Year: 2026
Type: preprint
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.
Source: ChemRxiv
Authors: Péter Németh, Christoph G. Salzmann
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7204917367