We prepared alpha -rhombohedral ( alpha -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 B_2O_3 with B, or mixtures of B and C between 1 to 10 GPa and 1200 deg and 1800 deg C. In the B-O system we synthesized boron suboxide (nominally B_6O) of high purity, crystallinity, and close to stoichiometric. Quantitative analyses give B_6O_(0.95) and B_6O_(0.77) for high-pressure and room-pressure samples, respectively. Between 4 to 5.5 GPa, B_6O formed as macroscopic near-perfect regular icosahedra (to 30 mu m in diameter).In the B-C-O system, intermediate phases were prepared showing evidence of solid solution between B_4C and B_6O. Boron carbide crystals, to 20 mu m, containing a significant amount of O, typically B_6C_(1.1)O_(0.33) and B_6C_(1.28)O_(0.31), were grown for mixtures in which B and C were reacted with excess B_2O_3 at 7.5 GPa and 1700 deg C. We also report the first conclusive bulk synthesis of a new boron nitride, B_6N_(1-x). This subnitride was synthesized from B and hexagonal BN at 7.5 GPa and 1700 deg C. XRD and PEELS shows that the boron subnitride has the alpha -rh. B structure and average composition B_6N_(0.92).
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