Phase relations in boron at pressure up to 18 GPa and temperature up to 2200 {deg}C

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}-B28 can be synthesized over a wide P-T range, and T-B50 is obtained at higher temperatures and similar pressures. The phase boundary of the {\beta}-B106, {\gamma}-B28 and T-B50 is determined at pressures between 7 and 18 GPa and the temperatures of 500-2200 {\deg}C. The results suggest that T-B50 might be an intermediate phase-metastable, formed for kinetic reasons (Ostwald rule) on the way from {\beta}-B106 to T-192 and {\gamma}-B28 to T-192.