High-pressure, high-temperature syntheses of super-hard α-rhombohedral boron-rich solids in the B-C-N-O

We prepared α-rhombohedral (α-rh.) B-rich materials in the B-C-N-O system at high-pressures and temperatures. Samples were synthesized using a multianvil device and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and parallel electron energy-loss spectroscopy (PEELS). The B-C-O compounds were obtained by reducing B₂O₃ with B, or mixtures of B and C between 1 to 10 GPa and 1200° and 1800 °C . In the B-O system we synthesized boron suboxide (nominally B₆O) of high purity, crystallinity, and close to stoichiometric. Quantitative analyses give B₆O_0.95 and B₆O_0.77 for high-pressure and room-pressure samples, respectively. Between 4 to 5.5 GPa, B₆O formed as macroscopic near-perfect regular icosahedra (to 30 μm in diameter). In the B-C-O system, intermediate phases were prepared showing evidence of solid solution between B₄C and B₆O. Boron carbide crystals, to 20 μm, containing a significant amount of O, typically B₆C_1.1O_0.33 and B₆C_1.28O_0.31, were grown for mixtures in which B and C were reacted with excess B₂O₃ at 7.5 GPa and 1700 °C. We also report the first conclusive bulk synthesis of a new boron nitride, B₆N_1-x. This subnitride was synthesized from B and hexagonal BN at 7.5 GPa and 1700 °C. XRD and PEELS shows that the boron subnitride has the α-rh. B structure and average composition B₆N_0.92.