Characterization of Elastic and Vibrational Properties of Dense BC$_{\mathrm{x}}$ Nano-Phases Synthesized under High-Pressure and High-Temperature*

We use Raman scattering to study cold phase transitions in the graphitic $g$-BC$_{8}$ phase and graphite under high pressure up to 84 GPa. It is shown that the $E_{2g}$ Raman active mode of graphite ($G $peak) can be detected up to 84 GPa. We demonstrate that (a) there is a phase transition in graphite and in $g$-BC$_{8}$ at 35 GPa and (b) above 35 GPa the graphite and $g$-BC$_{8}$ transform in a high pressure phase, fully \textit{sp}$^{3}$ bonded $a$-BC$_{8}$ phases, Below the phase transition a polynomial fit to the G peak position versus pressure data yielded the following quadratic relation; above 35 GPa it exhibits linear behavior for graphite as well as for $g$-BC$_{8}$ phase. A direct transformation of graphitic phases in the BC$_{\mathrm{x}}$ system with high concentration of boron (1.5 \textless\ x 8) under high pressure and high temperature was studied. It was found that graphitic phases transform to new cubic BC$_{\mathrm{x}}$ (c-BC$_{3}$, c-B$_{2}$C$_{3})$ phases in a diamond anvil cell (DAC) at high temperature, 2200 K, and high pressure, 31 GPa. The atomic structure, bonding between atoms, and nanostructure was determined using transmission electron microscopy (TEM), x-ray diffraction and transmission electron microscopy-electron energy-loss spectroscopy (EELS). Elastic properties of the BC$_{\mathrm{x}}$ phases were determined by Laser Ultrasonic and Brillouin scattering techniques.