Synthesis and Properties of Single-Crystalline Na₄Si₂₄

Na₄Si₂₄ is the precursor to Si₂₄, a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si₂₄ has potential to transform silicon-based optoelectronics including solar energy conversion. However, the lack of large, pure crystals has prevented the characterization of intrinsic properties and has delayed deposition-based metastable growth efforts. Here, we report an optimized synthesis methodology for single-crystalline Na₄Si₂₄ with crystals approaching the millimeter-size scale with conditions near 9 GPa and 1123 K. Single-crystal diffraction was used to confirm the open-framework structure, and Na atoms remain highly mobile within the framework channels, as determined by electrical conductivity and electron energy loss spectroscopy measurements. An epitaxial relationship between Na₄Si₂₄ and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy, is proposed to facilitate the growth of high-quality Na₄Si₂₄ crystals from DC-Si wafers mixed with metallic Na and could provide a viable path forward for scaling efforts of Na₄Si₂₄ and Si₂₄.