An Electrochemical Approach toward the Metastable Type II Clathrate Germanium Allotrope

Abstract

By using an anodic conversion process at 280 °C, the type II clathrates Na1.7(6)Ge136 and Na23.0(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 Cs8-xGe136 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 Na1.7Ge136 proved to proceed without parasitic processes and to comprise the material bulk. The hitherto existing preparation method for Nax→0Ge136 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.