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PreprintCC BYChemRxiv

Bulk hexagonal diamond remains elusive

  • Péter Németh1,2,3
  • Christoph G. Salzmann4
4 affiliations
  1. 1Institute for Geological and Geochemical Research
  2. 2HUN-REN Research Centre for Astronomy and Earth Sciences (MTA Centre of Excellence)
  3. 3Research Institute of Biomolecular and Chemical Engineering, Nanolab, University of Pannonia, Egyetem út 10, 8200 Veszprém, Hungary
  4. 4Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
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Abstract

Hexagonal diamond (HD) is an enigmatic polymorph of diamond whose crystal structure, diffraction properties, spectroscopic features and hardness remain controversial. [1–2] This carbon allotrope contains only hexagonal (h) stacking of puckered layers of carbon atoms and displays a complete absence of cubic (c) stacking that is found in the ‘ordinary’ cubic diamond. The use of the mineral name lonsdaleite as a synonym for HD complicates the situation further, despite several studies showing that the type material originally described as lonsdaleite does not represent a discrete phase. Obtaining pure HD would be a step change in diamond research that would resolve many long-standing controversies. A recent study claimed the bulk synthesis of 100-μm to millimeter-sized HD crystals by compressing and heating single-crystal hexagonal graphite. The authors based their identification and structural characterization on in-situ and ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. They interpreted their data in terms of ~100 nm triple-twinned HD domains. We identified problems with this interpretation and demonstrate that the material is not a highly ordered, pure HD phase, but instead stacking-disordered diamond (SDD) containing both hexagonal and cubic stacking. Our analysis also reveals a new type of intergrowth structure which provides fresh insight into the graphite – HD transition, and may help guide the future synthesis of pure HD.

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Cite

BibTeXRIS
BibTeX
@misc{nemeth2026bulk,
  title = {{Bulk hexagonal diamond remains elusive}},
  author = {Péter Németh and Christoph G. Salzmann},
  howpublished = {ChemRxiv},
  year = {2026},
  month = aug,
  doi = {10.26434/chemrxiv.15008123/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008123/v1},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Bulk hexagonal diamond remains elusive
AU  - Péter Németh
AU  - Christoph G. Salzmann
PY  - 2026
DA  - 2026/08/31
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15008123/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008123/v1
AB  - Hexagonal diamond (HD) is an enigmatic polymorph of diamond whose crystal structure, diffraction properties, spectroscopic features and hardness remain controversial. [1–2] This carbon allotrope contains only hexagonal (h) stacking of puckered layers of carbon atoms and displays a complete absence of cubic (c) stacking that is found in the ‘ordinary’ cubic diamond. The use of the mineral name lonsdaleite as a synonym for HD complicates the situation further, despite several studies showing that the type material originally described as lonsdaleite does not represent a discrete phase. Obtaining pure HD would be a step change in diamond research that would resolve many long-standing controversies. A recent study claimed the bulk synthesis of 100-μm to millimeter-sized HD crystals by compressing and heating single-crystal hexagonal graphite. The authors based their identification and structural characterization on in-situ and ex-situ X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. They interpreted their data in terms of ~100 nm triple-twinned HD domains. We identified problems with this interpretation and demonstrate that the material is not a highly ordered, pure HD phase, but instead stacking-disordered diamond (SDD) containing both hexagonal and cubic stacking. Our analysis also reveals a new type of intergrowth structure which provides fresh insight into the graphite – HD transition, and may help guide the future synthesis of pure HD.
ER  -