Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN

Details Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003jajap..42.1694y&link_type=abstract

Japanese Journal of Applied Physics, Volume 42, Issue 4A, pp. 1694 (2003).

Physics

5

Scientific paper

Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the essential details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. X-ray diffraction profiles of the products were well explained by a mixture of hDIA and cubic diamond (cDIA) with stacking faults. The mass fraction of hDIA and cDIA was 50:50 for the products annealed between 800 and 1400°C and it became 20:80 for the product annealed at 1600°C. Temperatures higher than 1600°C seem to favor the formation of cDIA or to induce the conversion from hDIA to cDIA. Structure refinement revealed that a decrease and an increase in the basal and apical distances of C-C and B-N bonds in hDIA and wBN, respectively, are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds largely depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both hDIA and wBN was refined to discuss their stability. The transition mechanism from graphite and graphite-like BN to hDIA and wBN is discussed by comparing the present results and those of previous simulation studies. Based on analogous features observed in the synthetic hDIA and lonsdaleite (natural hDIA found in meteorites), the formation mechanism of hDIA in meteorites is proposed.

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