Na 4 Si 24 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 Na 4 Si 24 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 Na 4 Si 24 and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy, is proposed to facilitate the growth of high-quality Na 4 Si 24 crystals from DC-Si wafers mixed with metallic Na and could provide a viable path forward for scaling efforts of Na 4 Si 24 and Si 24. ■ INTRODUCTION While silicon is a cornerstone of modern technology, the indirect nature of the fundamental band gap places limitations on desired applications. 1−4 However, several other silicon allotropes are known, especially under high-pressure conditions. 5,6 Among these, BC8/R8, Si 136 clathrate and hexagonal diamond (HD)-Si can be stabilized at ambient conditions. 5,7−13 In terms of optoelectronic properties, Si 136 is particularly encouraging as it has a wide direct or nearly direct gap near 1.9 eV, 14−17 though some electronic dipole transitions are forbidden, which detract from its potential photovoltaic merits. 18 Additionally, challenges with large-scale/film growth and sample purity of Si 136 have prevented major development thus far. 12,14,19 Moreover, numerous allotropes with exceptional optical properties are predicted to exist with energy not far above the DC-Si ground state, 4,5,20−22 suggesting potential for novel, metastable silicon allotropes with enhanced optoelectronic properties. Recently, a new open-framework allotrope of silicon, Si 24 , was discovered using a high-pressure precursor synthesis method. 23,24 In the first step of this process, a Na 4 Si 24 precursor was synthesized at high pressure. Na 4 Si 24 possesses the Eu 4 Ga 8 Ge 16 (Cmcm) structure type 25 with four formula units of NaSi 6. This structure type is also known for some alkaline-and rare-earth silicides, MSi 6 (M = Ca, 26 Sr, 27 Ba, 28 Eu, 29 Na 23,24). The structure consists of a clathrate-like sp 3 silicon host lattice comprised of five-, six-, and eight-membered silicon rings that form channels along the crystallographic a-axis. The topology is isostructural with calcium aluminosilicate zeolite (code CAS), 30 and the channels are filled with one-dimensional chains of metal guest atoms. This is unlike other known Na−Si clathrates such as Na 8 Si 46 (sI) and Na 24 Si 136 (sII), where the host Si lattice forms complete polyhedral cages around the Na guest atoms, making Na difficult to remove from the structure. Na removal from from sI is not possible, while complete Na removal from sII is extremely difficult,