Bulk Synthesis and Structure of a Microcrystalline Allotrope of Germanium (<i>m-allo</i>-Ge)

An easy to reproduce and scale-up method for the preparation of a microcrystalline allotrope of germanium is presented. Based on the report of the oxidation of a single crystal of Li₇Ge₁₂ the synthesis and structure determination of a powdered sample of Li₇Ge₁₂ is investigated. Besides the known oxidation of Li₇Ge₁₂ with benzophenone a variety of protic solvents such as alcohols and water were used as oxidants. Electron energy loss spectroscopy (EELS) proves that the reaction products do not contain Li. The structure determination of the powder samples based on selected area electron diffraction (SAED), powder X-ray diffraction, quantum chemical calculations (DFT-B3LYP level of theory), and simulated powder X-ray diffraction diagrams obtained using the DIFFaX and FAULTS software packages show that the microcrystalline powders do not match any of the existing structures of germanium including the rough model of so-called <i>allo</i>-Ge. It is shown that the structural motif of layered Ge slabs of the precursor Li₇Ge₁₂ that contain five-membered rings is retained in <i>m</i>icrocrystalline <i>allo</i>-Ge (<i>m-allo</i>-Ge). The covalent connectivity between the slabs and the statistic of the layer sequence is determined. According to B3LYP-DFT calculations of a periodic approximate model a direct band gap is expected for <i>m-allo-</i>Ge