It is well established that the high-pressure $\beta$-Sn silicon phase converts to the low-pressure BC8 phase during decompression, however the transformation mechanism linking them is less clear with the differing energy barriers and intermediates reported in the literature. In this work, we use density functional theory to find a thermally activated process consistent with experiments when the decompression rate is low. Nudged elastic band calculations identify the mechanistic transformations and show that the displacive reaction involves multiple changes of coordination, from six to eight to five to four. Close to the minimum energy path is a highly symmetric pentavalent structure which resembles molecular ${\mathrm{SiH}}_5^{-}$, and we name Si-220 after its space group. Although the structure is only stable at pressures above 10 GPa, it appears as a saddle point at lower pressures and can directly transform into all three low-pressure phases BT8, R8, and BC8. Although we do not expect Si-220 to be observable in the laboratory, knowledge of its presence helps to resolve the conflicting pathways and the multitude of BC8-like structures seen in experiments.

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• Crystal structure
• Crystal symmetry
• Metal-insulator transition
• Solid-solid transformations
• Structural phase transition
• Elemental semiconductors
• Semimetals
• Crystallography
• Density functional calculations
• Materials modeling