Recently we performed a systematic investigation of phase transformations of turbostratic graphite-like BCx phases of various stoichiometry (x = 1-5) at pressures up to 25 GPa and temperatures up to 2500 K in a laser-heated diamond-anvil cell using in situ angle-dispersive X-ray diffraction at beamline ID27. At pressures above 20 GPa in the 2000-2500 K range, the t-BCx phases (1 ≤ x ≤ 4) decomposed into boron-doped diamond (1-2 at% of boron) and boron carbides (B4C and B50C2) that were accompanied by the formation of cubic B-C solid solutions.
The diamond-like BC5 phase corresponds to the ultimate solubility of boron in diamond. It was synthesised under controlled conditions of pressure and temperature (24 GPa and around 2200 K) and then quenched to ambient conditions (Figure 1). The quenched samples were characterised using X-ray diffraction, Raman spectroscopy, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy.
Figure 1. Laser-heating sequence of diffraction patterns of a BC5 sample taken in situ at 24 GPa for various temperatures. The lowest pattern corresponds to t-BC5 at ambient conditions.
Figure 2. Suggested crystal structure of cubic BC5. The red and black balls represent the boron and carbon atoms, respectively. The boron atoms are randomly distributed throughout the diamond-like lattice.
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Principal publication and authors V.L. Solozhenko (a), O.O. Kurakevych (a), D. Andrault (b), Y. Le Godec (c), M. Mezouar (d), Ultimate metastable solubility of boron in diamond: Synthesis of superhard diamond-like BC5, Physical Review Letters 102, 015506 (2009).
(a) LPMTM-CNRS, Université Paris Nord, Villetaneuse (France)
(b) LMV, Université Blaise Pascal, Clermont-Ferrand (France)
(c) IMPMC, Université P&M Curie, Paris (France)