Growth of Millimeter Size B6O Single Crystals in a B-H3BO3 System at High Pressure and High Temperature

We report high-pressure synthesis of single-crystal boron suboxide (B6O), one of the superhard materials, using a reaction route between B and H3BO3 at 3–5.5 GPa and 600–2200 °C. At optimized pressure and temperature conditions of 5.5 GPa and 2100 °C, large B6O single crystals can be obtained with a crystallite size of more than 2 mm. Boric acid (H3BO3) was used as a starting material because it is easily decomposed into B2O3 and H2O at a lower temperature. The B2O3 produced by the decomposition should have a small particle size and thus have high reactivity, and the reaction byproduct H2O provides a good hydrostatic environment for the growth of B6O. Therefore, it is easier to synthesize and grow B6O by using a B-H3BO3 system as reaction raw materials. This work discussed the B6O growth mechanism in detail. In addition, photoluminescence experiments and first-principles calculations show that B6O is a direct band gap semiconductor material with good orange-red emission characteristics (the band gap is 1.83 eV). The results of this study indicate a method to grow large B6O single crystals that have great potential use in industrial applications.