Figure 5

Schematic diagram of the phase transition. (a) The enthalpy barriers as a function of pressure for various transition pathways from α-B₁₂ to hex-B_1/7 (α₁ and α₂) and from hex-B_1/8 to γ-B₂₈ (γ₁, γ₂ and γ₃), with the lowest enthalpy barriers for α₁ and γ₁ in Figure 4. The stepwise and concerted transformations are represented by red (α₁, γ₁ and γ₂) and black (α₂ and γ₃) lines, respectively (see Supplementary Figure S7). (b) The enthalpy diagram for the transition from α-B₁₂ to γ-B₂₈ at 50 GPa with the hex-B_1/7 and hex-B_1/8 allotropes as intermediate phases in the framework of the Ostwald’s step rule. The dotted line indicates the enthalpy barrier for the transition from hex-B_1/7 to hex-B_1/8, which is considered the upper bound because of the restricted supercell geometry (see Supplementary Figure S8).