B_xO: Phases Present at High Pressure and Temperature

Boron suboxide compounds are of interest because of their low densities coupled with high hardness. In the present study, we have attempted to determine the nature of the B_xO phases that occur in the field defined by pressures of zero to 1.5 GPa, temperature between 1200° and 1700°C, and the compositional range 2/3 x 24. Amorphous boron powder and boric acid B₂O₃ were the starting reactants for all the runs. The processing of the specimens was carried out in a controlled atmosphere furnace, a hot‐pressing assembly, and in a piston—cylinder high‐pressure apparatus within quasi‐hydrostatic and inductively heated cell assemblies. After processing at elevated temperature and pressure, for compositions over the range 2/3 x 6, B₂O₃ (identical to the hexagonal starting material) and B₆O (R3¯m) were the dominant phases present. For the compositions 6 x 24, B₆O and rhombohedral B were the primary phases identified. In general, the hardness of the processed composites was dominated by the occurrence of B₆O (approximately equivalent to B₄C). However, there is some suggestion of particularly high values of hardness on a very localized scale in specimens near the B₂₂O composition.