Bulk Crystalline 4 H -Silicon through a Metastable Allotropic Transition

We report the synthesis of bulk, highly oriented, crystalline $4H$ hexagonal silicon ($4H\text{\ensuremath{-}}\mathrm{Si}$), through a metastable phase transformation upon heating the single-crystalline ${\mathrm{Si}}_{24}$ allotrope. Remarkably, the resulting $4H\text{\ensuremath{-}}\mathrm{Si}$ crystallites exhibit an orientation relationship with the ${\mathrm{Si}}_{24}$ crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that $4H\text{\ensuremath{-}}\mathrm{Si}$ exhibits an indirect band gap near 1.2 eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline $4H\text{\ensuremath{-}}\mathrm{Si}$ in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.