  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.
