Lonsdaleite

Lonsdaleite (named in honour of Kathleen Lonsdale), also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Lonsdaleite was first identified from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond. It was first discovered in nature in 1967.[1][2]

Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967)[3] by compressing and heating graphite either in a static press or using explosives.[4] It has also been produced by chemical vapor deposition,[5][6][7] and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at temperature 110 °C (230 °F).[8][9]

It is translucent, brownish-yellow in color, and has an index of refraction of 2.40 to 2.41, a specific gravity of 3.2 to 3.3, and a Mohs hardness of 7–8.[10] The Mohs hardness of diamond is 10, and the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material. A simulated pure sample has been found to be 58% harder than diamond.[11]

Properties

Lonsdaleite has a hexagonal unit cell, related to the diamond unit cell in the same way that the hexagonal and cubic close packed crystal systems are related. The 'diamond' structure can be considered to be made up of interlocking rings of six carbon atoms, in the chair conformation. In lonsdaleite, some of the rings are in the boat conformation instead.

Lonsdaleite is simulated to be 58% harder than diamond and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa.[11]

Occurrence

Lonsdaleite occurs as microscopic crystals associated with diamond in the Canyon Diablo meteorite, the Kenna meteorite, and Allan Hills 77283. It has also been reported from the Tunguska impact site.

References

1. ^ Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a new hexagonal polymorph of diamond". Nature 214: 587–589. doi:10.1038/214587a0.
2. ^ Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a hexagonal polymorph of diamond". Am.Min. 52.
3. ^ Bundy, F. P. (1967). "Hexagonal Diamond—A New Form of Carbon". Journal of Chemical Physics 46: 3437. doi:10.1063/1.1841236.
4. ^ He, Hongliang (2002). "Direct transformation of cubic diamond to hexagonal diamond". Applied Physics Letters 81: 610. doi:10.1063/1.1495078.
5. ^ Bhargava, Sanjay (1995). "Diamond polytypes in the chemical vapor deposited diamond films". Applied Physics Letters 67: 1706. doi:10.1063/1.115023.
6. ^ Nishitani-Gamo, Mikka (1998). "Confocal Raman spectroscopic observation of hexagonal diamond formation from dissolved carbon in nickel under chemical vapor deposition conditions". Applied Physics Letters 73: 765. doi:10.1063/1.121994.
7. ^ Misra, Abha (2006). "Hexagonal diamond synthesis on h-GaN strained films". Applied Physics Letters 89: 071911. doi:10.1063/1.2218043.
8. ^ Nur, Yusuf (2008). "Facile Synthesis of Poly(hydridocarbyne): A Precursor to Diamond and Diamond-like Ceramics". Journal of Macromolecular Science Part A 45: 358. doi:10.1080/10601320801946108.
9. ^ Nur, Yusuf (2009). "Electrochemical polymerizatıon of hexachloroethane to form poly(hydridocarbyne): a pre-ceramic polymer for diamond production". Journal of Materials Science 44: 2774. doi:10.1007/s10853-009-3364-4.
10. ^ Lonsdaleite at Mindat
11. ^ a b Pan, Zicheng; Sun, Hong; Zhang, Yi; and Chen, Changfeng (2009). "Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite". Physical Review Letters 102 (102): 055503. doi:10.1103/PhysRevLett.102.055503. Lay summary – Physorg.com (12-02-2009).