Hexagonal diamond lattice
A small commercially 3D printed molecular model of a section of "hexagonal diamond" or "Lonsdaleite" lattice.
The model is made of coloured laser-sintered nylon power, and was manufactured by Shapeways' on-demand 3D printing service.
Lonsdaleite is named after crystallographer Kathleen Lonsdale, DBE, FRS (1903-1971).
The Lonsdaleite lattice
This proposed alternative configuration of diamond can be visualised by imagining a stack of perfectly-aligned hexagonal sheets of graphene. With this configuration in graphite, each carbon atom makes three bonds with its neighbours, and the "fourth bond" exists as a shared fuzzy cloud of virtual bonding between adjacent sheets. This allows the sheets to slip past each other easily. To convert ordered graphite into Lonsdaleite, we take alternate carbon atoms in a hexagonal graphene sheet and bond them downwards with their neighbours in the sheet below, then we take the remain 50% of atoms in the sheet and bond them with their neighbours in the sheet above. When we repeat the process with all remaining non-fully-bonded atoms across the other sheets, the result is a fully three-dimensional structure.
Properties of hexagonal diamond
Lonsdaleite is expected to be harder than conventional diamond: however, no large-scale samples are known to exist or have been synthesised for macroscopic testing. Specks of Lonsdaleite are believed to have been found in meteorites and identified by x-ray diffraction, and are supposed to have been formed from graphite under the extreme conditions of atmospheric capture, and some researchers have reported apparent synthesis of the material. However, it has been counter-claimed that a similar x-ray diffraction signature can be produced by faults in conventional diamond, and that these identifications may be spurious.
Links
- Clifford Frondel and Ursula B. Marvin Lonsdaleite, a Hexagonal Polymorph of Diamond Nature vol.214, pages 587–589 (06 May 1967) doi:10.1038/214587a0
- Diamond no longer nature’s hardest material (newscientist.com) – 2009
- Hexagonal Diamond - The "other" form of diamond (erkdemon.blogspot.co.uk)
- Péter Németh, Laurence A. J. Garvie, Toshihiro Aoki, Natalia Dubrovinskaia, Leonid Dubrovinsky and Peter R. Buseck, Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material Nature Communications volume 5, Article number: 5447 (2014) doi:10.1038/ncomms6447
Kathleen Lonsdale:
