Bulk Crystalline 4<i>H</i>-Silicon through a Metastable Allotropic Transition

We report the synthesis of bulk, highly oriented, crystalline 4H hexagonal silicon (4H-Si), through a metastable phase transformation upon heating the single-crystalline Si-24 allotrope. Remarkably, the resulting 4H-Si crystallites exhibit an orientation relationship with the Si-24 crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that 4H-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-Si in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.