Synthesis and properties of single-crystalline Na4Si24

Na4Si24 is the precursor to Si-24, a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si-24 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 Si-24.