Observation of a guest-free Si₄₆ clathrate-I framework from Ba_8-xSi₄₆ upon in situ vacuum heating

Silicon and its composites are key materials owing to their extensive use in the semiconductor industry. While the diamond-structured form dominates, other allotropes with superior properties at ambient conditions remain of interest. Toward this, Si₄₆ clathrate type-I crystals containing alkali/alkaline-earth metals have been extensively studied, but the experimental observation of a guest-free Si₄₆ structure has been challenging. Using advanced electron microscopy, we show experimental evidence of guest-free clathrate-I Si₄₆ framework from Ba_8-xSi₄₆ under in situ heating. We reveal the stepwise Ba evacuation process, starting with loss of Ba1 from the smaller cages to form Ba₆Si₄₆, followed by removal of Ba2 in larger cages to reach Si₄₆ that appears in the thin region of the nanocrystal with a thickness around 4-6 nm at 500 °C. Calculations give a quasi-direct bandgap of 1.89 eV and support the preferential evacuation of Ba1. The observation of this guest-free Si₄₆ framework opens up possibilities for applications in high-speed transistors, optoelectronic devices or solar cell technologies.