Phase relations in boron at pressures up to 18 GPa and temperatures 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-B ₂₈ can be synthesized over a wide pressure and temperature range, and α-tetragonal B ₅₀ (T-B ₅₀) is obtained at higher temperatures and similar pressures. The phase boundary of the β-B ₁₀₆, γ-B ₂₈, and T-B ₅₀ is determined at pressures between 7.0 and 18 GPa and the temperatures of 500-2200 C. The results suggest that T-B ₅₀ might be an intermediate phase formed for kinetic reasons (Ostwald rule) on the way from β-B ₁₀₆ to β-tetragonal B ₁₉₂ (T-192) and γ-B ₂₈ to T-192.