High-pressure, high-temperature synthesis and characterization of boron suboxide (B6O)

Boron suboxide, nominally B6O, was synthesized by reducing B2O3 with B up to 10 GPa In a multianvil press at temperatures between 1200 and 1800 degrees 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 B6O composition) in comparison to products of room-pressure syntheses. We describe the preparation of grains ranging from 20 nm to 40 mu m in diameter, as well, as the first synthesis of micrometer-sized B6O icosahedral twins and euhedral "crystals". The best materials are obtained for starting mixtures containing an excess B2O3 reacted at 1700-1800 degrees 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 mu m 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 B6O0.77 for our room-pressure material. The highest O occupancy, B6O0.96, is Obtained for the micrometer-size icosahedral particles prepared at 1700 degrees C between 4 and 5.5 GPa.