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Phase relations in boron at pressure up to 18 GPa and temperature up to 2200 °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 γ-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 β-B106, γ-B28 and T-B50 is determined at pressures between 7 and 18 GPa and the temperatures of 500-2200 °C. The results suggest that T-B50 might be an intermediate phase-metastable, formed for kinetic reasons (Ostwald rule) on the way from β-B106 to T-192 and γ-B28 to T-192.