Ultimate Metastable Solubility of Boron in Diamond: Synthesis of Superhard Diamondlike ${\mathrm{BC}}_5$

Here, we report the synthesis of cubic ${\mathrm{BC}}_5$ ($\mathrm{c}\mathrm{\text{−}}{\mathrm{BC}}_5$), the diamondlike B-C phase with the highest boron content ever achieved, at 24 GPa and about 2200 K, using both a laser-heated diamond anvil cell and large-volume multianvil apparatus. The synthesized phase is low compressible (bulk modulus of 335 GPa), conductive, and exhibits extreme Vickers hardness (71 GPa), unusually high for superhard materials fracture toughness ($9.5\mathrm{MPa}{\mathrm{m}}^{0.5}$), and high thermal stability (up to 1900 K); this makes it an exceptional superabrasive and promising material for high-temperature electronics.

Figure 1

(a) Representative diffraction patterns of the ${\mathrm{BC}}_3$ and BC samples quenched from 26 GPa and 2400 K. Both phases decompose into boron-doped diamond with $\sim 1.8$ at% of B (BDD), cubic ${\mathrm{BC}}_z$ phase (according to lattice parameters, ${\mathrm{BC}}_9$ and ${\mathrm{BC}}_5$ for starting $\mathrm{t}\mathrm{\text{−}}{\mathrm{BC}}_3$ and t-BC, respectively), and boron carbides ${\mathrm{B}}_4\mathrm{C}$ and ${\mathrm{B}}_{50}{\mathrm{C}}_2$. (b) Typical laser-heating sequence of diffraction patterns of ${\mathrm{BC}}_5$ sample taken in situ at 24 GPa and several temperatures. The bottom pattern corresponds to $\mathrm{t}\mathrm{\text{−}}{\mathrm{BC}}_5$ at ambient conditions.

Figure 2

(a) Lattice parameters of boron-substituted diamonds versus boron content. The dashed line represents Vegard’s law, while the symbols show the result of present work (circle with error bar) in comparison with other experimental (circles) [9] and theoretical (triangles [17]) data. The open circles in the inset show the lattice parameters and ideal mixing estimate for compositions of quenched $\mathrm{c}\mathrm{\text{−}}{\mathrm{BC}}_z$ phases formed during phase segregation of $\mathrm{t}\mathrm{\text{−}}{\mathrm{BC}}_x$ ($x=1–5$) at $p\mathrm{\text{−}}T$ conditions under study. (b) Raman spectrum of cubic ${\mathrm{BC}}_5$ at ambient conditions (514.5 nm excitation wavelength). The vertical bars represent the phonon frequencies observed by inelastic neutron scattering in pure diamond [19].