High-Pressure, High-Temperature Synthesis and Characterization of Boron Suboxide (B₆O)

Boron suboxide, nominally B 6 O, was synthesized by reducing B 2 O 3 with B up to 10 GPa in a multianvil press at temperatures between 1200 and 1800 °C. The samples were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and parallel electron energy-loss spectroscopy (PEELS). We used high-pressure techniques to synthesize boron suboxide of improved purity and crystallinity and less oxygen-deficient (i.e., closer to the nominal B 6 O composition) in comparison to products of room-pressure syntheses. We describe the preparation of grains ranging from 20 nm to 40 μm in diameter, as well as the first synthesis of micrometer-sized B 6 O icosahedral twins and euhedral “crystals”. The best materials are obtained for starting mixtures containing an excess B 2 O 3 reacted at 1700−1800 °C between 4 and 5.5 GPa. After the products were washed in water, well-crystallized single-phase product dominated by icosahedrally twinned particles to 30 μm in diameter was easily recovered. Oxygen occupancies ascertained from Rietveld refinements show data consistent with the chemical compositions determined by PEELS. These results give a composition of B 6 O 0.77 for our room-pressure material. The highest O occupancy, B 6 O 0.96, is obtained for the micrometer-size icosahedral particles prepared at 1700 °C between 4 and 5.5 GPa.