HIGH-PRESSURE, HIGH-TEMPERATURE SYNTHESES IN THE B-C-N-O SYSTEM - I -PREPARATION AND CHARACTERIZATION

We synthesized several alpha-rhombohedral B-rich materials belonging t o the B-C-N-O system using a multianvil press. B-C-O materials were pr epared by reacting mixtures of B, C, and B2O3 in the 5 to 7.5 GPa pres sure range and at a temperature of 1700 degrees C. Powder X-ray diffra ction and parallel electron energy-loss spectroscopy with a transmissi on electron microscope showed that the BxCyOz phases obtained are base d on the alpha-rhombohedral B structure. Crystals of composition B6C1. 1O0.33 to B6C1.28O0.31 ranging in size from 1 to 20 mu m were grown. S mall transparent lustrous red cleavage flakes were obtained for stoich iometries close to B6O. The growth of boron carbide nanorods was also observed in some of the run products. We report the first conclusive b ulk synthesis of a new boron nitride, B6N1-x, obtained by reacting B a nd hexagonal BN at 7.5 GPa and 1700 degrees C. The structure of this c ompound is derived from that of alpha-rhombohedral B (space groupe R(3 ) over barm$) and the refined hexagonal cell parameters are a(h) 5.457 Angstrom and c(h) = 12.241 Angstrom. (C) 1997 Academic Press.