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 1 800 degrees C. The samples were characterized by powder X-ray diffract ion, scanning electron microscopy, transmission electron microscopy, a nd parallel electron energy-loss spectroscopy (PEELS). We used high-pr essure techniques to synthesize boron suboxide of improved purity and crystallinity and less oxygen-deficient (i.e., closer to the nominal B 6O 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 ico sahedral twins and euhedral ''crystals''. The best materials are obtai ned for starting mixtures containing an excess B2O3 reacted at 1700-18 00 degrees C between 4 and 5.5 GPa. After the products were washed in water, well-crystallized single-phase product dominated by icosahedral ly twinned particles to 30 mu m in diameter was easily recovered. Oxyg en occupancies ascertained from Rietveld refinements show data consist ent with the chemical compositions determined by PEELS. These results give a composition of B6O0.77 for our room-pressure material. The high est O occupancy, B6O0.96, is Obtained for the micrometer-size icosahed ral particles prepared at 1700 degrees C between 4 and 5.5 GPa.