Lonsdaleite

It is interesting that sp$^3$ bonded carbon was found to exist not only as cubic crystals but also as hexagonal crystals (Londsdaleite). Lonsdaleite was first identified from the Canyon Diablo meteorite at Barringer Crater (also known as Meteor Crater) in Arizona in 1967. It is believed to form when meteoric graphite falls to Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Later lonsdaleite was grown in the laboratory [9].

For a long time, hexagonal diamond has been formed artificially only by static and shock wave compression of well-crystallized graphites[9,10]. Recently it was shown that hexagonal diamond can be obtained also from cubic diamond [11].

Cubic and hexagonal diamond, both being composed of sp$^3$ bonded carbon atoms, have a rather similar structure, differing only in the stacking order of the sp$^3$ bonded carbon layers. The angles between C-C bonds is 109 degrees and the interatomic distance is 1.54 Å for both these forms of crystalline diamond. The difference between c-D and h-D is only apparent when looking at the longer range structural properties of C atoms in the crystals. Figs.2.5 and 2.6 show the structures of ideal c-D and h-D crystals and their radial distribution functions. The similarity between these should be noted.

In contrast to the high pressure high temperature (HPHT) cubic diamond growth achieved under hydrostatic pressure, hexagonal diamond was observed to grow when uniaxial pressure was applied to liquid carbon during its solidification. Lonsdaleite is fundamentally less stable than diamond, therefore the hardness of lonsdaleite to be slightly less than that of diamond.

Figure: Structures of (a) perfect cubic diamond and (b) perfect hexagonal diamond. These viewpoints show the similarity between these structures. Differences can be seen by careful observation of the hexagons: at these angles each hexagon appears to have 2 short and 4 long bonds. In cubic diamond the short bonds are on opposite side of the hexagons separated by 2 long bonds, whereas in hexagonal diamond either 1 or 3 long bonds separate these 2 short bonds. We note that in fact the hexagons are not in a plane and all bonds are of the same length. The apparent lengths of the bonds are due to the viewing angle.

Figure 2.6: Radial distribution, $g(r)$, of perfect cubic diamond (top) compared with that of perfect lonsdaleite (bottom). The radial distribution functions were calculated for samples containing 64 atoms.