the familiar diamond. For decades, researchers have predicted that a dense arrangement called BC8 could become more stable than diamond under these extreme conditions. Yet previous experiments had not offered direct crystallographic evidence that BC8 had actually formed.

In “Single-crystal X-ray diffraction of BC8 carbon near terapascal pressures,” Liang Sun, Bo Chen, Zhiyu He, Pin Yang, Zanyang Guan, Guo Jia et al. report evidence that carbon crystallized in a distorted BC8 structure. Their results point to a crucial detail: the route carbon takes through pressure and temperature changes may matter as much as the pressure it reaches.

Why the path to BC8 matters

Diamond can persist at immense pressures, complicating the expectation that squeezing carbon alone will produce a new phase. The researchers therefore investigated a different possibility: carbon might first melt under shock compression, then crystallize as the pressure falls. In this scenario, the brief conditions during decompression could allow BC8 to form even if diamond remains stable along other paths.

To test this idea, the team compressed [100]-oriented single-crystal diamond at the SG-IIIp facility with a one-nanosecond laser pulse. A synchronized pulse of 8.4-keV X-rays probed the material, and detectors recorded the resulting diffraction. Measurements of the motion of the shock front helped constrain how pressure changed during each experiment.

Reading the diffraction pattern

The pressure did not stay fixed while the X-rays were collecting data. In shot 242, it fell from about 1.2 to 0.8 TPa during the exposure. Shot 366 sampled a similar, overlapping pressure range. Each diffraction record therefore captured a span of conditions rather than a snapshot at one precise pressure and temperature.

The images contained many discrete diffraction spots, along with calibration features from tantalum and reflections from surrounding, uncompressed diamond. The researchers mapped the spots by scattering angle and direction, then tested whether selected reflections could be assigned to BC8 lattice planes. Crucially, they asked whether several spots could be explained by one consistent crystal orientation. That provides a more demanding test than simply matching diffraction peak positions.

In shot 242, three proposed reflections—(200), (112), and (130)—were approximately consistent with a shared orientation, with an angular mismatch of 1.64 degrees.
In shot 366, the proposed (002), (12̅1̅), and (22̅0) reflections fit one orientation more closely, with a mismatch of 0.48 degrees.
When the researchers held the fitted lattice metrics fixed, leave-one-out tests predicted each omitted reflection from the other two within a two-degree criterion.

The orientation evidence was stronger for shot 366. Other proposed spot assignments were more tentative, so not every feature in the images carries the same weight.

A distorted BC8 structure

Across the experiments, the measured spacings were compatible with BC8 reflection families {200}, {112}, {220}, and {310}. The fitted lattice dimensions did not match an ideal cubic structure perfectly: symmetry-related spacings showed deviations. Even so, their relationships were consistent with a distorted BC8 framework. Reflections that would be forbidden for ideal diamond provided additional support for the assignment.

Using a conventional BC8 unit cell containing 16 carbon atoms, the researchers calculated densities of 7.19 ± 0.45 grams per cubic centimeter for shot 242 and 7.20 ± 0.46 grams per cubic centimeter for shot 366. The reported uncertainties are at two standard deviations. Together, the measurements give a representative density of 7.19 ± 0.45 grams per cubic centimeter.

The proposed crystallization window was 830–1000 GPa. Within the uncertainties, the measured density agrees with thermodynamic constraints from earlier work, including a comparison estimate of about 7.10 grams per cubic centimeter at 915 GPa.

What the result does—and does not—show

The authors interpret the observations as the first direct crystallographic identification of BC8 carbon formed from molten carbon during shock decay. The finding supports the idea that BC8 formation depends not only on reaching ultrahigh pressure, but also on how the material is compressed, heated, and allowed to cool.

Important uncertainties remain. Pressure and temperature changed during the measurements, and the experiment had limited spatial and temporal resolution. Some reflection assignments are less secure than others, and the observed distortions have not been explained. Variations in pressure, temperature, or stress could all contribute.

Even with these caveats, the work offers a new way to investigate carbon at extreme conditions: follow not just where the material ends up on a pressure scale, but how it gets there. If the interpretation holds, the fleeting transformation from melt to crystal may be the key to producing one of carbon’s long-predicted forms.

Created on 09 Nov. 2026

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

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Has Carbon Finally Formed BC8 at Terapascal Pressures?

Why the path to BC8 matters

Reading the diffraction pattern

A distorted BC8 structure

What the result does—and does not—show

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