Phase transition in BCx system under high-pressure and high-temperature: Synthesis of cubic dense BC3 nanostructured phase

We synthesized a cubic BC3 (c-BC3) phase, by direct transformation from graphitic phases at a pressure of 39 GPa and temperature of 2200 K in a laser-heated diamond anvil cell. A combination of x-ray diffraction, electron diffraction, transmission electron microscopy (TEM) imaging, and electron energy loss spectroscopy (EELS) measurements lead us to conclude that the obtained phase is hetero-nano-diamond, c-BC3. High-resolution TEM imaging of the c-BC3 specimen recovered at ambient conditions demonstrates that the c-BC3 is a single, uniform, nanocrystalline phase with a grain size of about 3–5 nm. The EELS measurements show that the atoms inside the cubic structure are bonded by sp3 bonds. The zero-pressure lattice parameter of the c-BC3 calculated from diffraction peaks was found to be a = 3.589 ± 0.007 Å. The composition of the c-BC3 is determined from EELS measurements. The ratio of carbon to boron, C/B, is approximately 3 (2.8 ± 0.7).