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

Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. XRD 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 > 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, resp., are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both h