The phase relations in boron have been investigated at high pressure and high temperature using a multianvil apparatus, and the quenched sample has been analyzed by x-ray diffraction, Raman spectra, and transmission electron microscopy. We demonstrate that $\gamma$-B${}_{28}$ can be synthesized over a wide pressure and temperature range, and $\alpha$-tetragonal B${}_{50}$ (T-B${}_{50}$) is obtained at higher temperatures and similar pressures. The phase boundary of the $\beta$-B${}_{106}$, $\gamma$-B${}_{28}$, and T-B${}_{50}$ is determined at pressures between 7.0 and 18 GPa and the temperatures of 500–2200 ${}^{\circ }$C. The results suggest that T-B${}_{50}$ might be an intermediate phase formed for kinetic reasons (Ostwald rule) on the way from $\beta$-B${}_{106}$ to $\beta$-tetragonal B${}_{192}$ (T-192) and $\gamma$-B${}_{28}$ to T-192.

• Received 18 September 2011

DOI:https://doi.org/10.1103/PhysRevB.85.014107