An Electrochemical Approach toward the Metastable Type II Clathrate Germanium Allotrope

[Image: see text] By using an anodic conversion process at 280 °C, the type II clathrates Na(1.7(6))Ge(136) and Na(23.0(5))Ge(136) were obtained from Na(12)Ge(17) as the starting material. An alkali-metal iodide molten-salt electrolyte complied with the reaction conditions, allowing for the formation of microcrystalline products. Characterization by powder X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy also revealed Na(4)Ge(13) as an intermediate and α-Ge and Cs(8–x)Ge(136) as byproducts, with the latter likely resulting from cation exchange between the starting material and electrolyte. Taking such minor side reactions and a small contribution of material without suitable electrical contact into account, anodic conversion of Na(12)Ge(17) to Na(1.7)Ge(136) proved to proceed without parasitic processes and to comprise the material bulk. The hitherto existing preparation method for Na(x→0)Ge(136) by gas–solid oxidation of Na(12)Ge(17) has thus been translated into a scalable high-temperature electrochemical approach with enhanced tools for reaction control, promising access to pure Ge(cF136) and Na(24)Ge(136) after process optimization.