An Electrochemical Approach toward the Metastable Type II Clathrate Germanium Allotrope

By using an anodic conversion process at 280 °C, the type II clathrates Na_1.7(6)Ge₁₃₆ and Na_23.0(5)Ge₁₃₆ were obtained from Na₁₂Ge₁₇ 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 Na₄Ge₁₃ as an intermediate and α-Ge and Cs_8-Ge₁₃₆ 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 Na₁₂Ge₁₇ to Na_1.7Ge₁₃₆ proved to proceed without parasitic processes and to comprise the material bulk. The hitherto existing preparation method for NaGe₁₃₆ by gas-solid oxidation of Na₁₂Ge₁₇ has thus been translated into a scalable high-temperature electrochemical approach with enhanced tools for reaction control, promising access to pure Ge(cF136) and Na₂₄Ge₁₃₆ after process optimization.