A New Natural, Super-hard, Transparent Polymorph of Carbon

Cubic diamond (space group Fd3m) and hexagonal lonsdaleite (space group P63/mmc) are the hardest natural materials known. Ab initio molecular dynamics calculations give more insight in details of the possible graphite-diamond inversion mechanism [1]. This process includes a pressure-induced sliding of the graphite atom planes towards orthorhombic stacking, from which a fast structure collapse to both hexagonal and/or cubic diamond takes place [1].

Shock-induced diamonds were reported from several impact craters [2]. Polished sections of shocked gneisses from the Popigai Crater, Russia contain transparent multiphase carbon platelets each displaying considerably variable relief (Figure 141). A large (70 µm) transparent platelet with very high relief on one side (height up to 10 µm above the section surface) was studied first by Raman and subsequently by X-ray powder-diffraction and fluorescence synchrotron at ID22. The part with highest relief displays uneven surface with rough gouges (Figure 141a). The 0.25 micrometre diamond powder used for polishing did not succeed in cleanly producing a flat polished surface and hence to erase the scratches and gouges formed during cutting of the section slab.

 

Fig. 141: (a) A Reflected light photograph focused 10 µm above the surface thus depicting the surface features of the new carbon phase with gouges and metallic lead oxide (Pb; arrow) scraped from the polishing disc; (b) Results of the synchrotron X-ray diffraction, imaging and X-ray fluorescence fine-scale mapping laid out on the microscopic photograph shown in (a) depicting the spatial settings of the various carbon phases. The figure shows also the size of the synchrotron beam (lower right, red) for comparison.

 

Synchrotron fluorescence point mapping revealed no other element heavier than carbon (except lead metal scraped from the polishing disc). X-ray diffraction mapping of the platelet indicated the presence of three distinct crystalline carbon phases in a remarkable concentric shell-like setting (Figure 141b), in addition to an amorphous carbon phase. The portion with the highest relief of 10 µm above the section surface is a new super hard polymorph of carbon. This new phase is enveloped by lonsdaleite (hexagonal diamond). The outer most 7-µm thick shell is secondary 2H graphite. Lonsdaleite occupies the portion with a lower relief than that of the new phase, thus manifesting its lower polishing hardness. The X-ray pattern of the highest relief portion is different from that of any known carbon polymorph. The pattern also contains the (111) and (200) reflections of metallic Pb. Twenty-three diffraction lines (Figure 142) were obtained from the super-hard carbon [2]. They could be unambiguously indexed in terms of a cubic cell (space group Pm3m) with a = 14.697(1) Å; cell volume a3 = 3174.58 Å3.

The Diffraction patterns of lonsdaleite and the new phase indicate the extremely small grain size of the crystallites (< 40 nm) of both phases and demonstrate a remarkable high degree of preferred orientation and texturing (Figure 142a). Nevertheless, obtained reflection intensities were used with fixed space group Pm3m and a variable number of atoms per unit cell for trail structures. However, the diffraction data and the calculated unit cell do not allow an unambiguous calculation of the density.

 

Fig. 142: X-ray diffraction pattern of the new carbon phase. (a) Diffraction picture of the new carbon polymorph demonstrating the preferred orientation due to shock-induced dynamic deformation (streaking of the diffraction spots). (b) The full X-ray patterns of the new phase along with the strongest lines of metallic lead (Pb (111) and Pb (200)) reflections (upper panel) and lonsdaleite (lower panel). The high background of continuum in the lonsdaleite pattern at 2 < 11° is due to the presence of an amorphous carbon phase.

 

Here we show that a natural shockwave induced by a large meteorite impact event led to the transformation of graphite to a new crystalline super-hard and transparent polymorph of carbon in gneisses from the Popigai crater, Russia. The synchrotron studies indicate that the new super-hard carbon species occupies the interior of a multiphase assemblage and is entirely enveloped by lonsdaleite and graphite. Polishing hardness of this new phase is greater than that of lonsdaleite. This species was neither encountered in a static or dynamic high-pressure experiment nor predicted by theoretical calculations.

References
[1] S. Scandolo, G.L. Chiarotti, E. Tosatti, Phys. Rev. B 53, 5051 (1996).
[2] A. El Goresy, L.S. Dubrovinsky, Ph. Gillet, S. Mostefaoui, G. Graup, M. Drakopoulos, A.S. Simiionovici, V. Swamy and V.L. Masaitis, Comptes Rendus 335, 889 (2003).

Principal Publication and Authors
A. El Goresy (a), L.S. Dubrovinsky (b), Ph. Gillet (c), S. Mostefaoui (a), G. Graup (a), M. Drakopoulos (d), A.S. Simionovici (d), V. Swamy (e) and V.L. Masaitis (f), Comptes Rendus 335, 889 (2003).
(a) Max-Planck-Institut für Chemie, Mainz (Germany)
(b) Bayrisches Geoinstitut, Bayreuth (Germany)
(c) Ecole Normale Supérieure de Lyon (France)
(d) ESRF
(e) ERCT, Montréal (Canada)
(f) Karpinsky Geological Institute, St. Petersburg (Russia)